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high gpm combi boilers? two units instead?

weil_fail Member Posts: 73
I've been planning to install a new boiler, and with some renovation, I can no longer vent anything to the roof. I had written off combi boilers because I will eventually have 5 bathrooms and 2 kitchens (mother-in-law suite), and I don't think a combi boiler would be able to keep up. so, the plan has been to go for a condensing boiler with indirect tank. however, I have been a little surprised at how expensive indirect tanks are. I can't for the life of me figure out why they're more expensive than a conventional water heater when they don't even have a burner. anyway, the high price tag of indirect tanks made me start looking back at combi boilers.

are there combi boilers that can handle 5 baths + 2 kitchens? I feel like it's reasonable to expect 3 or 4 simultaneous hot water draws, but that could be as much as 16GPM if 4 showers/tubs are going.

maybe the right answer is to use a combi boiler for 4 baths +1 kitchen, and buy a separate tankless for the mother-in-law suite? is 4 baths already more than a combi boiler can handle?


  • hot_rod
    hot_rod Member Posts: 14,797
    16 GPM, yikes! Are you sure your math is correct. What size water main and meter do you have coming into the house?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • weil_fail
    weil_fail Member Posts: 73
    well, it would be a 5 bathroom 2 kitchen, one laundry house with 5-6 residents. how should I calculate the gpm? I feel like no matter how you slice it, it's a lot
  • hot_rod
    hot_rod Member Posts: 14,797
    calculate the gpm of the showers and any other fixtures that would run continuously. Shower heads by code should be no more than 2.5 gpm.

    You would know better how often all the fixtures would be flowing at the same time.

    What tank or heater is in there now? Does it keep up with demand, or is this a new install?

    It would take a very large indirect and lots of boiler power to get you 16 gpm, if all showers run at the same time.

    Similar to a hotel loads, either store a lot for the dump loads, or generate it continuously with a tankless.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • bob eck
    bob eck Member Posts: 925
    I would install 1.5 GPM shower heads.
    Look at installing a Navien combi boiler. Then you can add one or two more Navien TGWH and they will work with the combi boiler. The Navien combi boiler can produce 4-5 GPM of domestic hot water and the Navien 199,000 BTU input TGWH can produce about 5 GPM each of domestic hot water. One Navien Combi Boiler plus one Navien TGWH would give you about 9 GPM add a second Navien TGWH and the total would be about 14 GPM.
    The Navien combi boiler and their TGWH can be common vented together so you would need only one wall termination.
    What is the incoming water supply size pipe? Is it on a city water supply system?
    Is this on a well pump? If so how many GPM does the pump and well produce?
    If you have well pump system I take it you would have a septic system. How good is the septic system? If using 12-16 GPM of hot water plus some cold water. That is a lot of water getting dumped into a septic system at one time.
    Find a good heating professional to help you.
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