Two-story, 1900 sq. ft. house built in 2004 to CA standards: 2x6 framing, tons of insulation, double-pane-low-e windows, etc. Furnace is a Bryant “80 Plus,” downflow, single-stage gas valve, single-stage blower, propane unit. Blower speed is adjustable by switching wires and is now set at med-hi.

The issue: The furnace constantly cycles on and off, being on about half the time, just to keep the house at 67 degrees on a 40 degree day. It is using about 175 gallons/month or propane, and we turn it off at night and when we leave the house.

I have had two furnace guys out to see why we get so much heat out of the 4 upstairs registers and so little from the 5 downstairs registers, all of which are 4 x 14 units. One guy said he would install a damper in the duct going upstairs. The furnace is the garage and the duct is easily accessible. The other guy said no way, that this would reduce the design airflow through the heat exchanger to the point where it would overheat.

"So much heat" in this case is about volume, not temp. The temp at the downstairs registers is 125 degrees (sticking a thermometer into the register), but the volume of air is very low. The upstairs registers blow huge volumes of air.

The upstairs duct work is all galvanized duct and the downstairs is mostly flex duct, suspended from floor joists in an uninsulated crawl space. Like octopus tentacles, all coming from a big box protruding through a crawlspace wall from the garage-installed furnace. Some of the runs are 20 ft. of 6” flex duct, all wrapped with fiberglass insulation.

HELP!! If I have a damper installed, will the furnace continue to deliver about the same volume of heated air, only now directed to the downstairs area where it is needed? Or is the flex duct too restrictive and the back pressure will cause the airflow to be reduced to a dangerous (for the heat exchanger) level?

BTW, both of these guys looked at the flex duct runs under the house and said “they were probably OK.” Whatever that means. I feel the furnace, at 80K btuh is oversize and under-featured (single stage) and that the duct work was not correctly designed for the application.

Now I'd love to hear a professional opinion on this, especially one that isn't going to cost a fortune!