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Help! Want an efficient HW & Heating Nat gas system

Hi there everyone, this is my first post on here.  Here's my situation.  Bought a 4 unit apartment building that is currently oil fired baseboard heat and hot water boiler for all 4 apartments.  I want to switch to natural gas as it is cheap in my part of SE Pennsylvania.  I had the following idea:  To heat and give hot water to all 4 apartments, I was going to put in a Tankless Navien 240 Combi HW Heater/Boiler system (199K btu).  This was to heat the 4 apts (total sq foot of 2000), handle 4 showers/sinks, and 1 dishwasher. 

After talking with my local heating supply store, they told me that system was probably going to be inadequate.  So right now, prior to your help, I am contemplating the following:

1) Install 2 Navien systems with an old HW heating tank to act as a tempering tank for the water.  I would separate the heat so that each Navien unit services 2 apartments only.

2)  Find a better and bigger combination unit system that can handle 4 apts with 4 separate thermostats.

3) Ditch the tankless boiler idea.  Go with a 1 Navien HW tankless system along with a traditional gas fired boiler. 

4) Pull my hair out, except I am already bald!

Anyone have suggestions? 


  • Zman
    Zman Member Posts: 7,574
    Why tankless?

    Here is what you need to to:

    Have a heat loss calculation if your building done.

    Measure you baseboard heaters to determine your available radiation.

    Check the flow rates of the showers in the units.

    Have a nice system designed that meets your needs.

    It sounds like you would be well served with a smaller modulating boiler like the Triangle Tube prestige solo 110 and an indirect hot water heater.

    In your situation the demand combo unit would have to be oversized to meet your potential domestic water spikes. It would then run very inefficiently as a heating boiler.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Not Probably

    Definitely will not work. Worse case is you need to make 12 gallons a minute of DHW. That would require a tankless that can produce 420,000 btu/hr. Better off with a condensing boiler and indirect water heater as Carl has suggested. Suggest you take Carl's advice and start with a heat loss.

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