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Gas furnace v hydro-air in remodel

We're in a 3100 sf 2-story cape house (with attached garage underneath) in Massachusetts and will be remodeling/expanding part of the 2nd floor with higher rooflines, etc. As part of the remodel, we'll be adding a new heating & A/C for the remodeled portion of the 2nd floor. Our current setup is a gas furnace (forced hot air) in the basement with central A/C. We also have a snowmelt system for a 1300 sf driveway running off a 5-yr old Viessmann WB2B-80 Vitodens 200-W boiler (net 230-240 BTUs).

the new remodel would have closed cell insulation in the roof and walls, so the attic would be conditioned space. the new zone upstairs would be ~1000 square feet.

We have 3 heating options for the remodel and are at a loss:

a) gas furnace in the attic

b) hydro air off the Viessmann boiler with a flat plate heat exchanger, isolating the snowmelt antifreeze on its own loop. the boiler would run hot water to an attic air handler for hydro air

c) hydro air off the Viessmann boiler with a 40-gallon indirect water heater as a heat exchanger. in this scenario, the snowmelt and boiler would run antifreeze/glycol mix and there would be a separate loop to the water heater. the water heater would run hydronic water to an attic air handler for hydro air.

the 2 hydro air options would have the benefit of being able to run a small garage unit heater (modine) as well. however, the hydro air options are substantially more ($5-10K) than the attic gas furnace.

Interested in thoughts on which to choose and why. Thanks!!!


  • Snowmelt
    Snowmelt Member Posts: 1,418
    Hydro air

    I. Like the hydro air option with the indirect.

    Or just have 2 more zones one for the indirect and one the secound floor heating the hydro air. The air won't be as dry as your traditional hvac equipment. Also don't have to run a gas line to the attic or cut another whole in the roof.
  • Snowmelt
    Snowmelt Member Posts: 1,418
    Hydro air

    I. Like the hydro air option with the indirect.

    Or just have 2 more zones one for the indirect and one the secound floor heating the hydro air. The air won't be as dry as your traditional hvac equipment. Also don't have to run a gas line to the attic or cut another whole in the roof.
  • icesailor
    icesailor Member Posts: 7,265

    Hydro-air/AC in an attic ROCKS in that application. Fish up a couple of insulated lines and it is a wonderful thing. Especially, the AC. How else would you get it up there?

    As for the humidity, "Heat flows to cold, dampness flows to dryness". Its the moisture you want in the winter. Put steam humidification in the first floor system for winter use and comfort. Mother nature will take care of the second floor.

    "Dampness flows to dryness".  With very tight and well insulated houses, you can't get long enough run times to make "normal" humidification work well. Steam humidifiers, run off of humidistats will give you run times to circulate the moist air without the heat. The humidistat on the unit keeps it from running when the humidity level is proper.  Just watch for water condensing on the inside of windows. The sign to cut back on the humidifier setting.
  • NewEnglandRemodel
    NewEnglandRemodel Member Posts: 2
    hydro-air limitations

    Thanks -- here are the specific concerns/thoughts

    a) cost/efficiency -- we're comparing vs. a Trane 97.1% AFUE two stage gas furnace (seems like that's comparable to the Viessmann in efficiency?)

    b) snowmelt - would we end up decommissioning the snowmelt by adding the extra load?

    c) If we go hydro-air, why not just use the flat plate heat exchanger to isolate the snowmelt? It takes up less space, there's not another water heater taking up space and possibly failing and sitting in the [colder/unconditioned] garage, and the Viessmann isn't running antifreeze/glycol. By the way, the water heater in the other setup is only being used as a heat exchanger, since it's holding boiler water it's not supplying hot water to the house (we have 2 other gas water heaters for that).

    d) isn't hot air always dry, whether it's dried by hot water coils or by a furnace?
  • SWEI
    SWEI Member Posts: 7,356
    Add radiant?

    Since you're adding on, you might consider adding some radiant emitters to increase comfort and reduce the winter CFM requirement. 
  • John Mills_5
    John Mills_5 Member Posts: 952

    Trane 60K 95% furnace will be GROSSLY oversized for 1000 sq ft built like a tank. I have 1000 sq ft stand alone house and heated at -15° out with high locked out or firing at 36,000 BTU. Friend with a foamed 1600 sq ft house had a Trane 60K 2 stage and it would only fire on low with 2 zones calling, She never did use the basement so the zone panel wouldn't let the furnace go to high. So if you go with a gas furnace, find the smallest you can get or you'll short cycle so much you lose efficiency. Or another selling point for the hydroair. 
  • Rich_49
    Rich_49 Member Posts: 2,766
    edited March 2014

    Have used this configuration before in a much larger home than you are discussing , works great hooked up to 5 Coils and plenty of DHW too .     You have the benefit of ODR and a wide range of modulation , by far the best combination I have ever used on a hybrid system . http://www.htproducts.com/phoenixairhandler.html
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • heatpro02920
    heatpro02920 Member Posts: 991
    Hydro air

    OK, there are a few more things to take into account, when comparing a hydro air {running off a std eff. furnace} and high efficiency furnace.

    First will your existing boiler be able to satisfy the added load?

    If so this is a good thing since it will bring you closer to not being over sized.

    Next is comfort, furnaces are dry heat, I prefer hydro air to a direct fired furnace

    Service, a hydro air is most likely going to go much longer with less service than a 95+ furnace.

    sizing, you are going to need a smaller furnace than is probably available for your space, depending on your heat loss, with a hydro air you can size it for the temp and heatloss you want...

    So in other words, I vote hydro air...

    As far as the isolated anti frz loop, you can install a frz stat, most likely your contractor will use first co. they sell a $20 anti frz add on that will stop you from freezing, when the unit is operational, I would pitch all pipes and add gravity drains in case the unit is ver down for a period of time, but if you can live with the anti frz its not a terrible option, and using a plate exchanger to separate them is the way to go, although expensive because you should install a 911s, prs relief valve, exp tank, back flo, ect on the coil side, I have seen some guys just install a water inlet and fill station...