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

  1. #16
    Retired Defense Industry Engineer jadnashua's Avatar
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    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.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  2. #17
    Licensed Grump GrumpyPlumber's Avatar
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    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.

  3. #18
    DIY Member Montalvo's Avatar
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    Default 250K Btu sounds high based on what I've got...

    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

  4. #19
    Retired Defense Industry Engineer jadnashua's Avatar
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    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.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #20
    Like an engineer alternety's Avatar
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    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.

  6. #21
    Retired Defense Industry Engineer jadnashua's Avatar
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    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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #22
    DIY Member Montalvo's Avatar
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    Default Again, thanks, guys!

    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

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    Licensed Grump GrumpyPlumber's Avatar
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    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.

  9. #24
    DIY Member Montalvo's Avatar
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    Default Update: the plumber's recommendation.

    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

  10. #25
    Like an engineer alternety's Avatar
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    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.

  11. #26
    DIY Member Montalvo's Avatar
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    Default Thanks for the response, alternety.

    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

  12. #27
    DIY Member Montalvo's Avatar
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    Default Pictures...

    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
    Last edited by Montalvo; 06-30-2007 at 10:15 AM.

  13. #28
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    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.
    Last edited by alternety; 06-30-2007 at 03:00 PM.

  14. #29
    DIY Member Montalvo's Avatar
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    Default Very good info, alternety...thanks!

    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

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    Licensed Grump GrumpyPlumber's Avatar
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    Quote Originally Posted by 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.
    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.
    "The biggest regrets we have in life are the chances we never took."

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