heater fan on today the switch was set to AC

FAN setting on t-stat........set it to AUTO

more likely the control board at the furnace. had the same thing a few years ago. easy fix, if you have two furnaces take the control board out of the other one if they both are the same, if that fixes it contact dinosour electronics for a new board, about 100.00

Do you have the integrated thermostat, similar to this one?



Single-Zone-LCD-Operation-Manual_17590.pdf

Mine has the same thing, and I encountered this issue too. Old-Biscuit has the right answer: The fan setting needs to be set to Auto.

Early one morning, I got up and turned on the heat, and the A/C fan kicked in! No heat (maybe), but a lot of constant noise. I finally turned off the A/C breaker to stop the fan.

Researching the issue after we got home, I found the fan setting was on something other than Auto. I'd probably tinkered with the thermo at one point before the trip as it is a new unit, and I'd never had an integrated thermostat before.

Hope this applies to yours as well.

The thermostat cannot control just the furnace blower- the fan auto/hi/low setting affects the air conditioner blower during furnace operation, but the OP has the furnace blower running.
It pretty much has to be the fan control board in the furnace- if the thermostat were calling for heat, the burner would come on as well as the blower, but blower only points to furnace board.

Chris Bryant wrote:
The thermostat cannot control just the furnace blower- the fan auto/hi/low setting affects the air conditioner blower during furnace operation, but the OP has the furnace blower running.
It pretty much has to be the fan control board in the furnace- if the thermostat were calling for heat, the burner would come on as well as the blower, but blower only points to furnace board.

I had a similar problem. The Fan relay on the furnace board was stuck CLOSED(ON). IF this happens, since the tstat did not call for heat, the system does not try to ignite the burner. What we found was, there was water intrusion and the Circuit board had rusty corrosion. This jumped the Fan wires and ran the furnace motor. Doug

Thanks for the update, Chris - now that I go re-read the OP, I see what I missed.

It does look like this is a control board issue.


--Bruce(occasionally jumping at conclusions).

dougrainer wrote:

Chris Bryant wrote:
The thermostat cannot control just the furnace blower- the fan auto/hi/low setting affects the air conditioner blower during furnace operation, but the OP has the furnace blower running.
It pretty much has to be the fan control board in the furnace- if the thermostat were calling for heat, the burner would come on as well as the blower, but blower only points to furnace board.

I had a similar problem. The Fan relay on the furnace board was stuck CLOSED(ON). IF this happens, since the tstat did not call for heat, the system does not try to ignite the burner. What we found was, there was water intrusion and the Circuit board had rusty corrosion. This jumped the Fan wires and ran the furnace motor. Doug

YEP........

Reading AND comprehending .......got FANS mixed up

wouldn't it be nice and helpful to all of us if op's would just post a few words about the solution and the outcome instead of just dropping it? just saying.

sorry its been awhile since ive been back on, I had only 2 days off last month and 10 hour days. what I found out I had an invasion of ants and they had gotten in my mother board controls and they caused my fans to kick on. one can of ant spray and a week with the fuse out cured my problem. Thanks for all the help.