Zoned Forced Air System Runs 2.5X Upstairs vs. Downstairs

Question

I bought a house built in 2005, ~4100 sq. ft.

Reasonably insulated to code, as required in Chicago, IL.

I've got an upstairs thermostat and downstairs thermostat. Both are Nest thermostats specifically, and I can track the daily run times.

Once we hit sub 30 degree days, I've noticed that the downstairs furnace will run for 6 hours but the upstairs furnace will run for like 13 hours, to keep everything at 68 degrees.

First thing, I noticed is that the upstairs thermostat is in the master bedroom and usually behind a closed door and the rest of the upstairs with open doors was a few degrees warmer due to better air circulation.

I adjusted our upstairs thermostat to sit at 65 degrees vs. 68 degrees downstairs to try to compensate for that.

Even with the changes, I haven't seen the variation shrink too much. The upstairs furnace just runs way way longer.

Is that typical in a bigger house like this? I always thought "hot air rises" -- in our old townhouse the upstairs would be smoking hot, to the point we need to close a damper.

This house seems completely opposite.

Aside from a possible HUGE hole in a duct somewhere that feeds upstairs, is what I am seeing normal?

I think if I measure the duct heat upstairs vs. downstairs it should be close...?
Intuitively there is a LOT MORE DUCTWORK for that heat to make it's way upstairs from the basement. Does that typically dominate for heat loss vs. "hot air rising"?

Answer 1

What you are seeing is actually normal IMHO and as a result of the second law of thermodynamics. Heat travels from hot things to cold things.

You are under the impression heat rises. That is only true for AIR. It is NOT true for solid objects.

I assume this is house-on-slab not house-on-crawlspace or house-on-basement.

Your downstairs is like a piece of melted cheese in a double-cheeseburger it is nice and toasty from the Earth and from the Sky. On the bottom is concrete and soil and carpet which is really dense and makes a good insulator as heat does not travel quickly through it. On the top is the ceiling and air space in between joists which is also a good insulator. PLUS on top of that ceiling insulator you have the upstairs which you are heating to the same temp - so the heat from downstairs isn't going to travel through the downstairs ceiling to the upstairs because the upstairs is already at that same temp. Then you have the walls which are insulated and they are close to the ground you likely have bushes and shrubbery and such that blocks some of the outside airflow around the structure, and you might have brick facing over part of it that insulates more, and you likely have less total window space. And those windows might even be triple paned.

Now for your upstairs. First I would bet you have more heat loss through the walls. Second I would bet you have larger total window area and you might even have cheaper double paned windows. And lastly your attic - even though you think it's insulated just how insulated are we talking? If it is below freezing outside and you can go into your attic and the temp is above freezing then your attic insulation is allowing heat loss.

The last thing you should do for the upstairs is check the return air. A big mistake people make with HVAC in residential settings is they pay a lot of attention to the hot air ducting FROM the furnace. Well it does no good to blow hot air into a room if you don't have some way for the cold air in that room to exit, preferably straight into the return air duct of the furnace. It's much better of course if you can suck that cold air out, which is done in commercial settings.