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

  1. #1
    DIY Member Montalvo's Avatar
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    Default Best hot water solution for radiant floor plus domestic hot water?

    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

  2. #2
    Master Plumber master plumber mark's Avatar
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    Talking go with the boiler

    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.......
    Last edited by master plumber mark; 06-18-2007 at 01:42 PM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Licensed Grump GrumpyPlumber's Avatar
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    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.

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6

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

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    Licensed Grump GrumpyPlumber's Avatar
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    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).

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    DIY Member Montalvo's Avatar
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    Default Thanks for responding...

    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

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    Licensed Grump GrumpyPlumber's Avatar
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    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.

  10. #10
    Retired Defense Industry Engineer jadnashua's Avatar
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    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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member Montalvo's Avatar
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    Default OK, let me see if I've got this right...

    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

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    Like an engineer alternety's Avatar
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    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.

  13. #13
    Questions from readers Guest's Avatar
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    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.
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    Jim DeBruycker
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    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.
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    Jim DeBruycker
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    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


  14. #14
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    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.

  15. #15
    DIY Member Montalvo's Avatar
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    Default Thanks for the help, alternety.

    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

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