Furnace won' maintain temperature

I have a six year old gas furnace and the old style round "dial" thermostat. I intend to keep the house at 68 - 69 degrees.

The system works fine if the outdoor temp is above approx. 30-35 degrees. If the outdoor temp falls below that, the indoor temp goes to approx. 66 degrees.

Another symptom: when I go away for the weekend, I set the stat down approx. 4 degrees. When I return and set the stat back to normal (68 - 69), the furnace will not run until the desired temp is reached, regardless of the outdoor temp. I have to overcompensate by setting the stat up to 74 or so. After about two days, the indoor temp will finally match the stat.

Apparently, I don't know enough about such systems, but it seems to me that the furnace should run until the stat is satisfied, regardless of the outdoor temp. Why isn't it doing that and how to fix?

 

Probably either the furnace is overheating on the limit switch or the heat anticipator of the thermostat is improperly set.

Either of these conditions will shut off the burners and prevent the long furnace run needed to maintain the temperature when it's cold out.

To check to see if it's the heat anticipator, take the cover off the thermostat. I'm supposing you will see the mercury capsule that switches the furnace on and off, and a pointer that can be set at various settings.

Turn the thermostat up and down until you understand when the furnace is being turned on.

Then set the thermostat for five degrees above the room temperature. See if the thermostat allows the long furnace run needed to increase the temperature or whether the furnace is shut off by the thermostat before the higher temperature is reached. Note --- you need to verify that IT'S THE THERMOSTAT that's shutting off the furnace.

If something else is shutting off the thermostat, it's probably the limit switch in the furnace.

 

Since you mentioned it takes a couple days to get to set temp, to me it sounds more like either the fire is being interupted, or it is puny gas flow.

Do you have propane or natural gas?

Have you tried to take the big cover off the furnace to observe what the burners are doing when you turn the stat up 5 degrees? Like, does it keep burning and burning and burning endlessly? Or does it have burner fire for only about 2 - 3 minutes, and then go out for a while. Then relight and the same thing, over and over and over? Then yes on possibilty of high limiting - and you can test for that.

Have you checked your furnace filter?,as a dirty one usually causes high limiting.