Gas furnace not blowing warm air at lower thermostat settings

Hello,

We have two gas furnaces, one in the attic for the upstairs, and one outside for the downstairs. We found that when we set the upstairs one to 69 degrees, when the fan turns on, it blows cold air. The higher we set the thermostat, the warmer the air.

I had the belief that the thermostat does not control the air temperature, but instead just controls when the warm air is turned on. We would like to not set our thermostat at 75 degrees and waste money, but would like to keep it at 69. Is this a case where it's just slower to heat at the lower temp, and before it can heat fully, it stops blowing?

We had an HVAC guy come to our house and all he did was clean the sensor rod, which has apparently not made any difference. We would like to not have to pay again to get this fixed correctly.

Thanks in advance!

 

No, the thermostat doesn't turn on the fan.


You should start by taking the front cover off the furnace and carefully observing the sequence of events that happens when the thermostat is turned up.

Report what you find along with the make and model of the furnace from the rating plate in the burner compartment of the furnace.

 

Right, I knew the thermostat doesn't turn on the fan itself, but when you increase the thermostat setting, it causes the sensor to realize that it needs to turn the fan on to bring up the temp to the desired level.

The main question I have is why I can't get good heat when I am at a lower thermostat setting. When I set it over 72, I start to get warm air.

 

Most furnaces wont turn the fan on unless the burners have been firing.


If you want to find out the reason for your problem, follow the directions you were given.

Guessing doesn't cut it as a diagnostic method.

 

Simple explanation of a regular thermostat is a switch that turns on and off at preset temps. It tells the system that the temp is out of range of it's settings (it's too cold or hot...or just right). Everything else is determined by the system. Thats why the questions were posed.

 

Yes.

The thermostat does not cause the fire in the furnace to burn bigger and hotter the more you raise the temperature, based on some kind of mechanical correlation between what the setting is and the fire temperature, in say the way the burners do on a gas stove.

BUT - You might have a two-stage gas valve that kicks out more fire when the set temp is a lot greater than the existing set point on the stat. The reason is say that you left your thermsotat alone at say 69 degrees. The fire would burn less intense, with less gas, but for longer, to cut down on uncomfortable oscillating temperatures at a more frequent rate. But when it either gets real cold out or you really want to ramp up the house temperature quickly, the first stage output can't handle that.