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Best options for hydro air backup heat?

jeffreyhsi
jeffreyhsi Member Posts: 7
I was ready to do an oil to gas conversion, when I decided to replace the 3 25yo ac condensers be also. I realized that i could use 2 heat pumps to heat the house instead, and it was recommended to have a backup heat source since I had already scheduled gas to be brought into the house. I live in Long island, ny.

There are two 60,000 btu coils in my air handlers in my attic, and I have 3 bathrooms, though usually only 2 will be used at any time because of my really young kids. I think that a tankless is the way to go, and was looking at the Navien 240e or the Bosch 151p combis.

I know that most of you will probably think that may not generate enough DHW for the house, but what's a better and cost effective solution? The boiler portion won't be used 99% of the year since it's just serving as a secondary heat source in case my heat pumps can't keep up in 20° weather.

Comments

  • jeffreyhsi
    jeffreyhsi Member Posts: 7
    Btw, to clear up any confusion, I'm using 2 new Bosch inverter ducted heat pumps connected to new ductwork in my attic.
  • newagedawn
    newagedawn Member Posts: 586
    a good back up is a wood or pellet stove, and 25 jumping jacks,lol
    "The bitter taste of a poor install lasts far longer than the JOY of the lowest price"
  • Steamhead
    Steamhead Member Posts: 14,834
    How much heat do you need? Did you do a heat-loss calculation on the house?
    All Steamed Up, Inc.
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • jeffreyhsi
    jeffreyhsi Member Posts: 7
    It's a 4500 sqft house. I know that everyone here says that contractors tend to oversize, and most people said that I needed around 160k BTUs, but my calculations suggested that 120k was enough.
  • JUGHNE
    JUGHNE Member Posts: 8,938
    Do you have 10 tons of AC/HP capacity?
  • jeffreyhsi
    jeffreyhsi Member Posts: 7
    8 tons of ac/hp capacity

    I'm going to spray foam the attic after all the work is done too to make it a conditioned and better insulate the house, which was built in 1950.
    JUGHNE
  • jeffreyhsi
    jeffreyhsi Member Posts: 7
    edited December 2017
    I guess my main question right now is which would be better for hot water? The Navien has a higher btu (199k) for hot water and has a flow rate of 4.5 gpm. The Bosch seems to max out at 155k and 4 gpm, but they seem to be more reliable? And also has a 5 year parts and labor warranty. It's only a $500 difference to get the Bosch, which other plumbers have given me much higher quotes for. But my plumber says that he knows the main Bosch guy on Long Island pretty well. Honestly, he's comfortable with both. Up to me, he said.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 10,253
    I would install a smaller boiler (say 80K max) and an indirect water heater. You will get better hot water with the storage and the 80K will provide plenty of back-up heating.

    Do it right and do a heat loss first
    jeffreyhsi
  • jeffreyhsi
    jeffreyhsi Member Posts: 7
    But wouldn't the small boiler be unable to keep us with the 2 60k coils?
  • Steamhead
    Steamhead Member Posts: 14,834
    Depends. How did you calculate the heat loss? What program did you use? Were you figuring on having the boiler carry the entire load in case your heat pumps went down? How were the coils rated- what water temperature entering and leaving the coils?

    Lots of details, but the details make or break the job.
    All Steamed Up, Inc.
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
    jeffreyhsi
  • mikeg2015
    mikeg2015 Member Posts: 1,183
    With hot water, you need to consider flow rates, delta T, water temps and airflow.

    Tankless units rely on low flow and very high delta T. they make terrible boilers because delta T on the coil in the AHU is 20-30F at most, but the maximum flow through the Navien is 7GPM including domestic.

    Save yourself a headache and go with a boiler. But, you'll probably need water temps over 140F return temps, so a mod-con might be a waste of money since it's backup heat.

    1) Figure out how much heat you need, assume the heat pump is not working (or power is out, can use a very, very small generator to power a boiler and pump). Sounds like 60k per coil.
    2) go to he coil specifications and see what water temperature and flow rate you need for that capacity.Factor in the airflow of course. Target fairly warm air temps for comfort.
    3) determine the what the return water temp will be. If over 140F return 160F supply, get a cast iron boiler.
    4) Get an indirect tank.

    jeffreyhsi
  • jeffreyhsi
    jeffreyhsi Member Posts: 7
    edited December 2017
    It seems that the return water temp is around 110, which should be perfect for a condensing boiler, right? I took a screenshot of the specs off of Bosch's website. Bosch seems to advertise their Greentherm as a perfect pair with their coils, actually. Perhaps it was designed to be used with a tankless?

    Can an indirect tank be used with a tankless? Does it nullify the efficiency of the tankless?
  • njtommy
    njtommy Member Posts: 1,105
    Yes you can use an indirect tank with a tankless boiler. Your probably better off using a indirect tank over a combi just to limit cycling on and off depending on your DHW load. Remember on Combi boilers They can’t handle the load of heating and DHW at the same time. They handle the DHW load first. So if you wash your hands it the cycles the heat off and restarts for DHW demands. It adds a lot of cycles to the boiler. Also you could get a bit of cooler air for a few mins out of the registers. Not a big deal to some. Others complain.

    I’ve heard a lot of great things about the bosch inverters. I would definitely install the heap pump and go with hydroair handlers and looking at the specs your good to go with a mod con boiler or combi.
    jeffreyhsi