Furnace problem -

Turn temp to 70
Furnace runs until 70 degrees
Maintains temperature for awhile - probably for a couple of hours
At some point it stops maintaining the temperature and keeps kicking on and off.
I've watched the thermostat - I'll actually turn the temp up to try to get it to kick in. The thermostat "flame" comes on, I'll hear it ignite, the furnace starts up and then runs for about 15 secs then shuts off. Thermostat calls for heat, igniter kicks in, furnace starts and runs for 15 secs then shuts off. So the Temperature isn't maintained.

If I shut the heat off on the thermostat, wait 10 seconds, then turn the heat on again - the whole cycle will start over again.

When I get home from work it is like 58 degrees. I shut the heat off, then on, then the furnace will stay on. I go to bed and wake up in the middle of the night and the temperature is in the low 60's.

A friend of mine does heating/air for a job but he usually works on commercial buildings. He has come over and tried a few different things but it keeps doing the same thing.

I read another answer to someone in the q&a section about the flame sensor being dirty and I'll mention that to him but I'm sure he has probably checked that. At one point he thought it might be a thermostat problem - so I'm going to buy a new one.

Is there anything else that can cause these symptoms?

I noticed a time delay fuse next to the on/off switch on the furnace - could that have anything to do with what is going on?

It is -8 in Minnesota right now and I just want to make sure my furnace stays on this week-end. My friend is going to come over tomorrow and take a look at it again.

Thanks,
Chris

Oh - the brand of the furnace is Amana