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Combi Boiler for radiant infloor heat and domestic HW

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sacres
sacres Member Posts: 2

Our retirement home is under design - SOG, single story, 5500 sf, 2 bedroom, 2 and half bath, in a moderate climate. We are thinking of 6 zones of infloor heating with two combi boilers, one at each end of the house. The domestic hot water would be divided between the two boilers as well. Based on square foot calcs (35 BTU/sf) one boiler would be 125k and one 70k. Thinking to upsize to a 150k and 100k to account for the domestic hot water load, but not sure if that would be too much or too little in the winter months. Welcome any comments.

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  • Jamie Hall
    Jamie Hall Member Posts: 23,661
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    Problem. You can't put 35 BTUh per square foot of a floor and walk on it. At least not comfortably.

    And I would suspect that a modern, well-built house won't need that much power anyway, in a moderate climate.

    So… better to go back to square one and do a proper heating load calculation for the house. The formal calculation is called a Manual J, and there is software for it — or your heating contractor should be able to do it.

    I suspect you will find that your boiler choices are oversized, possibly very oversized, so they would be cycling on or off or operating — if they can modulate — very near their low end.

    However… combis have an inherent little problem: while they may be oversized, often significantly, for the heating load, as I suspect they are in this instance, they may be marginal or even significantly undersized for the domestic hot water load. That 100K one you mention, for instance, will only just cope with one shower running at a time…

    So there may be something to be said for using more powerful on-demand gas hot water heaters for your domestic hot water, and only one boiler — sized to the load — for the heating demand.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EdTheHeaterManAlan (California Radiant) Forbes
  • hot_rod
    hot_rod Member Posts: 22,491
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    Most mod con combis now have turndowns around 10-1

    A WAG is s 120 or 150,000 would be adequate. Heating turndown to10 or 15,000

    Short cycling should not be an issue

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • GGross
    GGross Member Posts: 1,105
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    You should take the time to get a decent load calc. For some reference I do manual j in a cold climate, and most new construction is averaging 15-25 BTU/hr per sq ft in above/on grade levels, and those are usually a bit higher than reality. I think your 35 per sq ft is a guess at best and likely a very high one unless you are not insulating to code.

    Personally I think you could do it just fine with a single boiler and a tank, if you are already planning on 2 boilers it seems to make sense to me to use 2 boilers that are redundant with a tank so if one goes down you still have heat and hot water while you schedule repairs. Everyone has different priorities but for me redundancy would make more sense than 2 separate on demand systems for the money.

    EdTheHeaterManGroundUp
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,176
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    Even if you have a completed floor plan with the floor heat manifolds an distant ends of the building, that does not rule out one boiler for the job. You can run the manifold main supply and return the length of the building to get the heated water to the floors from the same boiler. If you are thinking of the distance for the domestic hot water to travel from one end to the other, then you can also have two 20 or 30 gallon indirect tanks. one at each end of the building. Indirects are usually fairly efficient as storing hot water compared to a vented water heater tank.

    Also zoning for that home design is easily done, if the guest room and guest bathoom is unoccupied most ot the time, then those rooms can be left at a lower temperature and the indirect tank can be shut off, so not to waste energy in the unoccupied area. Just remember to flush that unoccupied tank from time to time to keep the water fresh. And turn on the thermostat for the water heater at least 15 minutes before your guests need a shower.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • GroundUp
    GroundUp Member Posts: 1,959
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    Have you experienced a combi before? I couldn't be paid enough to have a combi in my house- personally. Your space heating BTU requirements are likely less than half of what you've suggested, but your DHW load is entirely unknown without more information. In order to properly size the equipment, an actual heat loss calc needs to be performed on each zone separately and it may or may not dictate a buffer tank, if any of your zones are smaller than the lowest firing rate of the boiler(s). I would agree with the above and consider a single indirect tank with perhaps tandem 80k-100k heat-only boilers.

  • Hot_water_fan
    Hot_water_fan Member Posts: 1,917
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    Tankless seems suboptimal here. You have space, use a tank or two!

    Alan (California Radiant) Forbes
  • sacres
    sacres Member Posts: 2
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    Thanks for all the input. The 35 BTU/SF came from an internet search on how to calculate in-floor heating requirements, probably sponsored by a boiler supplier. Using the manual J quick calc, also found online, of square footage (5500), occupant count (2), door count (9), and window count (69), results in a total load of 133,200 BTU or just over 24 BTU/SF (thank you GGross). The current HW layout indicates one boiler to serve the Master Bath, Laundry, and Powder Room and, at the other end of the house, the second boiler to serve the Kitchen and Guest Bath. My main concern was addressed by Mr. Hall, the ability of the combi boiler(s) to supply an adequate volume of hot water while also heating the home. The information I can find online regarding combi boilers does not specifically show flow rates. Is that calculated from so other specification value? It does seem the right answer may be to have a tank.

  • hot_rod
    hot_rod Member Posts: 22,491
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    a /150 combi would supply about 3.5 gpm continuously

    With a 77 degree temperature rise. So it comes down to how you use hot water


    139,000 btu/ hr in the heating side, 13?900 low fire modulation

    If you have a large tube with a valve flowing 5 gpm or more a combi would not be a good fit

    I have owned and lived with a number of combis, the work as expected

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream