Best hot water solution for radiant floor plus domestic hot water?

[Montalvo]

I have a 100 gallon commercial water heater (A.O. Smith) in my home that serves my radiant floor heating system (7,100 sq. ft.) plus hot water for two guest bedrooms and our kitchen. The water heater is now leaking (after only five years but, of course, out of warranty) and the radiant heating system installer has recommended against my replacing it in kind. He's suggested instead that I get either a 75 gallon water heater or a tankless water heater for the DHW and that I get either a high-efficiency boiler or a 75 gallon water heater for the radiant system. Now I'm trying to decide how to make the best decision based on economics.

My average annual gas bill is about $2,700 and it looks like about $1,700 of that is for the radiant heating expense, leaving $1,000 spent on DHW. The radiant guy says I'll be able to install everything but the boiler with fairly nominal costs for re-plumbing but that the boiler will require a PVC stack and, while the chase that currently encloses the water heater's stack could probably accommodate it, the piping would have to go up through through two floors and the attic and that would add to the cost.

And one last complication...I have an electric pump that circulates the hot water in the pipes to eliminate the long wait for DHW. Can that system work with a tankless hot water heater?

Based on this, can you help me decide:

a) tankless or hot water heater for DHW?
b) hot water heater or high-efficiency boiler for the radiant system?

I plan to stay in the house for another 20 years (if I live that long!).

Thanks for any advice you can offer.

Bob

[master plumber mark]

Personally, I would Go with the boiler,
I doubt that the tankless
would actually last 20 years..

the tankless will give you fits with the return
recirulaiotn line run..

and I would gamble that the tried and true boiler is
about the same price or less than a compairable tankless..


I am wondering
why not just another 100 gallon commercial heater
what is the reason for not doing that???????


2 75 gallon gas heaters is an ok idea too but you will probably
be wearing out the heater that heats the floor every 5 ;years or so.......

[jadnashua]

Go with a high efficiency boiler. Use it to heat an indirect water tank. You won't need as big of a tank since the recovery rate is much higher. The boiler efficiencies on boilers these days are near 95%. Unless you had an electric WH, your efficiency for that is unlikely much better than 70-80%, so you'll save because you can use a smaller tank, and get better efficiency. WIth the HE condensing boilers, they can exhaust up, but generally exhaust horizontally out. The indirect is usually set up as a priority zone. If you get a boiler with a modulating boiler and an outside reset control, it adjusts the boiler temperature for the conditions, some throttling down to as little as 10% of max output. This keeps the cycles down and the lower the temp, the higher the efficiency, approaching a theoretical 98% or so. Most of these HE boilers only run when there is a call for heat, so they don't sit with hot water in them all the time, which on the older ones meant a lot of waste in the summer. On mine, you can set it to allow a bigger excursion from the set point, so it doesn't cycle as much during the summer. On an indirect tank, the better ones have a loss of as small as 1/4-degree per hour, so standby losses are quite small - it may not need to run the boiler for many hours unless lots of water is used during the day. Once people get their showers done in the morning, it may not need to run until evening.

[GrumpyPlumber]

Hi Jim, let me start by saying hi...from what appears to me to be a very close neighbor.
My license is in a state just below us, currently getting ready to apply for this state.
Ironically I'm right at the tail end of installing a Buderus wall mount and couldn't help replying to this thread.
Some things to know about the condensing boilers....they state AFUE's of well in excess of 90% (up to 98%), but thats at a running temp of 130 degree's...ideal for radiant floors but little else.
The Buderus I'm installing loses approx. 1.25% efficiency per ten degree's above the 130 mark, so even though it would be approximately 90-92% efficient to run regular baseboard & an indirect it's still worthwhile.
What irritates me about this is that they don't openly state this ...in fact it was their tech dept that told me over the phone as I was calling with some questions about installation.
The price of the boiler alone is approximately twice what you pay for a draft vented counter-part...making a difference of about $16-$1800 more.
The gas co (at least in the state I'm licensed) offers a rebate of $800...after doing the math it actually pays itself back within 5 years, depending on square footage of the house - WITH the rebate.
One more factor in pricing boilers...Buderus allows for PVC exhaust venting, whereas others require you buy expensive stainless venting.

[jadnashua]

Mine seems to run at 120-140 except when firing for the indirect - it feeds radiant. When in this mode it seems like the condensate is a torrent coming down the pipe - it really does suck most of the heat out of the exhaust. We'll see how it holds up, but it does look pretty robust.

[poorplmbr]

have you looked into a combi unit like a Baxi Luna?

[GrumpyPlumber]

I just took a peak, briefly, at Baxi...they seem to be the same basic thing - condensing, wall mounted.
One thing about Buderus is it comes with a lifetime warranty (limited of course...but specifically good coverage on the exchanger).

[Montalvo]

Mark, my radiant installer said that he's found poor reliability with commercial installations in residential applications (mine is failing at 5 years) and that's why he recommended going with two separate units for DHW and radiant. And he was proposing tankless as an option only for the DHW, not the radiant. Are you suggesting that the radiant application may have been the reason my water heater failed at 5 years?

Jadnashua, are you suggesting that I get a boiler for supplying BOTH radiant and DHW? That's something that my radiant guy hadn't suggested and I'm curious about how it would work. Does the "indirect water tank" supply both the radiant and the DHW? Is it a standing tank of heated water, much like a water heater? And what's a "priority zone"? Also, how does the outside reset control work and is that only for the radiant? And if I keep my DHW circulation pump running all day (currently, it comes on for 15 minutes each hour), wouldn't that require that the boiler would need to cycle more frequently than twice a day? Sorry for all the questions but as you can see, I'm clearly no plumber!

GrumpyPlumber, if I use a horizontal exhaust on the Buderus, does it have to have a minimum upward slope or can it be truly horizontal?

PoorPlumr, I haven't looked at any brands yet but I'll check out Baxi Luna. Is it a boiler?

And in case it matters, I live in the Sacramento area with winter lows typically in the high twenties. The water heater is in a large equipment room off the garage and the longest run for DHW is about 70'.

Thanks again, folks!

Bob

[GrumpyPlumber]

Montalvo...if your plumber is working with radiant tied into a water heater and hasn't mentioned boiler options...I dunno 'bout that.

A water heater is intended primarily for potable use... 5 years sounds good for what it's being put through (constant firing in the winter months to keep up).
IF...and only IF you, for some reason, decide to stick with the water heater...ask your plumber what an "anode" is - he'll explain.
AO Smith generally comes with a 6 year warranty, but I don't think it's intended for use as a "boiler".
Do some math...a water heater runs at 60-65% efficiency...with a lifetime of 5 years for your useage and a cost to replace of anywhere from at least $1100 to $1,400 or more for a 100 gal - depending on type.
Even a low end draft type boiler runs at 80-82%...and many offer a 20 year warranty.
For roughly $2,000 more you can get a condensing boiler...they run at 90-95% and you can run both radiant and indirect hot water off them.
Condensing boilers come with even better warranties.
Take your average gas bill and figure it out...figure in the average cost per year for replacement as well as gas cost

The vent on a condensing boiler is suggested to slope at 1/4" /foot and needs a sanitary drain to tie into.

[jadnashua]

Basically, an indirect WH looks like a normal tank. Instead of electric or gas to heat it, it has what amounts to a big coil in it - a radiator. The boiler runs to make the WH tank hot. Since making hot water for a shower is usually more important that momentary heat for the house, it shuts the pump off for the house heat while it is needed to make water for say a shower. Since the boiler is tyically much larger heat source than that used to heat a normal tank, you get faster recovery and can usually end up with a smaller tank. You can get them with stainless steel tanks and a lifetime warranty...you should essentially never have to replace the tank.

You'd run the radiant heat with a separate pump, and use a mixing valve to keep the water temp for the radiant low enough so that when the boiler runs hotter, say to heat the WH, it won't overload the radiant circuits which normally are only in the 110-130 degree range, depending on design.

An outside reset control is used by the boiler control to adjust the heating curve of the boiler...if it senses it is only 40 degrees outside, it won't make the water as hot as it would if it sensed it at 0-degrees. By making the boiler adjust to the outside temperature, it runs longer and at the lowest temperature it can. The intelligent boilers will also monitor the return water temperature and adjust the level as well. This is the most efficient mode. Ideally, the boiler basically runs. On and off wears it out and after it gets everything hot, if it isn't needed anymore, you've got a whole bunch of hot water heating up the boiler room rather than the house.

[Montalvo]

GrumpyPlumbr, first let me explain that my radiant installer did suggest getting a boiler as an alternative, although only for the radiant system (relying on a WH or tankless WH for DHW). My system is gas so I assume that any discussion of an anode doesn't apply; is that right? And you mentioned the need for a "sanitary drain" in conjunction with a boiler's exhaust. Since my equipment room is above grade but below the level of my sewer system, I assume that I'd have to somehow connect the boiler's exhaust to the sump pump that pumps from the sink in that room up to the sewer line. Is that right?

And Jim D, it sounds like you're recommending that I get a boiler with a) an indirect tank for DHW and b) a separate pump with a mixing valve for the radiant system. But from your description, it doesn't sound like an outside reset valve would make sense in my climate. With mid-to-high 20 degree temperatures being the lowest we get and with a need to reduce the boiler output with a mixing valve on the radiant system, I assume that I could permanently set the boiler to the lowest setting to ensure a minimum amount of on-off conditions. If I did that, how would it effect the efficiency? And can you explain how I'd figure the sizing on the boiler and the indirect tank? I assume I have to start by figuring the maximum demand for both the radiant system and the DHW. But then what?

Maybe I'm trying to figure out too much of this on my own and should just call in some vendors to get their proposals. But I get nervous when I rely on folks intent on making money at my expense. And I've been living with a system that, although professionally installed, probably wasn't optimally designed to begin with.

Thanks, guys.

Bob

[alternety]

It would probably be good to get a heating guy that understands this stuff and get a proposal. Finding a good one could be tough.

You should not be mixing radiant heating water and potable water. It has potential for growing dangerous bacteria in your potable water. Incomming water quality may affect the heating system lifetime because of hardness, dissolved oxygen, and other stuff.

You have in-floor radiant. What material was used for the tubing. How is it attached to the floor. Embedded in a poured material, stapled to the underside, attached to the underside with metal plates? If stapled, is it properly insulated below the tubing? The method of attachment will affect the temperature of water needed. Do you have any data on the layout of the emitters. Is it all one zone? If it uses plastic tubing, what kind is it and does it have an oxygen barrier? Plastic without an oxygen barrier can cause corrosion in the boiler and associated equipment (e.g., pumps).

To size a boiler you need to do a heat loss calculation. A contractor who has any idea what he is doing can do this. There is a free software package on the slantfin web site if you want to play with it and get an idea of what is involved.

With a reasonably smart modulating boiler, and probably outdoor reset, you should not need a mixing valve for the heating. However, it is a good idea to keep the indirect DHW tank at 140 degrees to prevent development of things like Liegonella. Then use a tempering valve to bring it down to safe temperatures for the faucets. If you have the opportunity you could run the 140 degree stuff to a dishwasher or cloths washer.

There are a number of boilers that allow PVC venting. These sealed systems also have the merit of not taking combustion air out of the house.

[Guest]

Here's some background, alternety: My system was installed five years ago, uses PEX tubing without an oxygen barrier so it employs non-ferrous materials. It's stapled to the subfloor with a 1 1/2" gypcrete cover and insulated below, although several rooms of the house are on a slab so it's imbeded in the slab there. I have 14 zones with thermostats and I also have four central heating/AC units (six zones) and can use central heating to heat the house quickly such as when we return from a trip and have turned the radiant system down low.

Regarding your comment about mixing DHW with radiant heat water, I assume that Jim D's recommendation to use an indirect tank would ensure that the two were kept separate. Am I correct on that?

Bob

An indirect does not mix any of the boiler water with the potable water...it is heated by an internal hot water coil fed by the boiler - just like another zone on the boiler. As noted, this gives you more hot water faster since your system can dedicate the entire boiler's capacity to it if on a priority circuit (which momentarily disables the room heating pump). If you pick one with SS like the SuperSTor Ultra, it should be the last tank you ever buy. Because you don't have a super hot burner or element, you don't get the calcium deposits in it as much, either.
__________________
Jim DeBruycker
Important note - I'm not a pro

Thanks, Jim D, I think I understand the indirect tank but... Tell me more about the outside reset valve. Am I correct in assuming that an outside reset valve would be of little value for me because a) my in-floor radiant system's water temperature should never be hotter than about 130 degrees and b) the winters where I live are mild, with lowest lows in the mid-20's?

Bob

The outside reset is not a valve, it is an input to the boiler controller. It "resets" the control, telling it what is needed based on the outside temperature...the colder it is, the more likely it needs a higher temperature. The best comfort is when you run the system constantly. The outside reset would tell the system when it doesn't need the water at 130, for example, and on a mild day may run it constantly at say 100, verses 130. Add that to a modulating boiler which gets more efficient at lower temperatures, and yes, it is actually more useful there.
__________________
Jim DeBruycker
Important note - I'm not a pro

Jim, Bob...
I just fired one up today...wall mount buderus gb-142.
Bob...after seeing what you've typed so far (and boy there's alot to read).
You have 14 zones...how many square feet?
Bottom line...you sound like the perfect candidate for what Jims saying...which is exactly what I fired up today.
It's a little frustrating to go over and over the details...though I understand you want to know your options, the most efficient potential you have is this one.
I have done a good dozen or so tankless water heaters in the last couple of years...they run at the low to mid 80% AFUE (beats standard tank types at 60-65%), where condensing boilers get better than 90% and cover the heat also.
Beware...the "sticker price" will bowl you over...especially where it's not a "switch-out" boiler install. (same dimensions...same style boiler...easier install).
Also, if you have natural gas, you might be eligible for a substantial rebate - call them.
The outside temperature sensor that Jim mentioned simply helps the unit to run more efficiently in the summer months by letting it know to ease back on btu consumption...i.e. the indirect water heater takes less energy to reheat.
Grumpy Plumber

I was surprised at the number of controlled zones you have. I kind of related hot water heater powered and shared heating and DHW water with single or a small number of zones. How are you controlling the many zones in your house? One circulator and lots of solenoid valves or many pumps? You also indicate that there are differences in the thermal characteristics in some floors. In the slab on grade areas, did you insulate under the slab (you should have)? If you are using a fixed speed circulating pump and controlling zones with valves, you need to have a pressure bypass in the system. There are variable speed circulators with built-in pressure sensors becoming available that could eliminate the need for the bypass (my preferred solution). Also needs an expansion tank in the system.

If you have a situation where small zones can call for heat you may want a buffer tank to isolate the heat demand from running the boiler. This prevents short cycling of the boiler (bad for the boiler, and lowers efficiency). The demand is satisfied from the buffer tank. When the buffer tank water drops below a certain point the boiler recharges it.

Anyway, for as much as you have going in this system you should look into a smart control system. This type of system will learn the characteristics of a zone and provide the best operating conditions. It will know how much energy to pump into a zone to get it to the correct temperature without overheating (within limits caused by such things as sunlight and rapid outdoor changes). It will control the pumps and valves. It integrates everything. The outdoor reset, DHW, exercising the pumps and valves when they are not used enough to keep them limber, adjusting the temperature of the circulating water. Everything. They are expensive. My choice would be equipment from Tekmar. They have a lot of information on their web site. They will replace some of the smarts that will probably be built into the modulating boiler.

Of course, if you are happy with the way it works now, you can just stick in a new boiler. But I would do an indirect DHW tank.

This whole thing also serves as a warning about asking what you feel is a simple question. :-)

Alternety

[alternety]

The lack of oxygen barrier could be a problem with the boiler core. Some of them are aluminum, some stainless. I would be inclined toward stainless in your situation.

It is my understanding that there is oxygen diffusion even when the pipes are embedded in concrete or gypsum. The oxygen from fresh water and recirculating heating water may have been a part of the water heater failing early. It would be interesting to open it up when you take it out and see what it looks like.

Have you had a chance to look at the heat load software? One of the factors in determining how many BTUs your pipes can deliver is spacing (and thus length). Your control parameters will be the water temperature and the flow volume in the circuits. If they are not there already, you can put adjustable valves/solenoids on the manifolds to adjust each loop. Volume will be a function of pipe size and pump selection. If you had satisfactory heating performance with the water heater, that gives you a ball park for the BTU load. You do not want to oversize the boiler, so you should do the arithmetic. If the boiler is too big it will run shorter cycles. And that is not a good thing.

Make sure that it is large enough for the expected DHW demand. If you have some monstrous shower heads or some such thing, it could be a significant factor in boiler sizing. It is possible to get essentially infinite DHW. Look at the size of the indirect water heater (which is the way to go), recovery rate, characteristics of DHW load (like that 2 hour shower while perfecting your operatic performance).

For your boiler you want the modulating unit with essentially no water in the boiler itself. Many have only a gallon or so. This keeps the heat in places where it does not dissipate without useful gain (i.e., in insulated tanks or in the floor). This is important for summer DHW as well as overall effectiveness.

[Montalvo]

Here's more background in response to your questions:

House is 7,100 square feet, not including the four-car garage that's also heated. The only rooms on slab (and with no insulation beneath) are the garage, a guest suite, an exercise room and a project room; all the rest have PEX imbeded in gypcrete. The slab rooms are infrequently heated or seldom to much more than 50 degrees. The radiant system has what appears to be two pumps, 14 solenoids, three green control boxes (Taco mfr.), an expansion tank and a number of manual valves. Each zone is controlled by its own thermostat. About 4,000 sq. ft. is limestone and the rest is carpeted with a special radiant pad beneath (not slab foam but the next best). The heating system, relying on the 100 gallon water heater shared with DHW, seems to have worked fine, although we've had some shortfalls on hot water in the guest room during winter (weird...enough hot water in the bathroom sink but not in the shower).

Since the Buderus boiler was recommended by several of you, I called two of their distributors in my area and asked for installers who've bought that boiler. I've put in two calls, both to plumbers who specialize in radiant system repair/installation, but haven't heard back yet.

Based on the replies to my initial question, it looks like I want a high-efficiency SS boiler with an indirect SS tank for DHW, an outside reset and a mixing valve for the radiant (if it doesn't have one already or unless the "smart" modulating boiler makes it unnecessary).

The one thing that's still unclear to me is how my DHW recirculation pumps will work with the indirect tank. It sounds like I'll have to keep the indirect tank supplied with 120 degree hot water 24/7, since the recirc pump will pull hot water 15 minutes every hour (except 11:00 pm to 5:00 am) to heat the pipes and eliminate the wait for hot water in remote locations. I guess that's still better than heating a 100 gallon WH 24/7 like I've been doing.

Dang, this stuff can get complicated! Thanks again for the many insights.

Bob

[jadnashua]

The indirect basically becomes another zone. It is usually set up as a priority zone, so when it needs heat, it can get it since most of the time a shower is more comforting than a momentary decrease in room heat capability. WIth big slabs, the need for heat is mitigated by their large thermal mass.

You'll end up with two different temperature supplies - full boiler output for the indirect, and the output from a mixing valve to the rest. The modulating boiler will adjust the supply water temp to the outside and inside needs, but will jump to max when the indirect needs it to speed recovery, otherwise, it will run just where it needs to. On a mild day, my boiler water may only be 110 degrees, but on a cold winter day, it could average 150-180.

You might want to consider putting the recirculating system on a timer so it gets a rest late at night.

The boiler may not have to turn on each time the recirculation system runs; it will depend on how much water actually circulates and what its return temp is. If you run the tank at 140 with a tempering valve to decrease it to say 120, you end up with the effect of a larger tank but don't generate the liability of the super hot water.

I chose BUderus for a couple of reasons; their US headquarters is only about 10 miles away so parts should be quick if I ever need them, and the shop foreman I was dealing with had installed one in his home and liked it. There are other good units available.

[GrumpyPlumber]

Oh boy...dats a big boiler yer gonna need...at least 250K BTU for the space heating alone, without garage...tack on the DHW and garage - off the top of my head I dunno where you'll find a single condensing boiler that size, though I'm sure there would be....Your plumber will have some calls to make.

[Montalvo]

GrumpyPlumbr, the existing 100 gallon WH says "Input 199,000 Btu" and has a recovery of 193 gal/hr. It's been serving our needs for five years now, including a near-record cold period this past winter. Based on that, and the fact that a high-efficiency boiler would be more efficient, shouldn't I be able to get a boiler that's under 200K Btu?

Jim D, the recirculating pump is on a timer which allows me to set "on" in 15 minute intervals and I do have it shut off late at night. You mention that on a mild day, your boiler water may be only 110 degrees. If the DHW needs to be 120, doesn't that suggest that the boiler always needs to be at least 120? And since I won't use the radiant heat at all for almost 8 months/year, will it mean that I'm heating both a boiler and an indirect tank throughout the year? If so, is that still more efficient than getting a boiler for the radiant heat and a WH for the DHW?

Sorry if I'm a little slow in getting all this...

Bob

[jadnashua]

The boiler runs based on the need. When the indirect says it needs heat, to get it quickly, it jumps the output temp to near max, which, depending on the boiler, is usually something near 200 degrees. that's why you need the mixing valve so you never overheat the radiant loops. The high-efficiency, condensing boilers only have a few quarts of water in them on average, and they don't stay hot unless there is a call for heat. The reset control helps with that. When it is warm outside, it knows it doesn't need standby heat and basically lets itself cool off. It will only come on if there is a call for heat, or, if in self-protection mode, if it is cold, but your system is turned down. That protects it from freezing. Mine often reads room temperature this time of the year since it isn't producing heat for the radiant circuits. It is only when the thermostat in the indirect calls for heat does it fire. So, while the boiler is on constantly, it only fires when there is a need. It can modulate its output so it only produces enough heat to get the delta T at an efficient place. It does not modulate when the indirect calls for heat, it runs at 100%. That can be changed in the setup. So, there are basically no standby losses by keeping the boiler running all the time, it only fires when there is a call for heat, and because it only holds a very small amount of water, you don't waste much when it does shut off. Older boilers with a fairly large tank didn't work that way. They maintained a fairly significant volume of water hot all the time, so there were more losses. They also could not stand the shock of cold water coming into a hot boiler...the new ones can go from cold to hot fairly quickly. They do this while operating in the 90% efficient range. Last I looked, there were only a couple of boilers that qualified for the federal income tax credit, and for my need, they were all too big. Your needs may fall into that category where you can get one, and that may sway your decision on brand and model. The requirement is that it must be >=95% AFUE. Mine is a little over 94%, and didn't qualify. I was able to get $1100 back from the utility company as a rebate, though, which helped. This varies by region and utility.

[alternety]

You really! need to do a load analysis. The water heater you have should not be assumed to be the correct size. There are a lot of people out there that don't know how this works. One of them may have sized the water heater.

You need intelligent control. If you do not feel that your heating system is smarter than you are; you probably have not gotten it right yet.

The proper control being done, the boiler runs in two different modes. Heating DHW and heating the house. If you use 140 degree hot water (recommended) the boiler will probably run 180 or higher when DHW demand is present. Efficiency will be lower when this is happening. But probably still better than a water heater for a number of reasons.

When heating the house, outdoor reset will adjust the water temperature. My house heating water was running at about 80 degrees last winter (fairly mild climate-low around 20 degrees F). I have a well insulated and pretty air tight house. YMMV.

If you got it right (controls) the boiler knows when it is heating the house and when it is heating DHW. No mixing valve on the heating side is needed. The boiler knows what temperature to generate. Sort of like the Shadow, but less combative. There are some techniques that do use a tempering valve but I do not see the merit of these approaches.

Some people use constant circulation to "even out the comfort levels" but I am not a fan of this approach. You use more electricity. This leans a bit toward belief vs results. There are those that will disagree with that statement.

I designed my system in accordance with what I have told you. Seems to work fine. I tried to use a variable speed pump controlled by pressure drop to deal with zones opening and closing without the pressure bypass. Failed because of serious noise from the variable speed circulator. New pumps may fix that. In my opinion, all pump motors should be brushless permanent magnet devices with appropriate intelligent controls.

[jadnashua]

The in-floor radiant stuff should never see the full output of the boiler, although it is fine if it is lower than the maximum (which depends on the system, you never want the floor surface to be more than say 80-degrees or so for most surfaces). Without some way to temper the boiler's output after it just finished heating the indirect at 180-200 degrees, you must have some sort of mixing valve in the system as far as I can determine, otherwise, you'd have the system pumping that too hot water to your radiant loops. Now, it probably wouldn't stay at that temp for long, but could cause problems. The fanciest systems use a computer controlled mixing valve and maybe a low-loss header assembly to account for significantly variable flow rates (as different zones open/close). When just running the radiant, you can usually tell the computer the max temperature to allow, but as soon as the indirect calls for heat, that is overriden. Normally, the priority circuit turns off the other circulators to allow all the heat to go to the indirect, but when it switches back, there's still a shot of hot water in that primary loop that will make its way to the radiant unless there is some way to temper it.

[Montalvo]

More good information...I'm feeling like a sponge reaching full capacity but I'm sure there's lots more I don't know.

One of the guys called this afternoon and is planning to come out next week to look at my system. He said Buderus doesn't make a SS boiler (only aluminum) but he has another brand he'd recommend. He explained that the indirect tank can be over-sized for a nominal cost (he thought probably 60 gallon, before doing an analysis) because it is so much more energy efficient than a WH when in stand-by mode. He said that my system with O2-permeable PEX was clearly a better way to go because my pumps, etc. were of higher quality material.

I'll let you know what he proposes.

Bob

[GrumpyPlumber]

Bob, you're probably ok with 200K...I forgot the fact that your in a warmer styate than I.I might still suggest something bigger , but thats your decision to make along with your plumber.

[Montalvo]

The plumber finally came out and recommended that I install a potable water boiler from Phoenix rather than a boiler and an indirect tank. He pointed out that a boiler and indirect tank would cost more to buy and would involve more re-plumbing, since he'd have to remove the hydronic system's heat exchanger in order for it to receive water directly from the boiler.

He's working up a bid but thought that the potable water boiler with an 80 gallon SS tank and perhaps 130,000 Btu burner would run almost $10K (not including installation). The only encouraging part is that I'll be saving on my gas bill.

I'd like to hear your thoughts on his recommendation.

Thanks,

Bob

[alternety]

You need to get a heat loss calculation done. No one can make an accurate comment about specific equipment sizing without knowing this. Use the software I mentioned. It is not real hard to do.

I am not a plumber/heating person.

I searched for Phoenix water heaters and the only thing I found was a Phoenix heater from HTP. Is that what you are talking about?

I don't remember you saying where you actually are located and some idea of the heating season; other than it is fairly warm.

In your last post you talked about an installed heat exchanger for the radiant. I do not remember you stating this before. This separates the DHW from the radiant water. This makes the use of a single source of hot water for both uses safe.

I don't really understand why removing a heat exchanger would be particularly costly process.

There are fuel saving functions available with a modulating boiler and outdoor reset. Correct water temperature in the radiant zones should be properly controlled. That may be able to be done with with a mixing valve on the floor side. If the required temp is much lower than your hot water temp, this would become harder to do. A modulating boiler can take care of providing proper temperatures as heating demand varies. The hot water heater does not.

Just as an example of the scale of things. I live in an area where the lowest temps in the winter are in the 20 degree F range. I have about 6000 sq feet heated. It is very well insulated and tight. My calculated boiler size was in the range of 80 KBTU. I put in a boiler with about 120 K because of some high energy use devices attached that have no association with indoor temps or DHW. It has only been running with the house occupied for about 6 months. I have no real way to evaluate actual performance as yet.
Most of the time when I went to the utility room and looked at loop water temp it was below 110 degrees. Many times in the 80 degree range.

Actual conditions render examples like this less than exciting; but you get some idea how one could work.

You might also post a few pictures of your existing equipment for people to get an idea of the space.

If you can get the basic information together, perhaps a pro would be willing to ballpark a system cost for you. Just a SWAG. Perhaps a response via a personal email.

[Montalvo]

Yes, the Phoenix boiler you found from HTP is the one that the plumber is recommending (here's a link: http://www.htproducts.com/products/phoenix/index.html). He actually recommended either that OR the HTP Munchkin boiler with an indirect tank and we talked about the trade-offs. But with both products being high-efficiency with long predicted lifespans, the discussion came down to whether the Munchkin would provide enough incremental savings on my gas bill to offset what looked like almost a 50% higher initial cost over the Phoenix. Based on that, he thought the Phoenix would be a better choice.

I'm going to try using the heat loss calculation software to see how it compares with what he recommends. I believe that his appraisal was based on learning that my current 100 gallon, 199K Btu WH had been, in fact, serving my needs satisfactorily for the past five years.

Living in northern California with a lowest winter temperature in the mid/high twenties, my gas bill may not be as bad as what you experience up north. The house is five years old, well-insulated 2X6 exterior walls and low-E double-paned Pella windows so my potential for offsetting a higher initial installation cost through greater system efficiency is limited.

I'll snap some pictures of my current set-up and post them tomorrow so you can see what my plumber was seeing. And I didn't realize that a boiler will require TWO PVC pipes: an air intake and an exhaust. While I think we can make that work, he also told me that I'd need to run a pipe for the condensate from the boiler to a drain. The good news is that I have a drain in the middle of the floor of this room; the bad news is that a pipe will have to run across the floor from the boiler and dump into the drain. I asked why we couldn't just let the condensate run across the sloped cement floor to the drain and he explained that the condensate water is acidic and would eat away at the concrete floor.

Also, he mentioned that I have a mixing valve that sends maximum hot water through the recirc system to heat the pipes but then sends a set temperature water when a valve is opened. He stated that he doesn't like to use mixing valves, preferring instead to use the scalding prevention adjustment at the shower valve instead. (I think I'm stating what he said correctly but I might have it wrong.) Any thoughts on this?

Bob

[Montalvo]

As promised, here are some pics of my water heater and hydronic system:

www.geocities.com/pics2send

All this equipment is in an 11 X 16 foot equipment room that also houses my house network and lighting relay panels, audio/video rack, security system, furnace, freezer (next to water heater) and elevator equipment.

Bob

[alternety]

A couple of points. I can't really give you numbers to support some of the things I am going to say. The arithmetic is complicated and requires test data.

First the easy stuff. Yes the water is acidic. It may also have some other things in it from impurities in the fuel. If you must run it over the floor, pass it through a bed of marble chips first to neutralize it. It will probably stain the floor and may be a bit of a humidity problem. When these suckers run in full condensing mode it makes a lot of water.

Yes you can run high temp DHW and use temp controls (anti-scald) devices at the point of use. This actually has some benefit in that all the hot water piping gets exposed to temps high enough to kill dangerous things that can grow there. It would also be good for the dish and clothes washers. However, this means everything that could scald needs to be protected. That is a lot of new, and probably pricey, fixtures to install. And of course she who must be obeyed must like the new fixtures. I actually thought about doing this but it got lost in the process. All my tub/showers have the right fixtures, but sinks do not.

Now for the harder stuff.

I do not believe that the efficiency numbers you see for the water heater are directly comparable to the annual efficiencies of the modcon (MODulating CONdensing) boiler for heating your house. I am not familiar with the internals of the condensing Phoenix unit, but I will describe the useful attributes of the modcon.

The condensing process in the boiler is needed to extract the water vapor from the exhaust gasses. Thinking back to high school, this captures the latent heat of vaporization of the water from the exhaust. For this process to occur, the temperature of the condensing chamber must generally be in the 120 - 130 degree F range. Velocity of gas flow, configuration, etc. affect the actual performance as well. The point of this is that you will be keeping the hot water heater above 120 degrees from the discussion about scalding. And 140 is needed to keep little gremlins from growing in the water. You are probably more likely to be in a range where full condensing may or may not occur. Stratification and incoming cold water will help this.

With the modcon, the temp of the generated water is adjusted to be the minimum needed to heat the house based on the conditions seen by the outdoor reset. This can keep temp well below what the hot water heater must maintain. Demand from the DHW indirect would run above the condensing point. If the emitters and heat loss are well matched the modcon will be in full condensing recovery most of the heating season. The ability to use outdoor reset may account for something in the range of 20% of the gain enjoyed by switching to a modcon from an older simple boiler.

A separate boiler can run into issues of short cycling if small zones call for heat. There is no appreciable water in the modcon so sometimes it is useful to include a buffer tank between the boiler and the zones.

I believe (from reading opinions of heating guys) that the theoretical optimum for a heating system is when the boiler runs 24X7 on the design day (with constant water circulation).

OK. All this said: you have selected a pretty cool hot water heater. In simple terms it is pretty much equivalent to a modcon with a buffer tank. You have separated the DHW and heating loop water via a heat exchanger, and you have dealt with oxygen in selecting system materials. It can not adjust temp for weather conditions (e.g., heat load variance). Using the constant and higher temp for heating will cause the zones to run in shorter cycles. This has some wear and tear affect on pumps and valves but I do not think it is enough to really be an issue. You will have a pump circulating the water from the heater to the heat exchanger and one circulating through the heating tubes and heat exchanger. If you were happy with room temp control before: leave the control system alone. Smarter thermostats that understand thermal mass delay will help if you are not happy. If you have an intelligent system installed, you may be able use outdoor reset to adjust heating water temp. There are controlled variable mixers that can work on the heating side. They are limited by the difference between the temp of outgoing and returning water in the heating loops. A good system can learn the room characteristics and adjust the zone behavior. Bunch of bucks.

If the water in the previous boiler was kept hot my mistake (i.e., no one actually determined what temp water was needed on the design day [worst case heating day] you may be able to run the hot water heater at a lower temp (e.g., 120) and heat just fine. It is better to use 140, but there are millions of DHW systems running at 120. Part of that is because of the energy and safety people pushing that operating point.

My gut feeling: the payback for going with a modcon vs the water heater is probably pretty long. The water heater (Phoenix) seems like an attractive option. Remember - I am not a pro and I said gut.

I just looked at your pictures. A nice neat installation. What is that device directly above the heat exchanger for? It looks like it is between the DWH and heating circuit.

[Montalvo]

The suggestion about the marble chips sounds like it might be worth a try to see whether I'm better off with a damp floor as opposed to a pipe sticking out into the middle of the room.

And I feel better hearing your endorsement of the Phoenix as a reasonable alternative to the boiler with indirect tank option. I sounds like it's necessarily tough to predict what system design will optimize efficiency and whether my utility bill savings will provide a reasonable ROI. And I'm sure it's complicated by the fact that my most common usage is vastly different than the design day usage. It's just my wife and me in this house...until the 17 house guests (our max, so far) arrive to remind us why we have seven bathrooms!

You mentioned thermostats and I'm curious about that. I've been using the nighttime setback on my radiant thermostats (Robertshaw 9610) but wonder whether it saves or wastes energy to lower the temp at night when it must re-heat all that thermal mass the following morning. And the factory setting for the temp differential on the thermostats is 1 degree. The manual says to increase the differential if the system turns on too often but I never know when the system is cycling on and off because it's silent. Any thoughts on that?

Bob

[GrumpyPlumber]

The in-floor radiant stuff should never see the full output of the boiler, although it is fine if it is lower than the maximum (which depends on the system, you never want the floor surface to be more than say 80-degrees or so for most surfaces). Without some way to temper the boiler's output after it just finished heating the indirect at 180-200 degrees, you must have some sort of mixing valve in the system as far as I can determine, otherwise, you'd have the system pumping that too hot water to your radiant loops. Now, it probably wouldn't stay at that temp for long, but could cause problems. The fanciest systems use a computer controlled mixing valve and maybe a low-loss header assembly to account for significantly variable flow rates (as different zones open/close). When just running the radiant, you can usually tell the computer the max temperature to allow, but as soon as the indirect calls for heat, that is overriden. Normally, the priority circuit turns off the other circulators to allow all the heat to go to the indirect, but when it switches back, there's still a shot of hot water in that primary loop that will make its way to the radiant unless there is some way to temper it.
I meant to comment earlier...with your boiler there's supposed to be a seperate manifold isolating the zone to protect the exchanger when used for radiant.

[alternety]

The general feeling I have seen about radiant set-back is not to do it. One of the reasons is the delay when using high thermal mass emitters (e.g., concrete floor). Does your room actually get much cooler before the system needs to start heating again? It can also lead to overshoot when it is reheating the thermal mass. The degree (sorry) of success is also Dependant on the intelligence of the thermostat.

The physics is that if the room is cooler, less heat will be lost to the outside across the insulation layer. With forced air, the responsiveness makes the effort profitable. For a conventional heat source not using power vented combustion air, a longer cycle on return from reset will probably make that period of operation a bit more efficient.

My guess is that if you were using a modcon, you may cause the boiler to run at higher temps and may actually reduce burn efficiency a bit while bringing the mass back up to temp.

It is another one of these hard to quantify numbers. It involves interaction of complex factors. Mass, thermal loss characteristics, comfort, boiler performance characteristics. There are complex modeling systems that can evaluate these things and calculate the expected results. You are more likely to use a thermometer, wet finger, walking barefoot in your skivvies, and reading your fuel bills. Less scientific, but the results should be good. But not as quick as a simulation. Your situation for tracking heating cost is difficult because you are using the gas for DHW and maybe cooking.

If you are comfortable in the house with the existing thermostats and setback scheme, it probably is not making a big practical difference one way or the other (with respect to the cost of replacing lots of equipment). As I think about it, there is one issue you may have with the water heater and heating setback. Because of the shared heat source, you can not identify when DHW load is occurring. A smart boiler will stop heating the floor when DHW demand occurs. By demand I mean when it would have to recharge an indirect hot water heater. Morning showers and setback recovery may be an issue unless the boiler can supply BTUs for all the concurrent loads. You can figure this out by finding the inlet temp on the water heater (e.g., your water supply temp) and desired output temp. Figure out the flow rates of your showers and fixtures, decide how long they are going to be used, if they are going to have simultaneous use, anything else taking hot water concurrently, and add them all up. This gives you the worst case flow. Calculate the BTUs needed to raise that many gallons to the required temp. That is how many BTUs you need to have to keep the showers warm if you are not heating the house at the same time. You could mitigate the heating load by using your thermostat setbacks to turn heating off when you have hordes of guests showering. Remember that we have been talking about output BTUs; not the input to the boiler, which is what the ratings you see represent.

Something I was going to mention earlier. Some areas will not allow a hot water heater to be used as a heating system source. Obviously you have on already, but I thought I would mention it. Note that the Phoenix is designed to do this.

Perhaps you heating guy can provide you some input on these issues.

[alternety]

I had a better idea about DHW vs heating demand. Make sure the piping at the output of the tank for DHW has a accesable place on it near the system. You may need a loop to stop convection. Attach a sensor to the pipe and when I gets hot from running DHW, use the sensor to disable the heating system.

A possible sensor would be a high temp limit switch. It just clamps on and has a switch and adjustable temp.