Solving 2nd floor overheating with a hydronic hot air system
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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