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Can we split tankless on-demand hot water unit for hydro air & domestic hot water?

Can anyone suggest a good way to use one whole-house on-demand lp gas tankless unit (Rinnai for example) and split its use so that it provides tank-less on-demand domestic hot water for a small house AND also provides hot water for an existing central air system (conventional with ducts and an outdoor condenser - one zone) which we want to convert to a hydroair heating system? We live in Putnam County, NY and we currently have an electric hot water heater and electric baseboard heat. Can we install the on-demand tankless in place of the elec. water heater and hook it up to the air handler (install a hot water coil), get the lp gas to it and install controls that make it work properly? We don't want to install a boiler and an indirect storage tank because we don't have the physical space for the equipment. Plus, we think it'll be efficient and have a short-term pay back on the cost of buying and installing the equipment. Crazy? Complicated? Easy & smart? Thanks! Mike


  • heatpro02920
    heatpro02920 Member Posts: 991
    Are you going to do this your self?

    Or higher a pro...

    I have done this, but be warned if it is not done correctly {especially with lp} you will soot up that heat exchanger from close deltas...

    First I will explain the process and parts...

    Use something like a ru98 rinnai tankless, a 3 way zone valve a flat plate heat exchanger a switching relay, thermostat, ss circulator, aquastat, and 1 regular circulator...

    So you plumb the tankless so that the hot out tees into the dhw piping and the fphx, then you have the cold in tee into the domestic cold with a check valve, you bridge the 3 way valve between the supply and return and use the aquastat to control it so when the delta comes back too close and would make the unit fire low and soot up it redirects it back into the air coil until it returns cold enough to fire the tankless at a good rate.. Over size the coil so that it can work at a low temperature, do your heat losses correctly, and plan the job well and it can work...

    BUT, I can almost promise you it would be cheaper to install a small 90+ furnace in the duct work, it will be more reliable and work better with less thinking...

    Just install a small water tank or a small tankless in the basement, then a 90+ furnace in the attic or where ever your ac is... done and easy..

    If you are interested in using the rinnai, I have drawings of how to make it work and I have done it,
  • heatpro02920
    heatpro02920 Member Posts: 991
    edited April 2014
    My drawings are in the pc at the shop

    here is a fast one I just made, this would be the best way to get it done, as you can see there are a lot of parts, and its not going to be as cost effective as you would think.. Beleive it or not you can get even more complicated than this, I have done this and tried and tested this to death...

    I have a system out there I built very similar the gentleman actually repped Rinnai and loves the product, he wanted me to install it in the closet of his guest room {large room with a full bathroom, it was 600 sq ft total with a heat loss under 20K btus. I used a first company air handler with an additional water coil stacked on top of it {couldnt oversize the air handler too much because I would have had to oversize the a/c coil too which would have led to issues on that size}...

    That system has worked well for years now and is propane... I went a few steps further with his, I installed a flow sensor on the hot water line so it would cut a call for heat and make hot water priority, I wired it with pre/post purge circuits so the air handler had hot water in it before it ran the fan and ran the fan until all the heat was gone at the end of the cycle...

    His air handler side was ran low pressure with glycol so no frz circuit was needed, and since it did not mix with the dhw it was ok..

    Keep in mind, these are expensive parts, you will need lead free fphx, circ, zone valve, ect everything on the dhw side will need to be lead free..

    I would look into a properly sized tankless for DHW and a properly sized lp 95% furnace for heat, Ill bet the budget will be similar, and if you go with a small 30 ga direct tank, Ill bet the budget comes out less...

    Now you can do this without the fphx and I have done it, but it doesnt work as well, and if your coil ever lets go and you have full domestic pressure feeding it, you may drown, and your insurance co will not be happy about the damage a domestic water line in your attic does...

    good luck, like I said it will work, and can be setup nice, but its not a great practice...
  • Snowmelt
    Snowmelt Member Posts: 1,387

    You can go to the takagi sight, then go to the digital catalog, then go to page 37 , the only thing I would do different is more the expansion tank to the bottom of the air seperator
  • Mike_W1
    Mike_W1 Member Posts: 8
    reply to HeatPro

    Thanks for taking the time to really get into it! Yes, based on all the scary stuff you write I can see that, IF we do this, I'll need a highly skilled heating mechanic. More study and research on my part is needed to pin down the scope and then I'll reach out to a couple well-referred contractors to set a budget baseline. Then we'll cost/benefit the deal with a 15 year time-frame as a basis. Thanks again and I'll be back here down the road with an update. Be well.
  • Mike_W1
    Mike_W1 Member Posts: 8
    thanks to snowmelt too!

    I will check that out as well!
  • heatpro02920
    heatpro02920 Member Posts: 991
    you are welcome

    I would get prices both ways a propane high efficiency furnace like the Armstrong 97mv. Or the Lennox 98 or any almost 99% efficient furnace with variable speed and modulation should last 30 years and coat very littleto run... Plus for short money you cam add a case coil and condenser for ac... I know a combo or nodded takless sounds like a good way to take care of Dhw and heat but as ice sailor would say "you are stepping over 100 dollar bills to pick up pennies"