Ok this one has me puzzled. Been having this issue on and off all this winter, and happened a couple of times last winter too.

It'll suddenly be cold in the house, i'll go to the thermostat and turn up the target temperature, however, it doesn't click. So then turn it way down and i'll hear the click somewhere in the low 60's, somehow as the temperature in the house has dropped, the thermostat has been triggered, but the furnace doesn't start.

So now that I have the dial on the thermostat way low, i'll turn it back up to the early 70's hearing the click, and it still won't activate. I can do this as many times as I like, back and forth, it won't activate the furnace, but here's the strange part...

It will trigger the furnace to turn on only if I move the temperature arm very slowly.

I can't think of an explanation for this so I'm hoping someone can explain why this is and how I can fix it?

Details:

Changed the thermostat, didn't help. Both thermostats were mechanical, just basic $20 ones.

Our furnace is a coleman, it's a gas furnace which is also an AC, central air system.

The entire system is probably close to 14 years old now, no problems until now.

Never had this issue with the AC, never had any kind of issue with the entire system for all the years I've lived here.

Thanks!

 

What's the model number on the furnace. If you move the stat switch to ON does the blower start?

 

If you jump R and W at the stat will it run?
Geo

 

Coleman furnaces have their own distinct problems.
Usually the FAN ON control is at the furnace along with the HEAT/COOL selector.

We may need some pictures of the inside of your furnace to identify it.
http://www.doityourself.com/forum/el...-pictures.html

 

The Fan on/auto is at the thermostat and works fine, although I virtually never have the fan set to on. Not even sure why it's even an option to be honest.

I've never actually tried turning the fan on when it's in the problematic state I shall try that next time I have an issue. But I can turn the fan on or off with no issues.

The heating/cooling selector is also at the thermostat.

As for Jumping R + W, you want me to try that when it's in it's problematic state? If so how would I go about doing that safely?

I will get the model number for you guys asap.

Thanks!

 

Also provide the manufacturer and model of the gas valve controller as it may have been changed out being the furnace is 14 years old. The controller provides the sequence for lighting the furnace when the thermostat calls for heat.

 

If you can find the thermostat wires at the furnace just take a short piece of wire,with the ends stripped and touch the Red and White terminals, if the burner fires up then the issue is between there and the stat.
Geo