Guide
Furnace running but no heat
If the blower runs and the air stays cool, the problem is almost always in the few seconds when the furnace tries to light.
First, the fan setting
A thermostat with the fan set to On runs the blower nonstop, including between heating cycles, and the air feels cool during those gaps. Set it to Auto and give the furnace a few minutes before going further.
How a gas furnace lights
A failure at any step stops the sequence. The blink code on the board usually says which one.
The thermostat asks for heat
A low-voltage signal reaches the control board.
The inducer starts
A small fan clears the heat exchanger and sets up draft in the vent.
The pressure switch closes
It proves the vent is drawing before anything else happens.
The igniter heats or sparks
A glowing element or a spark readies the burners.
Gas flows and the burners light
The valve opens for a few seconds of trial.
The flame sensor proves the flame
No signal, and the valve closes again.
The blower starts
After a short delay, warm air reaches the registers.
Usual suspects
Burners that light and go out within seconds point to a dirty flame sensor. A furnace that runs its inducer and never lights points to the igniter or the pressure switch. On a condensing furnace, a frozen or plugged condensate line and snow over the side-wall pipes both stop the sequence at the pressure switch.
If burners light, run a while and then shut off as the blower keeps going, the high-limit switch may be tripping from poor airflow. Check the filter first.
When the stove and other gas appliances are out as well, the gas supply is the issue, and Xcel Energy is the call.
Heat in some rooms but not others
Closed registers, a duct that has slipped apart in a crawlspace, or a damper left shut after summer can starve one end of the house. The furnace itself may be fine.
Write this down for the call
- The blink code and how many flashes.
- Model and serial number from the rating plate.
- Whether the burners light at all.
- Any smell, or a new sound.
- When the furnace was last serviced.
Cold house? Make the call.
Describe the symptoms and give your street. A heating and cooling business that covers the Fort Collins area, independent of this website, handles everything after that.