I have a 13 yr old Carrier Weather Maker 8000 gas furnace. It heats fine. However, earlier this week, after reaching the set temperature, rather than shutting off the blower kept running, eventually blowing cool air. What's crazier, even after switching the thermostat (a programmable type) to Off the blower keeps running.

I've checked the circuit breaker, even flipping it off and on.
I changed the batteries in the thermostat.
I followed the directions and powered down the furnace, including shutting off the gas, waited the allotted time, then powered it back up.

The blower keeps running. The only way to shut it down is to flip the on/off switch located on the furnace itself, which I do before going to bed.

Any ideas?

Thanks.

Try this: increase thermostat setpoint above room temperature to initiate heating cycle, and listen for heating cycle to begin. From that point, does the blower stop after about 30-40 seconds, then re-start about 30-40 seconds later? Or does it run continuously throughout this test? Post back with results.

I know this sounds crazy but I have to ask is the fan switch in the on position on the thermostat? If not but the system is heating you have a stuck relay on the control board. No danger but it will need to be replaced to get the blower to cycle.( I run my blower 24/7 it never shuts off)

Quote:

---

Originally Posted by T-Top

I know this sounds crazy but I have to ask is the fan switch in the on position on the thermostat? If not but the system is heating you have a stuck relay on the control board. No danger but it will need to be replaced to get the blower to cycle.( I run my blower 24/7 it never shuts off)

---

Not so crazy at all. If the blower doesn't pause during the test, either the relay contacts have fused or the timing logic of the board has failed. If the board is good, blower will pause regardless of fan switch setting.

Quote:

---

Originally Posted by KC13

Try this: increase thermostat setpoint above room temperature to initiate heating cycle, and listen for heating cycle to begin. From that point, does the blower stop after about 30-40 seconds, then re-start about 30-40 seconds later? Or does it run continuously throughout this test? Post back with results.

---

The blower runs continuously.

Faulty circuit board. On the upside, the system will still heat if a new board isn't in the budget right now.

Quote:

---

Originally Posted by KC13

Faulty circuit board. On the upside, the system will still heat if a new board isn't in the budget right now.

---

I appreciate your help. Thanks.

Is this circuit board in the thermostat on the wall??

I had this same problem a ouple of years ago and the service call cost me more than the part. I replaced the thermostat and it worked great, but today I'm back to the blower not shutting off again.