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Community and Q&A

Source to guide me through sizing radiant floor & hot water boiler?

airfix | Posted in General Questions on

Does anybody have a source to help guide me through the calculations required to size a boiler for my basement radiant floor system?

It is going to be a combi boiler tankless water heater with a 50 gallon storage tank.  Per my heat loss numbers I need 29,000 Btuh heat load plus whatever I need for my domestic hot water.

I’m really just trying to keep a check on the radiant contractor who I think is over sizing the boiler by a long way.  He proposed a 135,000 Btuh unit – the LAARS Mascot FT model.  


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  1. Expert Member
    Peter Yost | | #1

    Hi Steve -

    Your instinct seems sound: if your contractor can't show you that he or she used ACCA Manual J to calculate your home's heating load, you need to find a different contractor.


  2. Zdesign | | #2

    Navien NCB150E has enough turn down ratio to handle 29k BTU of heating load and has dual outputs, one for domestic and one for heating.

  3. GBA Editor
  4. Expert Member
    Dana Dorsett | | #4

    If the radiant floor is just for the basement it's highly unlikely that it will need 29,000 BTU/hr (unless it's 5000 square foot or larger basement.) A code minimum basement's heating load is usually well under 10 BTU/hr per square foot of conditioned space, but until you run the Manaual-J it's shooting in the dark.

    The maximum fire of tankless combi burners is usually several times the space heating load numbers, necessary to be able to provide the domestic hot water. 135,000 BTU/hr is only enough to support a 4 gpm bathtub fill at a 70 temperature rise, and might not be enough for a lhouse with 2 or more bathrooms, or a large tub to fill. The minimum fire output of the LAARS Mascot FT MFTCW140 is about 26,600 BTU/hr which is a bit on the high side even for a tankless combi, and much higher than optimal for a 29,000 BTU/hr heating load, since it will almost never modulate.

    What is the point of going with a tankless if you have a 50 gallon buffer tank on either the potable or heating side?

    For sizing a modulating condensing heating system the size of the radiation really matters. If any zone can't emit most or all of the minimum-fire output at condensing water temperatures it will be cycling on/off during extended calls for heat. If that cycling is rapid it takes a large bite out of the efficiency, and puts wear and tear on the boiler, shortening it's service life. For some napkin-math analysis on that see:

    1. airfix | | #7


      So is your opinion that my 1600 sqft basement is should should have a heatloss of much less than 29,000 BTU/HR with the insulation mentioned in a post below? Per your 10 BTU/hr I should need only 16,000 BTU/hr and my insulation is better than code. How could my heatloss guy have got the numbers so wrong?

      You also imply that it is better for me to have a boiler dedicated for the radiant floor and then have an separate on demand water heater because I can size each to their respective job functions. So I should look for a boiler that does about 30,000 BTU/hr and an on demand water heater in the 150,000 BTU/hr range to accommodate 2 showers at the same time?

      Do you know if you can get an on demand water heater that can accept a hot water return line so that I can have a recirculating pump to warm up the water for a long runs?

      It was my understanding the tank proposed in the system by my general was to smooth out those times when you are using more than the output capability of the on demand side of the combi boiler. In those times you'd draw from the tank and the combi system would replenish the tank. Actually this is how it would work all the time.


  5. user-2890856 | | #5

    You should really look into not using a combi since you also have a proposed 50 gallon tank . Combis have a few problems such as three way valves that fail and are in most cases terrible to replace , too high a low end modulation creating short cycle problems and longevity . I suggest a 10:1 TDR boiler like the HTP UFT 80 or Lochinvar KHN with the 50 gallon tank . they will have low end modulation below 10,000 BTUh and will make your DHW also . Remember to store in the tank at 140* minimum and mix down for your desired temp at faucets

  6. airfix | | #6

    Well this is super frustrating for me. I can't seem to find professionals in my area to do their job.

    I didn't think I needed to look at my heat load anymore because at the good suggestion of this forum I hired someone who specializes in doing only manual J's to do the heat load. They have nothing to do with installing or selling equipment.

    I'm in climate zone 6A. My walk out basement is 1600 sqft of heated space with 10 foot ceilings. It has a garage on one side, the back wall is entirely below grade the other side wall is partially covered (30 to 30% grade). There is approximately 163sqft of windows. Insulation in below grade walls is 1.5" polyiso on inside of concrete with 2x4 studs filled with BIBS. Above grade insulation is 2.5" exterior polyiso with 2x6 studs filled with BIBS. The whole house is 4000 sqft with 5 bathrooms.

    The manual J shows 29,000 Btuh heating load for the basement only.

    Dana good article on the condensing boiler but it all stems from having an accurate manual J which you indicate I may not have.

    After consulting 3 or 4 professionals regarding systems with and without the 50 gallon tank they all said they install buffer tanks because nobody is happy with a true on demand hot water system. It takes too long to get up to temp and can't keep up the temperature if there are 2 people in the shower and a washing machine or dishwasher running. Both house owners that I know who have tankless systems advise against going tankless.

    The thought behind the storage tank I believe is to buffer those high demand situations.


    1. Expert Member
      Dana Dorsett | | #8

      >The manual J shows 29,000 Btuh heating load for the basement only.

      Seems insanely high even for a 1600' code-min walk out basement, and your wall-R is somewhat better than code.

      Can you scan and post the Manual-J?

      A combi or tankless water heater with a buffer tank is a kludge, not a well engineered system. Richard McGrath's recommended solution is exactly what a true hydronic heating pro would do. The UFT-80W even comes pre-plumbed with a secondary port for the indirect water heater and the controls to support it. The KHN has more bells and whistles, but takes more plumber-time to install. From a value point of view the UFT is hard to beat- the boiler is priced comparable to or even cheaper than most combi-boilers, but easier to install than a combi + buffer.

      1. airfix | | #9


        Thanks for your expertise and guidance on this. Attached is a redacted manual j for the whole house.


        1. Expert Member
          Dana Dorsett | | #11

          The 99% outside design temp for Park City UT is +12F, not 0F. The median of extreme lows is 0F, according to this document:

          The also used wall descriptions that differ substantially from what was built. From the text of your Manual-J:

          "Bg wall, light dry soil, 2"x6" wood int frm, concrete wall, r-19 cav ins,
          8" thk, 1/2" gypsum board int fnsh: Bg wall, light dry soil, 2"x6" wood int
          frm, concrete wall, r-19 cav ins, 8" thk, 1/2" gypsum board int fnsh"

          That's a code min wall, not your wall.

          Your below ground wall isn't a 2x6 R19 wall with a U-factor of 0.053, as used in the Manual-J. You have something like R25 at center cavity, and with the 1.5" polyiso you have less than half the thermal conductivity of the framing fraction of the wall description they used. Without formally calculating it I'd hazard your below-ground wall losses are at least 1/3 lower than stated.

          "Frm wall, cmnt brd ext, 3/8" wood shth, r-30 ins, 1/2" gypsum board int
          fnsh, 2"x6" wood frm, 16" o.c. stud: Frm wall, cmnt brd ext, 3/8" wood
          shth, r-30 ins, 1/2" gypsum board int fnsh, 2"x6" wood frm, 16" o.c.

          They punted- made like it was an R30 continuous insulation wall, which may be closer to your actual "whole-wall-R" performance than their below ground wall approximation. The U0.033 is probably within 10% of reality.

          "21A-32c: Bg floor, light dry soil, 8' depth, carpet flr fnsh"

          What no insulation under the slab, just a rug? In a RADIANT slab? Surely you have at least a couple inches of EPS under the slab!?!

          Go over it carefully, pick it apart, and don't be afraid to ask whoever ran the calculation to explain anything that seems amiss. Hopefully they used the manufacturer's U-factors on all the windows (looks like the might have.)

          The total upper levels 2615' with 53,379 BTU/hr of load seems high too. I have 2400' of fully conditioned space above a 1600 (but insulated) basement in my way-below code 1920s antique, and the heat load for the whole house at 0F outdoors, 70F indoors is about 40,000 BTU/hr (confirmed by both Manual-J and fuel use) if I were to actively heat the basement. (It idles along in the mid-60s F all year round.) Most of the windows are U0.5-ish wood sash single-panes + clear glass storms, so even though you may have more window area than me, you shouldn't have significantly more window losses. Being a walk-out your lower level would have more above grade loss than mine but 29K for just that level still seems high.

          For the sake of argument let's assume the 82K number is correct (which it isn't). The 82K @ 0F outside/68F in drops to about 68K at your 99% outside design temp of +12F. Per ASHRAE recommendations of 1.4x oversize factor at the 99% load, you'd be looking at a boiler with 95,000 BTU/hr of output. Something like HTP's UFT-100W would be about right (95K out at condensing temps, 87K out at high temp) , or at most a Lochinvar KHB-110 (105K out at condensing temps, 96K out at high temp.)

          In reality you'd probably do just fine with an 80K-85K (input BTU) boiler, correcting for the errors in the Manual-J.

          1. airfix | | #12

            Yeah as you can see I was on revision 4 of my Manual J because of issues I found out to this point. I already had him revise up the out door design temp from -20 to 0. Salt Lake City is not a good representation for Park City. Salt Lake City is at 4200ft my main floor elevation is 6900.

            Most people use Logan design temperatures for park city which has a 99% heating dry bulb of 0.3F. I thought Evanston might be more appropriate which has a 99% dry bulb of 2.6F so I thought using 0F was pretty good.

            I had him put in manufacturers window numbers. Then on another revision I had him supposedly update it for our continuous insulation on the outside and the basement. On one of those runs I noticed that the heat loss to air infiltration went up. When I queried him he said he fudged the air flow for the program to show the right sized furnace. I've not really talked to him since. I'm at a loss. I thought paying someone who only does this stuff for a living would be the way forward rather than mucking it up myself.

            Under the slab I have 3" of XPS insulation and on the stem walls below the slab I have 2" of XPS insulation. The garage is fully insulated with 2" under the slab. The garage is going to be partially heated. I'll have a wall mounted gas burner to keep it between 30-40F and provide quick heat for when I'm working in it in the winter.

            For my basement I show 1.5" Polyiso, derated to give R-7.5 and 2.3" of Bibs for R-13.65 for a total of R21.6 in the cavity. I looked at the section through the studs and found about R-12 for 2x4s and 1.5 of poly.

            My above wall assembly is 2.5" of polyiso plus 5.5" of BIBS for a total of R-34.


  7. tommay | | #10

    The whole house is 4000 sqft with 5 bathrooms? Well, 1600 ft^2, something like 20 x 80, or since you propose a recirculating line 100 x 16 ft? With 10 ft ceilings, half underground, a bunch of windows, next to a cold garage and a walkout? That basement is almost the size of a small ranch. So 29,000 BTU for heat, which seems low, and proposing 150,000 BTU for HW, your contractor may be on to something. Experienced installers can usually determine boiler sizing without going through all the mumbo jumbo.
    What do you heat the rest of the house with? Why a separate heating and HW system for the basement, with two showers, WM, DW? Sounds like you want to put an outboard on the titanic.
    If you could give us a better idea of the living conditions that this basement is going to see it might make things easier. eg, partitioned bedrooms, kitchen, bath etc. or is it one big open basement. Is the radiant already installed in the floor or is this a new install? Why not baseboards? Any passive gain through all those windows?

    1. airfix | | #15


      We have a different living situation than the norm. The bottom line is we have a fully grown mentally handicapped daughter and we're setting her up with her own basement apartment so she can feel independent but yet have us close by. It has a living room, kitchen, bedroom and bathroom. We have an newly adopted 16 year old boy who is in a bedroom also in the basement. In the hall there is a bathroom for the 16 year old and for me when I come in from the garage all dirty.

      I have 2 elderly parents with mobility issues that are sure to be living with us within a few years. I've set them up with a master and on suit on the main level. Also on the main level is our master and bath plus an office. We have generously sized living room and kitchen.

      The garage will be insulated and partially heated with a natural gas burner for quick temperature rises to working conditions. Normally it will be around 30 to 40F in the garage until I go to work on my cars.

      There is only one hot water system for the whole house. We vacillated over using radiant heat in the basement for a while but the wife wants finished concrete floors and we decided to pay for the radiant. We had planned radiant up stairs (also with finished concrete floors) but we had a ridiculously warm summer here and the wife would likely divorce me if I was too cheap to put in AC. With the AC decision made we ditched the radiant and concrete upstairs in favor of forced air. No AC in the basement.

      It's a new install. Radiant pipes are likely to be laid within 2 weeks if the weather complies. Hardly any passive gain through the basement windows. Only one basement window will get sun and that will be from about 10am to 2:30 pm in the winter.

      I'm just trying to do the right thing, hoping to build a pretty good house and looking for guidance along the way. Paying extra for insulation just makes sense to me and it's important to size my heating equipment to account for the fact I've got extra insulation. I'd say "many" contractors in my area and all that I've talked to have no idea how to size a system for a well insulated house. The always go back to "this is how we always do it". Which is leading to my frustrations.


      1. tommay | | #16

        So this basement apartment hasn't been built yet? Can I assume you are planning on pouring a slurry on top of the existing basement floor to incorporate the radiant tubing? Hopefully they zone it with tempering valves so you can even out the heat if necessary. Have you considered hot water baseboard heaters?
        Where are the existing utilities located? Are they centrally located in the basement? Forced hot air so, Natural gas? How big is your existing HW heater? Natural draft/ chimney or power vented? How about your water service, well w/ pressure tank? What size water main? What size piping to your HW heater?

        1. airfix | | #17


          It's a new house. Just finished waterproofing and the footing. We're in snow melt mode so I can drop in the gravel, sub slab insulation and vapor barrier. Then we'll set up for radiant tubes then pour the concrete slab. I set up my mechanical room so it is as central as possible in my house and right below my kitchen.


          1. tommay | | #18

            Obviously, the gas furnace for the first floor is gonna happen. Radiant is being installed. Chimney?
            So back to the original question, heat and hot water for basement. If it were me, I would get a 50 -80 gal gas HW heater. I would also get a gas, tankless HW heater to use as a preheat or in parallel to the tank to ensure plenty of HW for the entire house. It also adds redundancy in case power goes out, you still have HW from the tank or if the tank fails you got the tankless.
            I would also get a tankless for the radiant and split it into two or more zones. Properly sized of course. Then, if you find one tankless will not handle the heat load, you can purchase another one later on and either stage them or use separately for each zone. Even with two you still may come in cheaper than a small boiler, especially with a built in tankless which has to run year round, and once again, redundancy comes into play in case one fails.
            Of course some pipe sizing and valve controls need to be thought out to ensure proper flow and isolation if needed.

          2. Deleted | | #34


  8. Expert Member
    Dana Dorsett | | #13

    Steve: I apologize for the outside design temperature error- I caught that just a few minutes ago and was going to post a corretion, but you beat me to it!

    >"...he fudged the air flow for the program to show the right sized furnace"

    Clearly a "service" not worth paying for. (Sorry you had to experience that.)

    That kind of fudging is exactly counter to the instructions in "the manual", which require that every credible factor that could lower the load number be included.

    >"For my basement I show 1.5" Polyiso, derated to give R-7.5"

    That's excessive/conservative- your ground temperatures aren't going be low enough for severe a derating. Below frost line it could rightly be UP rated on performance.

  9. airfix | | #14

    Fair point on the de-rating Dana. Any chance you want to consult on this to help me over this hurdle?

    1. Expert Member
      Dana Dorsett | | #19

      >Any chance you want to consult on this to help me over this hurdle?

      I'd ask Richard McGrath for that (he's actually in the hydronic design biz,I'm not.)

      More important than precision the Manual-J is the size of the indirect, with that many adults showering & bathing daily. If it's going to be multiple large tub fills in succession (or in parallel) it might even call for a bigger boiler.

      1. airfix | | #20

        I was thinking more for getting the Manual J correct. I'll seek out Richard too, somehow.

        1. Expert Member
          Dana Dorsett | | #22

          Please rest assured that ANY tight, better than code min (or even code min) 4000' house can easily be heated & cooled with an 80KBTU/hr condensing boiler. Typical load/area ratios for code-min houses (even fully above grade slab on grade ranchers) come in 12BTU/hr per square foot & under @ 0F. A ridiculously overglazed code-min house might come in at 15-16 BTU/hr per square foot, which would still be under 65K. Your house is probably under 50K @ 0F due to the amount of below grade wall area that is and the better than code walls. It might even come in below 40K, but I wouldn't count on it.

          Competent heating designers will run their own room-by-room load calculations or hire a competent person to run those numbers in order to get the proper room radiation. If one accepted the existing Manual-J and installed radiation capable of emitting the full calculated 82K the error is likely to be sufficiently proportional room to room that you won't end up with temperature imbalances within zones. It's not a disaster to have more radiation than the boiler can emit, but more radiation is more expensive.

          1. airfix | | #24

            Except in mine it is only a 1600 sqft basement that is being heated with the radiant that that should be less then a 40kbtu/hr unit?. The upstairs 2400 sqft is heated and cooled by forced air. No cooling in the basement.

    2. Expert Member
      Akos | | #21


      It is always nice to get the numbers correct, but I can tell your the 29K heat load for the basement is out to lunch (about that much heats a 2000sqft mostly insulated century home here in Toronto).

      Looking at the number of bathrooms and possible hotwater usage, I would suggest get a large power vented high BTU storage water heater and run the floor heat for the basement off a plate heat exchanger from it. If you want something more efficient than a power vent, go for a high output condensing tank (ie HTP RGH-100F).

      With the heat load from the basement, there is no point in getting a boiler/indirect. Not only is it expensive, they are extremely inefficient outside the heating season.

      1. Expert Member
        Dana Dorsett | | #23

        While less efficient than during the heating season, indirects are not "extremely inefficient" when tied to a low-mass condensing boiler. Even with higher mass boilers they aren't terrible if the cast iron boiler has a heat purge control. See the tested summertime net efficiency in water heating mode of system 3 in Table 2 of this document:

        1. Expert Member
          Akos | | #26


          I think relevant to this discussion is #11 which is a modcon. 59% summer time efficiency is not "good".

          From personal experience comparing two triplexes, one with a tankless DHW and the other with a modcon+indirect, the summer time fuel use is about 40% less for the tankless.

          I stand by my statement. Indirect is inefficient and expensive to install.

          There is no reason to use one when tankless or condensing tank DWH are cheaper to install and also more efficient.

          1. Expert Member
            Dana Dorsett | | #27

            It's not clear that in this instance a water heater solution would be cheaper than a mod-con + indirect, but a competent hydronic installer would be able to assess that. (Richard McGrath is partial to using HTP's modulating water heaters as combi heaters in lower load homes.)

            In Appendix 11 they noted that the post-purge on this aluminum heat exchanger boiler (probably a Weil McLain) only purged the boiler temp down to 150F though the storage temp in the tank was 135F, which was clearly part of the low hot-water-only performance of that system. The world has become a lot smarter about that in the past 10 years, but I suppose you COULD still set it up to perform that miserably.

            In a high hot water use applications the overall duty cycle will be higher than tested in the Brookhaven study, which would improved the overall efficiency even if the control aspects were not fixed.

            Other resources for getting a reasonable Manual-J and complete system design down to the last pump & valve on the system would be Morgan Audetat, of Badger Radiant Designs in Minnesota:


            I believe Morgan is semi-retired but still doing consulting work. In the past he has done several completed design packages that included the load calculations, all via internet, phone and mail. With a complete design package the project can be put out to competitive bid among local installers. As it happens Morgan installed a UFT-080W in one of his own houses in the past couple of years, but has ample experience designing systems around water heaters too.

            Another resource might be Rob Brown, currently working at Rockport Mechanical in Maine:


            In his previous job he did some amount of remote design work- not sure if that is something that Rockport Mechanical does as a service, or if Rob would be willing to moonlight on weekends for projects like this, but he's pretty good at ferreting out the real numbers then engineering the solutions rather than hacking. (Rob designed one of the first Daikin Altherma air source hydronic heat pump solutions ever installed in the US.)

          2. Expert Member
            Akos | | #28

            Last I priced a modcon plus indirect the part cost was about 2x of a condensing tank + plate HX and 3x of condensing tankless.

            For low load hydronic setup, it is way easier to mess up a modcon install. You need a good design, good install and setup/comissioning. That is something that we all know is extremely rare.

            Condensing tank (or power vent tank) plus plate HX is hard to mess up. About the only thing you can do wrong is over-pump the plate HX and cause increased cycling on the water heater. Even if everything is over-sized, the setup still works with better efficiency than a well designed modcon+indirect. It will also be efficient all year around and provide longer hot water draws in a multi bathroom setup.

  10. Jon_R | | #25

    If this is tubing in a 1600 ft2 concrete floor and the boiler can modulate below 40K btu/hr , you can ignore concerns about an over-sized heat source and short cycling.

    With condensing heat sources, there is significant efficiency loss if you heat water to say 140F when you really only need 90F for a radiant floor.

  11. Expert Member
    Dana Dorsett | | #29

    Akos: The UFT-080W runs about $1.7K USD, quantity 1. The UFT-100W is still under $2K. Even the smallest stainless condensing water heaters with 76K burners run about $2400. An 80 gallon version runs ~$3K. Bigger burner versions start at about $3.5K (even for smaller tanks.)

    It's not really a slam dunk either way- let a competent hydronic designer make the call.

    As a DIY, sure it's harder to screw up a water heater approach, but this isn't a DIY. There are plenty of DIY installtions of the UFT series boilers out there though- it's almost idiot proof. But of course the more idiot-proof you make something the more creative the idiots become...

    1. Expert Member
      Akos | | #30


      For the modcon, need to also include the cost of the indirect tank:

      -modcon + indirect (box store price Rinni M120SN + SSU-45 ) ~3500
      -condensing tank +plate HX (box store WGRGH20NG100F ) ~$1800
      -tankless + plate HX (Takagi T-H3-DV-N ) $1200

      DIY or not, I'm sure you can get a modcon/indirect designed and installed properly, I just don't think it happens often in the real world. Even then it is not as efficient as the cheaper options in a low heat load house.

      1. Expert Member
        Dana Dorsett | | #31

        WGRGH20NG100F for a house with 5 bathrooms? You're kidding, right?

        M120SN + SSU-45? That's a mighty small indirect for that house too.

        The 120K Rinnai mod-con is about $400 more expensive than the UFT-120W and only has a 5:1 turn down to the UFT's 10:1.

        Let the designer spec the solution.

        1. Expert Member
          Akos | | #32


          This was from a spreadsheet I had for a 3 bath rental. The parts would be undersized for a 5 bath unit.

          My intention was not to design the system for the OP, I'm just pointing out that there are other options that are more efficient and cheaper.

          P.S. The WGRGH20NG100F has a 130gal first hour rating, I think it would be marginal for 5 showers, but would handle 4.

          1. Expert Member
            Dana Dorsett | | #33

            >"The WGRGH20NG100F has a 130gal first hour rating, I think it would be marginal for 5 showers, but would handle 4."

            ...and less than one tub fill in any reasonable amount of time at the incoming water temps in Park City UT.

            With such limited buffering capacity and a 100K burner it won't handle even two simultaneous 2.5 gpm showers for the US average 8 minutes, let alone 4, but it'll handle 1. That's fine for a 1-bathroom apartment with a miniscule heating load, small tub and a low flow showerhead. Since there is no way to give priorty to the shower, when the heating system is pulling more than 30K from the thing it won't support even one 2.5gpm shower.

          2. Expert Member
            Akos | | #38


            You are right, it would be undersized. I used the wrong spreadsheet which was for an open system with a radiant slab, so the incoming cold water was tempered before it got the the heater. You get a lot more GPM out of the unit with 65F incoming water.

            Lets actually actually look at the OP's situation.

            Looks more like a 5-plex with a small radiant load then a 5 bath McMansion. I find a in a 3 plex a 60 gal 45kBTU power vent is enough (or at least nobody complains), we can probably get away with a 80 gallon with a big burner (100 would be better but then you are in commercial territory$$$) with a daily hot-water usage of around 200 gallon (10 people).

            Hydronic load is calculated at 29k, my feel is it is between 15k to 20k, but for calculations lets go with 29k BTU. For simplicity lets assume 3 months of heating at full tilt.

            With 1600 sqft of radiant slab at 70F room, that is 18 BTU/sqft (which is a ludicrous number for a well insulated basement) with 79F slab temperature. High mass slab so cycling should be minimal with even with a low turn down modcon.


            With the 140k 95% modcon and 50 gal tank (marginally sized for a 5 plex with a 140k burner, 75 gallon tank would be better) as originally suggested. For heating, the RWT is low, efficiency would be around 95%, for hot water optimistic assumption 75% efficiency.

            Assuming 80F rise for the hot water, 200 gallons/day is 1.7therms/day with a 75% efficiency.
            Heat is 7.3 therms/day at 95% efficiency.

            Year energy use 1277 therms (this is not accurate, just using it for comparison).

            Cost is $2000 for the modcon, $1500 for the indirect plus say around $300 for the extra zone for the indirect. Total around $3800.

            CONDENSING Tank

            Readily available in a box store is WGR080NG076, 80 Gallon 75k btu 95%. Heat load is low enough for a small plate HX say a 6.5"x3" 20 plate is more then enough. With a 140F tank you need around 85F return temp not to affect stratification, so that is around 1GPM circulator, depending on the pressure drop on the HX a Taco 003 would probably work. The efficiency of the water heater would be slightly lower for heating as the 95% is for colder water input. Let say 90%.

            Hotwater (200 gal/day 80F rise) 1.4 therms/day
            Heat (29k btu) 7.7 therms/day

            Year Energy Use 1204 therms.

            Cost $2800 for the tank, $200 for the plate HX plus $250 for the pump.
            Total aorund $3300

            POWER Vent Tank

            Box store 75 gallon (G6-PVT7576NV) 0.7 energy factor. When used for heating these typically run around 80% efficient.

            Hotwater (200 gal/day 80F rise) 1.9 therms/day
            Heat (29k btu) 8.7 therms/day
            Year Energy Use 1476 therms.

            Cost is $1400 for the tank, $200 for the plate HX plus $250 for the pump.
            Total around $1900.


            5 Bath makes this hard. Most likely 2 x 200k BTU will work but depends on the actual shower rates. I've seen this setup in multi bath residential with big tubs.

            Takagi T-H3-DV-N 95% efficient x 2 . Same plate, but would need a slightly larger pump so Taco 006. The min fire on them is 15000BTU, so probably best to run the floor heat from only one of the units.

            Hotwater (200 gal/day 80F rise) 1.4 therms/day
            Heat (29k btu) 7.3 therms/day
            Year Energy Use 1168 therms.

            Cost is $2100 for the tankless, 200 for the plate HX plus $280 for the pump. There would be some additional cost in gas piping with that many BTUs, so the option is not as cheap as it seems.
            Total around $2600

            There are also some other options to bring the cost down (for example going with an RGH-100F in parallel with a 100gall buffer tank in thermosyphon would handle the bath loads for less than the larger condensing tank).

            The modcon+indirect is still the most expensive option and far from the most efficient.

            The OP is better with going with one of the cheaper options and for the money saved add on an extra zone or two of floor for the main floor to heat the kitchen and bath area. Also not fully heating the basement slab would be a big cost save. My guess around 50% of the slab needs to be heated, this way at least the high foot traffic areas would feel warm.

            For a low heat load house, there is no point in going with modcon plus indirect.

  12. airfix | | #35

    Thanks for all the input. Most of the specs and differences between the boilers is over my head at this point. While I am a mechanical engineer, aerospace is a bit different and my time is very limited at the moment to get myself up to speed quickly.

    I need help on this quickly as I expect we will be putting sub slab insulation down next week and laying the radiant tubes for the 135,000 btu/hr combi unit and 50 gallon storage tank soon after.

    I'd really like to get a consultant to help me propose a new system to my radiant floor guy but I can't seem to get hold of anybody and I'm short on recommends except for Richard McGrath that Dana suggested above.


    1. Expert Member
      Dana Dorsett | | #36

      >" I'm short on recommends except for Richard McGrath that Dana suggested above."

      I've also recommended Robert Brown, and Morgan Audetat, not just Richard McGrath, and provided links to websites through which they can be contacted.

      Rob works here:

      Morgan owns this business:

      Richard's linked-in page:
      I think he works here :

    2. Deleted | | #37


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