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Matthew, HVAC Technician
Category: HVAC
Satisfied Customers: 2014
Experience:  14 years experience in the HVAC field of service, installation, Manual J & D calculations
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Furnace running but blowing cold air. What to do?

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My furnace keeps kicking on and off and running constantly but it's blowing cool air. What could be the problem?

I need you to go down to the furnace with the thermostat set for the heat to come on and turn the switch to the furnace off then back on. Tell me what sequence happens and when it kicks the burners off or if it even lights them.
Customer: replied 7 years ago.
The motor kicked on and then about a minute later the burners kicked up and then another minute or so later the blower kicked on.
everything lights like normal and then sometime before the heat call is satisfied it kicks everything off or just the burners off then back on? We need to catch it when the burners drop out and see what the failure code is.
Customer: replied 7 years ago.
The burners were kicking off and back on but the blowers would continue to run. I had just changed the filter right before I posted my initial question, and since then the furnace has run full cycles like normal and the house has heated up. Could a dirty filter have caused it to short cycle or is there something else going on? This only started yesterday and happens more at night when the temperature dips very low outside.
a dirty filter very well could be the problem. Furnaces have what is called a high limit sensor safety device built into them that monitor the heat build up in the cabinet. If this heat is not pushed out of the furnace and kept moving then it will trip this limit cut the burners off and then re-lite them when the blower has pushed the built up heat out of the furnace and gets the sensor temperature back down to a safe operating range. With a dirty filter it is starving for return air and can not push the air out of the furnace fast enough to meet this safety requirement. The blink code would blink high limit open code while the burners were off then go back to normal when the limit reset itself. This would be more prevalent at night because the furnace is running for longer periods of time and building up more and more heat.
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