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Community and Q&A

Domestic hot water and space heating

Vladimir Polyakov | Posted in Mechanicals on

I am trying to put together a system that can deliver domestic hot water and heat the house. The colonial house is in Brooklyn, NY ( zone 7), 29′ x 33′ and consists of 2 floors + basement. I am doing complete renovation and already removed all walls throughout the house and will use closed cell foam insulation in all walls and roof. I plan to run Pex in joists for both floors and I am thinking of using baseboard in the basement.

My questions are:

1. I read a lot on this website about issues with oversizing boilers, so I am trying to see if a boiler with around 8k-80k BTU/h will not short cycle , like for example HTP UFT-80W .

2. I found several companies online that design and pre-fabricate the whole domestic heat/water system, and in their quote I see they use Takagi T-H3-DV on-demand water heater, 15,000-199,000 BTU. Will this water heater short cycle in my scenario? Is it better to use a dedicated boiler instead?

thank you for any help

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  1. Steve Knapp CZ 3A Georgia | | #1

    Vladimir. I believe Brooklyn is climate zone 4A. See this article for more info on that topic.

    I'm sure other folks will post to address your question.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    This is a big topic. The nature of your questions makes me wonder whether you will be able to design your own hydronic heating system. I suggest that you consult with a mechanical engineer or a reputable HVAC contractor with experience designing hydronic systems, because you probably don't want to learn everything about this topic, starting from scratch.

    Briefly, several manufacturers sell "combi-system" water heaters that are designed to provide domestic hot water as well as hot water for a hydronic space heating system: for example, the Vertex from A.O Smith, the Phoenix from HTP, or the Polaris from American Water Heater, or the Combi2 TTW from Bradford White.

    Before you select an appliance, however, you need to (a) verify that your local building department will approve this type of appliance for space heating -- some jurisdictions require the use of a separate boiler -- and (b) perform a room-by-room heat load calculation (which is always the first step for any heating system design).

    For more information on load calculations, see these three articles:

    Saving Energy With Manual J and Manual D

    How to Perform a Heat-Loss Calculation — Part 1

    How to Perform a Heat-Loss Calculation — Part 2

    For more information on some of the pitfalls associated with using a single appliance for domestic hot water and space heating, see Using a Tankless Water Heater for Space Heat.

  3. Vladimir Polyakov | | #3

    Thank you very much for all the responses.

    I calculated the rate of heat loss in my house to be approximately 25k BTU/h.
    New York outdoor design temperature of 15°F. ΔT of 55°F.

    I was looking at this combi boiler as an option:

    SKU: ZWB-28-3 Brand: Bosch

    This Bosch unit has BTU inputs ranging from 34.6 to 100.6 MBH .
    So given my calculated heat loss of 25k, does it mean this
    unit is not a good option?

    Another option I found was this boiler:

    As far as pre-fabricated goes, I found the following companies online :
    (they are using tankless water heater for heating)

  4. User avatar
    Dana Dorsett | | #4

    Why closed cell foam? It has 2x the polymer per R as half pound open cell foam, and uses much more damaging HFC blowing agents (instead of water), and adds very little to the "whole wall " performance when used as cavity fill in a wood framed structure, less than R2. For less money you could do the cavity fill as open cell foam, and buy back the whole-wall R with fan fold XPS siding underlayment on the interior side, behind the wallboard.

    Closed cell foam makes sense for insulation the interior of foundation walls, where it's low vapor permeance is an asset rather than a liability, but almost everywhere else there are greener methods of achieving the same performance.

    Low mass combi boilers are terrible solutions to low load homes- the don't modulate low enough to be modulating except in mid winter, and have fairly lousy domestic hot water performance. Tank type combis can work out well, or site-built combi based on something like the HTP Light Duty stainless steel tank modulating water heaters.

  5. Vladimir Polyakov | | #5

    Thank you Dana.

    What about using closed cell foam in between roof rafters or do you think open cell in walls and roof?

    By cavity fill do you mean like half an inch ? And like 2 inch of XPS? My wall studs are 2x4 and roof rafters 2x6.

    Thank you

  6. User avatar GBA Editor
    Martin Holladay | | #6

    For more information on all of the different ways to insulated a sloped roof assembly, see this article: How to Build an Insulated Cathedral Ceiling.

  7. Steve Knapp CZ 3A Georgia | | #7

    Vladimir. If you decide to use foam in your roof assembly, closed cell is better, especially in cold climates. See this article for more detail:

  8. Vladimir Polyakov | | #8

    this is all very informational. thank you very much.

    Can I conclude from all of the above that for a small house like mine, <2000 sq feet, there is no point in getting tankless water heaters or mod con boilers to heat the house?

    I will be seriously considering HTP Phoenix Light Duty as per Dana's advice, probably a 50 gallon version. Is Polaris water heater similar to it? Are there other alternatives?

    Also, will these boilers work well with radiant heat ( pex in the floor joist) or with Air handler is a better option? ( just an fyi i wanted to have about 6 heating zones and hence though radiant heating is best suited for that )

  9. User avatar
    Dana Dorsett | | #9

    There are mod-con boilers that work just fine in low heating load homes, but there are no wall-hung combi boilers that are a good fit for both the space heating and potable hot water loads. The most common solution is a small mod-con boiler with a indirect fired hot water tank as a separate zone, with a zone controller that gives priorty to the hot water tank when it's calling for heat, ignoring thermostat calls from space heating zones until the water heater is finished.

    Mod-cons work fine with radiant floors too, provided the system is designed correctly. Designing heating system via web-forum almost never results in an optimal design- it really takes a hydronic designer with the proper design software, and the experience to make judgement calls. Something like the NTI TX51 can modulate down to under 6800 BTU/hr out at minimum fire yet can also deliver more than 50,000 BTU/hr when needed (say, for heating hot water). But to get it to behave correctly in your system requires some system design, as well as post-installation adjustment. It's not just a plumbing job.

    The Polaris has a very low temperature swing on it's internal controls, and can short-cycle on you. That is fixable by design (or a soldering iron, replacing a resistor on the control board, violating the warranty), but it's easier to get there with a modulating hot water tank such as the Phoenix Light Duty, which is more flexible in the setup.

    In US climate zone 4 you only need ~30% of the total R of the roof to be low-permeance foam, but you can also do the whole thing with open cell foam if you use a "smart" vapor retarder such as 2-mil nylon (eg Certainteed MemBrain), or half-perm "vapor barrier latex" primer paint on gypsum board. For a 2x6 rafters the depth is 5.5", and with 2" of closed cell (R12) you could could then fill the remaining depth with R15 rock wool or fiberglass. The R-value under the roof deck between the rafters would be R27, but the foam would be 32% of the total, which is more than 30%, and sufficient for skipping the interior vapor retarder- standard latex paint would be enough. If you installed 5.5." of open cell foam you would be at about R27 between the rafters, and while that SEEMS like a lot lower performance than R27, it isn't. The rafters would be conducting more than half the total heat in the R27 case. Raising the performance of the insulation doesn't change the amount of heat going through the rafters, so the average performance increase is far less. If you go with open cell foam now it would be well below code min, but if you added rigid insualation above the roof deck when it's time to re-roof you can improve it (a lot!).

    With half-perm paint or a smart vapor retarder you would avoid the moisture ping-pong effect that sometimes happens in attics insulated with open cell foam without vapor retarders. With only 5.5" of open cell foam the roof deck isn't as cold as with a full R49 code min, and doesn't take up as much moisture either since total moisture uptake in winter heavily curtailed by the vapor retarder (whether v.b. paint or nylon sheeting.) Nylon sheeting would allow the moisture to leave faster than with v.b. paint, but would still block the high daily swings in attic relative humidity during the spring/summer.

    At a 7% framing fraction (typical for 24" o.c. rafter spacing) and half pound open cell foam, with typical roofing and ceiling gypsum you'd be at about U0.050. If you later added 3.5" of fiber faced roofing polyiso above the roof deck later it would squeak under the code-max U0.026. (You can often get used roofing polyiso for 1/4-1/3 the cost of new goods from reclaimers.) That is a lot cheaper than 5" -5.5" of closed cell foam, and a lot greener too. Polyiso is blown with relatively benign pentane, and using reclaimed goods would be even greener. (I can usually find used but nearly-perfect 3.5" or 4" roofing polyiso for about $20-25 per 4' x 8' sheet from reclaimers near me.)

  10. Steve Knapp CZ 3A Georgia | | #10

    Just to complicate things :-) . Let me suggest that you could probably heat and cool your home for a lot less using mini split units. Wouldn't you like to have AC on 90 degree days? You could use a heat pump water heater to produce hot water very efficiently.

  11. Vladimir Polyakov | | #11

    Steve, I do have one mini split heat pump already installed in the basement and I really like it. But if I consider having them heat the whole house, means having multiple units constantly on, and I am not sure what my electricity charges will be.

    As for boilers I really like the idea of mod-con with indirect water tank. Just not so many choices I can find with low BTU. Phoenix light duty prob is a good option. NTI TX50 or Phoenix units I am yet to find the pricing on these units as I can't find them sold online.

  12. User avatar GBA Editor
  13. Steve Knapp CZ 3A Georgia | | #13

    If you are doing a complete renovation down to the studs, you have an opportunity to make your house very energy efficient. An efficient house requires much less energy to heat and cool. If your renovation isn't that extensive and the "new" house will remain under-insulated, your current strategy may be best. Either way, you would be better off engaging an experienced HVAC engineer or system designer to run the calculations. There are a lot of factors that go into creating comfort. If you skimp on the that expense, you are likely to end up with a system that is neither efficient nor effective for conditioning your interior space.

  14. D Dorsett | | #14

    in Brooklyn electricity prices are much higher than most locations, and it's hard to make a financial case for heating with mini-splits rather than condensing gas based on operating costs alone. The marginal cost of heating with gas is quite a bit cheaper than heating with mini-splits in that local market. You can still factor in the lower cost of the equipment though, since the mini-split would cover both air conditioning and heating. It's not a simple equation.

    But, if you have sufficient access to the sun, the levelized cost of solar energy can be quite a bit cheaper than the residential retail rates. But depending on where you are relative to the substation you may be able to get quite a bit in subsidy from the state & utility to offset the upfront cost of the solar array, bringing the average cost of power to something like half. Under the rapidly evolving New York "Reforming Energy Vision", the utility is QUITE interested in avoiding the cost of upgrading the Brooklyn Queens substation by having a lot more solar, demand response, and microgrids. This too is not a simple equation, but if your roof has reasonable shading factors for solar, it's worth looking into before deciding how to heat the place. If you can get the installed cost low enough with subsidies and the net-metering or other remuneration is favorable heating with mini-splits can end up being quite a bit cheaper to run than heating with natural gas. See:

  15. Vladimir Polyakov | | #15


    I am doing renovation down to the studs and will use all the suggestions I got here to insulate my walls and roof. My outside walls are 2x4 studs and as per Dana's suggestion I will try to spray 1 inch or less of open cell foam to seal al the cavities etc, and install about 2 inches of XPS board. Will that be enough insulation for the walls?

    On the topic of HVAC engineer, I am in the process of getting estimates and so far I am seeing very little knowledge from companies or suggestions similar to what I got on this forum, and pricing close to 15k to build a radiant heat system. Not to mention everyone is oversizing the specs.

    I probably want an engineer design the system and I can build it myself or with the help of experienced plumbers that I know. There are also companies online that sell complete systems, all is needed is just hang it on the wall pretty much.

  16. User avatar GBA Editor
    Martin Holladay | | #16

    You wrote, "There are also companies online that sell complete systems, all is needed is just hang it on the wall pretty much."

    Not really -- I suspect that there are far more ways to screw up this job than you realize. But if you have friends to help you, and if your friends have experience installing hydronic systems, you may avoid the pitfalls.

  17. User avatar GBA Editor
    Martin Holladay | | #17

    Your plan for insulating your walls doesn't make much sense. If you want to do a flash-and-batt job (or a flash-and-fill job), you don't want to use XPS for the "fill" part of the job. You want to use fiberglass batts, mineral wool, or cellulose.

    For more information on this insulation technique, see Flash-and-Batt Insulation.

  18. Steve Knapp CZ 3A Georgia | | #18

    Vladimir. If the venders aren't providing information consistent with this site, they are not providing good advice. In particular, listen to Martin and Dana since they are particularly knowledgeable.

  19. D Dorsett | | #19

    I am not recommending closed cell foam or cut'n'cobbled foam AT ALL. To air seal 2x4 walls a full cavity fill of open cell foam (trimmed flush with the stud edges) will air seal better than 1" of closed cell and deliver both higher thermal efficiency & better moisture resilience than a flash-inch spray + 2" of XPS. I

    It would also have FAR less environmental impact. A full 4" of open cell uses the same amount of polymer as just 1" of closed cell, and is blown with water instead of climate damaging HFCs.

    If you choose to go with an unvented roof, 2" of closed cell on the underside of the roof deck would be more than enough moisture protection, then fill the rest with batts or blown fiber. Better (and greener) than that would be to put 3" of rigid polyiso above the roof deck, and all open cell on the under side (filled to the full rafter depth.)

  20. Vladimir Polyakov | | #20

    Donna, are you suggesting 3 1/2 inches of open cell in 2x4 wall studs and 2 inches of closed cell foam in the roof (I have no vents) and fill the rest with faced insulation batts( or unfaced?) , please confirm.

    Thank you

  21. Steve Knapp CZ 3A Georgia | | #21

    Vladamir. You really should read these articles to better understand the advice you are being offered.

    These articles include links you should check out as well. If you don't have time for this amount of research, you should hire a qualified architect or energy consultant to guide your next steps.

  22. User avatar
    Dana Dorsett | | #22

    The best value for the walls is a full-fill of open cell foam. Depending on the siding type it may or may not need an interior side vapor retarder. More information is needed to make that decision. But if it needs an interior vapor retarder either half-perm paint (sold as "vapor barrier latex" primer) on the wallboard s good enough (and cheap.) Somewhat better would be a continuous layer of 2-mil nylon (Certainteed MemBrain) installed over the studs & insulation, but behind the wallboard. It costs about 12-15 cents per square foot, but you would be saving far more than that by going with open cell foam rather than closed cell.

    For the roof deck, using 2" of closed cell foam (and no more) would provide adequate protection from wintertime moisture accumulation, and would leave 3.5" of cavity in which you can install inexpensive fiber insulation, completely filling the rafter bays, With 2x6 rafters and 2" you would have 3.5" of depth remaining, which is enough room for R15 batts (which is fine). Since more than 30% of the R-value would be the low permeance foam, you would not need an interior side vapor retarder to keep the fiber insulation dry in your climate. The R-value & U-factor would still be well below the current IRC code minimum performance, but would not reduce head room. To later bring it up to IRC code performance would take about 4" of rigid polyisocycanurate above the roof deck.

    So yes:

    Walls: 3.5" of open cell foam, plus vapor retardent paint on the wallboard, or 2-mil nylon (preferred) behind the wallboard.

    Roof: 2" of closed cell foam + 3" of fiber insulation, no vapor retarder. When it's time to re-roof, you can save a lot on foam costs by shopping around for reclaimed 3.5" or 4" roofing polyiso, or factory seconds goods, eg:

  23. Vladimir Polyakov | | #23

    Dana, in one of your responses you said this...

    "Something like the NTI TX51 can modulate down to under 6800 BTU/hr out at minimum fire yet can also deliver more than 50,000 BTU/hr when needed (say, for heating hot water)."

    Will 50,000 BTU be enough when heating with cast iron radiators or baseboard ?

  24. User avatar
    Dana Dorsett | | #24

    The type of radiation does not matter, as long as the radiation can emit enough heat to cover design heat load with 180F or cooler water. That is a fairly low hurdle- nearly all forced hot water systems do not need the water to be that hot to cover the heat load, even before the house undergoes upgraded insulation & air sealing. Most existing systems are WAY over designed, and can get by just fine with 140F or cooler water at the 99% outdoor design temperature.

  25. Vladimir Polyakov | | #25

    Thank you Dana, here is another clarification.

    "The best value for the walls is a full-fill of open cell foam. Depending on the siding type it may or may not need an interior side vapor retarder. More information is needed to make that decision."

    I have vinyl siding.

  26. User avatar
    Dana Dorsett | | #26

    With vinyl siding and no exterior foam or other low-permeance layers such as foil or polyethylene on the exterior, you will NOT need an interior vapor retarder in your climate. If you re-side later, put at least R2.5 foam on the exterior (though R6 polyiso would be better, and would bring performance up to IRC 2015 code minimum performance.)

  27. Vladimir Polyakov | | #27

    Thank you very much Dana.

  28. Vladimir Polyakov | | #28

    Hi again!

    Can I ask the following question please: "If designing a system with HTP Phoenix Light Duty boiler or similar boilers with an internal water tank, and the purpose is to use this boiler for both radiant heat and DHW, does it mean DHW in faucets may have cycled through the floors tubing many times prior to that?"

    Thank you

  29. User avatar GBA Editor
    Martin Holladay | | #29

    You can design a system either way. A so-called "open sytem" -- which is illegal in many jurisdications -- uses the same hot water for both purposes. The water you shower with may indeed have circulated through the PEX in your floor before reaching the shower head.

    A so-called "closed system" uses a heat exchanger to keep the two water loops separate.

    If all of this is new to you, Vladimir, I strongly urge you to hire an experienced hydronic system designer rather than taking a do-it-yourself approach. There are a lot of ways to screw up these design details, and some errors can injure the health of your family.

  30. Vladimir Polyakov | | #30

    Thanks Martin,

    So if I want a separation of water then I am better be using a boiler with indirect water tank, correct?


  31. User avatar GBA Editor
    Martin Holladay | | #31

    I'm not familiar with the HTP Phoenix Light Duty boiler. Some so-called combi appliances have separate output taps for space heating and domestic hot water, with an integrated heat exchanger that keeps the loops separate. In any case, it's always possible to hook up an indirect tank, with or without an external flat-plate heat exchanger, to any boiler.

  32. Vladimir Polyakov | | #32

    I want understand my options and want to see why was I recommended a Phoenix light duty boiler in above conversations?
    To me it looks like a low BTU boiler with an indirect water tank is the right way to go about it. Please correct me if I am wrong.

    I know Bosch Greenstar 12-57k BTU boiler can also control indirect water tank temperature from the boiler control panel. Can any other boiler manufacturers do the same?


  33. User avatar
    Dana Dorsett | | #33

    A hot water heater is inherently self buffered- the thermal mass of the water guarantees that the system will never short-cycle- the minimum cycle time is long.

    If you can find a modulating boiler that can fire at a rate under half your (as yet not calculated, given that the wall stackup & U-factor isn't fully resolve) 99% design temperature heat load, delivering the heat at condensing temperatures using the (as yet unspecified) radiation that would be fine. The Bosch Greenstar series can definitely be used with indirect hot water heaters, but can your radiation deliver the full min-fire output at condensing temperatures without short-cycling the boiler?

    If your actual 99% heat load is 20K, not 25K you'll want something that modulates to under 10K if using a modulating boiler, not the Greenstar 57's ~12K. If it can't modulate to track the load at least half of the hours of the heating season there's little point to the modulation.

    But with 300-500lbs of thermal mass in a water heater to work with modulation doesn't much matter. The heating system will just sip (heat not water- you definitely want it to be isolated from the potable with a heat exchanger) from the thermal mass of the tank at whatever duty cycle the heating system needs, but only firing the burner when the tank's temp drops below it's setpoint. You don't get quite the same steady cushy warmth of modulating system, but you don't cycle the burner into an early grave or low efficiency no matter what the radiation or water temp.

    To design the system well you need to first have a fairly accurate estimate of the heat load, then measure up or specify the radiation that is to deliver the heat, to come up with both the water temperature at which it can deliver the heat. When looking at a modulating boiler you then need to calculate the water temperature at which the radiation can no longer deliver the minimum fire output of the boiler. Read and understand this bit o' bloggery:

  34. Vladimir Polyakov | | #34

    So to separate DHW from water used for heating using Phoenix light duty I would still need add'l indirect water tank?

    With Bosch Greenstar 12-57 I mentioned indirect water heater as a separate zone( as thermal mass ) so it will heat the tank in similar fashion as Phoenix, isn't it?.

  35. Richard McGrath | | #35

    Vladimir ,
    What you need using the Phoenix Light Duty as a combination DHW / Space heating source is 2 circulators and a flat plate heat exchanger . However , since you are in NYC and assuming you will be having the job inspected you will require a boiler or some equipment that possesses an H Stamp if it is to be used for heating . Many areas of the country including many larger cities have yet to recognize that low temp heating systems do not require a high temp boiler and can utilize other sources for space heating .

    Stay away from the Radiant floor company and their ideas . Besides code compliance issues with MOST ALL authorities having jurisdiction ( model codes) the fact that cold incoming potable water runs through the floor or panel which has been heated to a usable temp for heating robbing that heat and leaving you to heat it again is counter intuitive to radiant heating and what most would consider efficiency .

    Do yourself a favor and contact a heating contractor whom is VETY Familiar with radiant heating , performing heat loss calcs taking your particular assemblies into account along with a whole host of other building science issues . This person should be able to offer what you need and advise you properly , you would benefit greatly from this .

    Might I suggest contacting Ezzy Travis ? He just finished installing 2 of my designs in your vicinity and is quite capable . 1-347-436-6593 . Cannot hurt to reach out in an effort to get it right the first time . Good Luck

  36. Vladimir Polyakov | | #36

    Thanks Richard, I will contact this person.

    It seems at this point I do need heat loss calculated so that I can choose between BTU Tx51 7-57 BTU or Bosch Greenstar 12-57 . Either setup with an indirect water tank.

    This way DHW is separated from boiler water.

    It seems as if I can even omit heat loss calculation all together and just go with NTI boiler just because it
    Is lower on the low BTU side, i.e. 7k.

    Interestingly, I don't really see anyone selling the NTI boiler.

  37. Richard McGrath | | #37

    I have no idea why you would choose either of those boilers with all the choices available on the market . NTI and Bosch / Buderus both have sub standard customer service and warranty claims should they eb necessary are akin to getting paid by an insurance company , Deny , Delay , Defend .

    Look at HTP UFT or Lochinvar KHN , both 10:1 TDR and both companies are a pleasure to do business with and both are proud of their products . Superior materials also .

  38. D Dorsett | | #38

    "So to separate DHW from water used for heating using Phoenix light duty I would still need add'l indirect water tank?"

    No, you don't need an indirect when using a hot water heater as the heating system source (if that's even allowed in NYC, per Richard's comments.) In this type of system you need a plate-type heat exchanger near the Phoenix LD to separate the potable water (in the tank) from the heating system water (on the other side of the heat exchanger.)

  39. Vladimir Polyakov | | #39


    Is there a schematic for what you described?


    I was looking for a boiler that has controls for indirect water heater from the boilers control panel which should simplify installation. You basically can set the two temperatures on the boiler. Radiant plus DHW.


  40. Vladimir Polyakov | | #40


    So with an indirect water heater it seems the boilers input BTU number is not critical any more and it can start from twenties or even thirties, right?
    Is it why you suggested Phoenix light duty which has its BTU start from 25k?


  41. Vladimir Polyakov | | #41

    Hi all! And thank you very much for providing some answers on this website.

    I have a follow up question.

    I used Radiant Works program to estimate the heat loss in my house, and in total I got only 16k heat loss and this includes basement plus two floors.

    Just to refresh I am designing a low mass system with under floor joist based radiant heating.
    The house is small, 20'x35' (New York). I plan to have six zones of heat and some zones are very small , for example a room 12'x9' which has heat loss calculated to be at 2,000 BTU and another room at 3200.

    So, to avoid short cycling, as advised above, I started looking into a Versa Hydro boiler or a boiler with low BTU inputs. Do you think Versa Hydro is a good option for the scenario, whereas I may have just a a few small zones calling for heat?

    are there any other options other than Versa Hydro which is a little pricey but as I read in some other articles a more idiot proof boiler?

    Thank you

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