Noticing an issue with our furnace or thermostats. We have a two-story house with a thermostat on each floor and a gas furnace. Typically when the downstairs thermostat clicks on, obviously the heat comes out of the downstairs vents primarily. And when the upstairs thermostat clicks on for heat, the heat is coming out of the upstairs vents primarily

But as of yesterday, we noticed that when the downstairs thermostat clicks on it is actually pumping the hot air through our upstairs vents. Which effectively is making our upstairs like a sauna and our downstairs is freezing. We can turn both thermostats off for a few hours and when we turn them back on it seems to fix the problem for a few hours but inevitably within a few hours we will notice the upstairs is roasting again and we are freezing downstairs.

Do you think this is a thermostat problem or possibly the control board of the furnace/ac unit? I've already tried switching the thermostats and seeing if anything changed but even with the thermostat switched upstairs and downstairs, it's still heating the upstairs regardless of which thermostat clicks on.

Waiting for a better HVAC guy answer but let's gather some 411
Q: are your thermostats wired to furnace or wireless?
Q: type (or photo) of thermostats
Q: brand of HVAC (elect-cool, gas-heat)?
Q: zip code or general location and outdoor temps?

Not saying what you are seeing isn't possible, but ducts don't work like that. One unit is for upstairs and has independent ducts, and another is for downstairs and has independent ducts. There isn't really any common duct where they could mix. What might be happening is, if you have wireless t stats, maybe one t stat is controlling both somehow. If your t stats are wired, I guess you could have some kind of short in both wires and one activates both. That is pretty unlikely.

Turn both t stats off. Go sit in the attic and have someone turn on one t stat and see what happens. Then switch and try the other.

I thought the same thing initially, considering an upstairs unit and a downstairs unit, but then wondered if OP actually has a single unit that uses powered damping for zoning. Wasn't clear when I read the OP.

tgivaughn said:

---

Waiting for a better HVAC guy answer but let's gather some 411
Q: are your thermostats wired to furnace or wireless?
Q: type (or photo) of thermostats
Q: brand of HVAC (elect-cool, gas-heat)?
Q: zip code or general location and outdoor temps?

---

Thermostats are wired.
They are generic Honeywell thermostats
Gas furnace, electric cooling
Fort worth area. About 16 degrees right now

Been in the house 10 years now. Only one outdoor unit. Furnace in attic. Every time heat kicks on downstairs in the past, the hot air coming from the downstairs vents is powerful. Minimal hot air comes out of the upstairs.
When heat kicks on upstairs, the hot air comes out strong from the upstairs vents and just a minimal amount from the downstairs vents.

But as I mentioned, now the hot air comes out strong from upstairs vents even when the downstairs thermostat activates to turn on the heat. I guess I could try to turn off the unit from the switch in the attic and see if that resets something. I've tried turning off both thermostats for a few hours. That actually seems to work for a little bit, but then it goes right back to only blowing hot air upstairs and the downstairs starts getting cold

Possible issue with damper and redirection.

I align with Ben's thoughts on a power damper that throws heat to one floor or the other, as a thermo "calls" for heat.

Also align with cena that the damper controls are a mess, probably needs an electronic part replaced so damper responds correctly as it once did.

FYI: we do less & less of these damper HVAC designs over the years.
If you ever replace your system (~20 year life in Aggieland if used a lot)
then think about two units, no damper and after spending more at first, it's an EZ bet you'll be saving money monthly on energy bills + get better zoning/comfort IMHO

I also agree that this is a damper problem. I have had this same issue and I use nest thermostats. Either the damper is stuck open in one position or the nest confuse the damper. This confusion is what happened to me. Due to not great ductwork in my house to the first floor (and not really fixable), my airflow to the first floor wasn't sufficient, and the upstairs got most of the air flow.

In talking to my hvac guy, he suggested that nests can confuse the damper if they are both within 2 degrees of each other. So now, I just mainly use the downstairs thermostat to control the house temp, since the upstairs gets adequate airflow even when the upstairs thermostat is not calling for it.

Since you would normally get some airflow upstairs anyway, try to turn the upstairs temp to 5 degrees less that downstairs and see if that helps and resets the damper to an open position for the downstairs. They heat will rise anyways