Gas Furnace blower problem/Blower only works when Thermostat Fan Switch is in "ON" Position not in "Auto"

Question

My 1984 Comfortmaker Gas Furnace is having blower problems. I replaced the thermostat thinking that would fix the problem, but no luck. The problem is the blower only comes on if I put the "Thermostat Fan Switch" in the "On" position. If I put the Thermostat Fan Switch in the "Auto" position the burner comes on for about a minute (the blower doesn't start) then the burner turns itself off, I'm assuming it is turning itself off due to the blower not starting. If I keep the thermostat Fan Switch in the "ON" position it works fine. Any idea what might be wrong?

Answer 1

yes...you're WAY over due for a new furnace and one night you may be sorry. (ours tried to kill us last Jan. 05 with this tactic)
One night the gas is NOT going to turn off and you may kill everyone in the house. a 23 yr old furnace is death waiting to happen.

Answer 2

In addition to the wall-mounted thermostat, your furnace has two internal thermostats. The contacts on one thermostat control the blower, and will close when the heat exchanger has reached a preset temperature after the burner has come on. The contacts on the other thermostat are in the circuit that controls the burner, and will open to turn off the burner if the heat exchanger has overheated. These two thermostats may be separate, or in the same unit.

It is the first thermostat that is your problem, in that the contacts are not closing or may be burned. We had the same problem some years ago, before I replaced our furnace with a higher efficiency model. After a little bit of troubleshooting with a meter, I found this thermostat whose contacts never would close. It was a rather simple part to replace, and it solved the problem.

I bought the replacement part from Johnstone Supply. Hopefully you can find a store near you from their web site below. Take your old thermostat (I think it may be called a fan limit control) into the store to make it easier for them to find a replacement. As I recall, the temperature settings were a little bit different for the replacement I bought, but it didn't seem to matter.

Answer 3

If you had the same problem before you changed the thermostat, you probably have a problem in the electronics of the furnace, A burner should come on, and when it has heated the heat exchanger, a temperature sensor should activate the blower fan. The reason it heats up first is so not to blow cool air throught the house, it also keeps the blower going after the burner shuts off for a while to keep warm air moving and not warp the heat exchanger.

You need a Furnace repair guy to look at your furnace, Also,Get him to check it for Carbon Monoxide emissions...This is extremely important. also 25 years is all you can really expect out of a furnaces life.

Answer 4

Furnace Blower

Answer 5

You got a lot of good advice. It could be a couple different things. Your best bet is to have a technician to look at it and confirm the exact problem. I would suggest a new furnace, only because of the age of the unit and the repair will probably cost you between 300.00-400.00 dollars. Or it could be something small. It will probaly be cheaper in the long run if you replace the furnace, verses the continued repair cost you will be making on it.

Make sure you properly maintain your unit. Get it check out twice a year and change your filters monthly. Get a preventative maintenance agreement on your systems. I recommend this to everyone for your safety and care of your equipment. This is where a lot of people go wrong, they only have there system looked at when they have a problem.

Answer 6

Most furnaces have multi speed fans in them. Normally the low speed winding is connected to run when the t'stat calls for heat with the fan switch in the auto position. When the fan switch is placed in the on position the higher speed winding is being used. We use the different speeds because we need to move different volumes of air for heating and for cooling. It may or may not be the t'stat at fault here, but I suspect not. The fan is energized by the fan relay. The low speed winding is wired through a set of normally closed contacts on the relay and through a temperature sensitive switch. The higher speed is wired through a set of normally open contacts. In the heat mode, When there is a call for heat the gas valve is energized and the burner lights, the heat exchanger is allowed to warm up, the temperature sensitive switch closes and turns on the fan on the slow winding. If you turn the fan switch on the t'stat to "on" you energize the fan relay and the normally closed contacts open and the normally open contacts close. Now the fan's higher speed winding is being energized instead of the low speed winding. You may have a bad winding on the motor, a bad run capacitor, a fauty fan relay, or a faulty t'stat. Unless you know how to check these things out, you will need a professional. It shouldn't take more than a few minutes for him/her to tell you just what the problem is and what it will take to fix it.

Answer 7

The blower is turned on by a thermostatic switch mounted on the furnace wall. It is simply called a fan switch. replace it. about 10.00 - 15.00 at a retail parts store

Answer 8

when u replaced ur thermostat, did u hook the wires up right? i am assuming on a furnace that old it is only 2 wires. but to me, it sounds like u need a new blower motor for it. at which point you might as well replace it given its age.

Answer 9

You have a bad blower control relay board it may or may not be part of the ignitor control panel. Plan on replacement parts to exceed $100.00

Answer 10

you have a temperature switch that tells the fan that the heat exchanger is hot enough, so turn on.
that's not working, so the fan does not know when to go on.
dangerous condition, call a HVAC tech for service to replace this part.