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Community and Q&A

Solving 2nd floor overheating with a hydronic hot air system

Lizzieplants | Posted in Mechanicals on

We built a near passive house 2 years ago (R60 attic, R40 wall, R20 under slab, Intus windows, etc Ach 1.6) in upstate NY zone 5. We unfortunately participated in a NYSERDA/GTI demonstration project where they installed a NuAir hydronic hot air system (with integrated HRV) fed by a Noritz tankless gas heater and solar thermal (they paid for all the equipment). The mechanical room is on the second floor.

The first winter we immediately noticed a lot of temperature stratification and had the hot air system zoned (first floor open plan 800 sq., separate master wing 384 sq and second floor home offices, guest room and bath 774 sf) which improved the temp difference on the first floor between the LR and Master and improved the A/C temps on the second floor in the summer.

The problem is in the winter when the second floor zone runs as high as 80 degrees in my office with heat off in that zone. The total heating load of the house is less than 15,000 Bthu per the Manual J (Zone 1: 2,562 Btuh Master suite, Zone 2: Second floor 3,894 Btuh , Zone 3: LR/DR/Kit 5,375 Btuh).

Unfortunately our builder let the GTI installers put in a larger NuAir (rated for 35,000 Btuh) than what was originally specified which is part of the problem. It appears that our bodies, the computers, network and office equipment along with the mechanical room on the second floor provides more than enough heat for the second floor. We still need the NuAir system to run upstairs because of it is also our HRV. If we heat with the wood stove on the first floor (heating off) the second floor becomes comfortable which indicates that the problem is not heat rising up the stairs but system leakage (all ducts were sealed and are inside).

Folks who have looked at the system recommend re-piping the system to lower the water temp to the NuAir to 90 degrees (now 120 degrees) and adding ventilation on the second floor to get rid of the excess heat. These options are quite expensive and I hate the idea of penetrating our sealed attic and blowing hot air out of the house. We do have 7.35 Solar City PV but with living and working from home 24/7 we use more than what we produce.

Based on our observations my thought is to install other heating sources in Zone 1 and Zone 3 and set the NuAir thermostats to 50 degrees to use as a back up source of heat. The options are baseboard or wall electric heat which is cheap and easy to install but expensive to run, hot water radiators or a ducted mini-split with ducts to Zone 1 and Zone 3. Your thoughts on the most cost effective solution.

P.S. We tell everyone considering a well insulated house to install mini-splits

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  1. GBA Editor
    Martin Holladay | | #1

    Permanent solutions are likely to be expensive.

    I'll start by stating the morals of the story:

    1. Heating systems should be right-sized, not oversized.

    2. It's almost always a bad idea to deliver ventilation air through heating ducts. Ventilation systems need dedicated ventilation ductwork.

    If you can afford to do the work, the best approach would be to install a new ventilation system that has separate ductwork from the heating system.

    My guess is that if you did that, and disabled the NuAir equipment, you could easily heat your house with a single ductless minisplit on the first floor.

  2. Lizzieplants | | #2

    Lesson learned.

    Putting in a new ventilation system with new ductwork is beyond our budget. Won't it be cheaper to just use the NuAir and existing ductwork as an HRV (turn off the heating part) in the winter. If we disable the NuAir then we would have no AC on the second floor. The AC works fine in the summer. The way the house is designed we need two zones on the first floor or a ducted minisplit system. When we run the wood stove in the LR the air doesn't make it to the master suite and it is too cold.

    Of the secondary sources (electric resistance, radiator or ducted mini split) which would you recommend?

  3. GBA Editor
    Martin Holladay | | #3

    I'm really not very familiar with the NuAir, so I am reluctant to provide advice on how the appliance could be altered. If the heating function of the NuAir can be easily disabled, so that the appliance is turned into a ventilating appliance in winter and an air conditioner in summer, that approach might work well.

    Since you now have a type of hydronic heating system, the cheapest way to install new heating elements might be to just add a few hydronic baseboard units where needed. But I hesitate to suggest the best way to proceed without knowing more about your house and looking at a floor plan.

  4. Lizzieplants | | #4

    I agree that would be the most efficient. I can check with the NuAir folks. I would think just setting the thermostats low (55 degrees) might work as well with a second set for the radiators A floor plan is attached.

  5. GBA Editor
    Martin Holladay | | #5

    The master bedroom clearly can't be heated with a single ductless minisplit in the living room.

    If you can keep use your NuAir to heat the master bedroom by setting the thermostat of that zone at 70 degrees F, while keeping the thermostat of the other zone at 55 degrees F, I suppose that you could heat the main section of your house (living room and kitchen downstairs, and the second floor rooms) with hydronic baseboard units. This approach would require you to install a new circulator and the necessary controls for the hydronic baseboards.

    I'm not sure from your description whether you can use the NuAir for heating just one zone, while still getting the desired benefit of ventilation distribution in all rooms.

  6. Lizzieplants | | #6

    I believe when it is not calling for heat all the zones open for air circulation and the HRV.

    Attached is a trial we did last winter using space heaters and the wood stove to see the effect of using supplemental sources. The best temps on the second floor were when we weren't using the system in both the LR and the master.

  7. charlie_sullivan | | #7

    There's got to be a better solution than punching holes in your envelope!

    But it's hard to figure this out remotely. A couple of questions:
    1) How does the zoning work? It's one Nu-Air but with motorized dampers?
    2) What model Nu-Air is it?

    There are about 5 ways your upper floor gets heated:
    1) Parasitic--humans, computers, etc.
    2) Solar gain
    3) Heat from downstairs wafting up
    4) Standby losses from equipment in the closet upstairs
    5) Heat from the ducts which somehow comes up upstairs even when upstairs ins't calling.

    1) and 2) are for free, so short up upgrading to more efficient models of computers, there's not much to be done.
    3) Is unlikely to be your main problem, but you might be able to reduce it with air sealing--the classic scenario is that the air leaks in the basement or first floor, cooling the first floor, and the heating system has to work hard to maintain temperature on the first floor with that cold air coming in. There's then a flow of warm air from the first floor up to the second, equal to the amount of cold air flowing in below. It sounds likely that you've had a blower door test and have a pretty tight envelope, but I thought it would be worth mentioning. Also, your wood stove experiment indicates that this isn't a major problem.
    4) You might be able to help things out a lot by insulating all the pipes in the utility room. It can be fussy work to fit insulation around the joints, valves, etc., and that combined with the argument that the heat isn't lost but is doing useful work heating the house, leads to most contractors omitting it. But in your case heat loss there is a problem. Perhaps you have some pipes insulated, but some valves, etc. left uninsulated--whatever the situation, better insulation could help.
    5) Without knowing how the zoning is set up I'm not really sure what is going on there. And I'm skeptical that changing the water temperature would might just run more often and on a higher fan speed and give you just as much of a problem.

    If you took Martin's suggestion and installed radiators downstairs--baseboard or panel--you'd shift the balance to more heat downstairs. Taking that to the extreme and shutting off the water supply to the Nu-Air completely would eliminate 5) as a problem, but that might not be necessary. With some radiators downstairs, the Nu-Air would run as a heater less often, and so you'd get less of the problem.

    But starting by insulating the pipes really well seems simpler.

  8. charlie_sullivan | | #8

    Seeing the recent comments, I think a possible design would be an upstairs Nu-Air thermostat setpoint of 70 (or your preference), a downstairs radiator setpoint of 70 (or your preference) and a downstairs Nu-Air setpoint just a few degrees lower. That way you would still have the Nu-Air able to kick in downstairs if you didn't have enough radiators to meet the demand.

    But I still think pipe insulation first.

  9. Lizzieplants | | #9

    One NuAir EN500 DX (was supposed to be a EN400 DX) with motorized dampers, 3 thermostats. My office with windows on two sides and the most equipment is the warmest room upstairs. Solar gain does contribute a degree or two. Today is cloudy and it is 76 degrees in my office, yesterday it was sunny and up to 78 degrees.The heat mode upstairs is off.

    Insulating the pipes is a good idea. Thanks for the input.

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