Is a damper to limit upstairs air flow OK?
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!
The duct is probably sub par but after the damper is installed the system should be balanced with a flow hood to get the proper cfm to each floor then set and then do a temp rise test to see if the furnace is above or within manufactures specs temp wise. If it is then great if not then the blower could be sped up, if that gets it within specs great. If not some duct modifications should be done to get it within manufactures specs. The damper will have to be adjusted in spring to deliver more ac upstairs less downstairs if you have ac. Or you could have a motorized damper system installed that will self adjust operated by a thermostat in both the upstairs and downstairs locations.
The way this is set up is the furnace sits on a box. Out of the box come two ducts, one directly up to the second floor and the other out the backside of the box to the ducts in the crawlspace that service the first floor. When you say "the duct is probably sub par," which duct are you referring to? Wouldn't a damper in the duct going to the second floor accomplish the same effect as a flow hood?
As long as the motorized damper is wired to open if the plenum temperature reaches a temperature that is getting close to the furnace limit, I see no reason why such a damper would not help this situation.
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A balometer (flow hood) measures the cfm (cubic feet per minute) of air coming out of a supply register/diffuser or entering a return grill.
Originally Posted by riverguy
Flex duct should not be used more than 3-6 feet at the end of the run of rigid tin ductwork.
The static pressure is too high causing more air to be delivered upstairs as there is less restriction.
Install manual dampers on each run to get balanced air flow to each zone as well.
Reply on my "flow hood" misunderstanding . . .
OOPS. I thought a flow hood was a movable baffle to divert airflow from one duct to another.
As logical as it sounds, I was afraid somebody would tell me the long flex ducts have to go. So all I need now is a good HVAC guy out here in West Sonoma County, CA!
Originally Posted by koolkahuna
Where in Hawaii were you working?
I had a job in Kauai that I finally refused when the paperwork took over seven years to get me there.
Your furnace is running too hot. The flex (as mentioned before) is very restrictive, however, my guess would be to look more closely to the return air side. I bet you're really short on RA.
What size (diameter) is the flex runs and the supplies to the second floor? All 7" I hope. Nine 6" supplies are hardly enough, especially when 5 are in flex.
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