Furnace fires up but no fan/heat

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Jsmith2022

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I have a gas furnace (forced hot air) that was working fine a week ago. This week I noticed it wasn't blowing any hot air, in fact no air at all when the heat is on. I can hear the furnace fire up and run when I switch the thermostat to "heat on" but no warm air comes out. I thought the fan was not working - if the thermostat fan switch is moved to "on" from "auto", still no warm air. To be sure I turned the furnace off via the HVAC switch and turned the fan on, to my surprise the fan was working. I changed the batteries on the programmable Honeywell thermostat, then finally replaced it with a new Honeywell programmable. Same symptoms, furnace on but no warm air. Fan turns on when heat is in off position. I am hoping this set of symptoms will identify a failing part in the furnace. The unit is a Carrier Crusader, gas from the 90's, heat only. I cannot find a model number markings. Any help would be appreciated!
 
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Breplum

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You have established that the fan is good.
The heating sequence is that flame rod needs to detect flame on fire-up. That is a pick and hold function.
You need to watch the fire-up sequence.
Definitely pull the flame rod assy and use emery cloth to thoroughly clean it.
 

fatima mehru

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It seems like the furnace is igniting, but there's an issue with the heat distribution. Given that the fan works independently of the heat, it's likely not a problem with the fan itself. You might want to check the limit switch, which could be preventing the blower from turning on. Additionally, inspect the thermostat wiring and ensure it's correctly configured.

Check the air filter, as a clogged filter can impede airflow. If these checks don't resolve the issue, it could be a malfunction in the furnace's control board or another internal component. Considering the furnace's age, seeking professional HVAC assistance is advisable for a thorough inspection and diagnosis.
 

Fitter30

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Some use a independent fan switch, some a timer ,some a timer on the main board. Brand, model and pic of the wiring diagram on the inside of blower door. Furnace blowers with ac thermostat fan on is high speed, heating is medium or low. Running on high isn't a problem in heat just cold draft with burner off.
 
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