Help! Furnace won't ignite without fan on. - http://www.hvacmechanic.com/ Forums
Original message
| Ixnay888
 |  "Help! Furnace won't ignite without fan on."
, posted Tue 21 Oct 17:01  
My furnace runs the AC perfectly, however when the heat tries to kick on it makes lots of clicks and you can smell "plastic". However, if I'm running the fan and then the heat turns on, it ignites right away.
I don't really want to be running my fan 24 hours a day, so I'd like to know what is wrong, I'm thinking its a heater fan or something on the furnace not turning on. Any ideas how to fix this?
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Replies:
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| techshawn
| "Re(1):Help! Furnace won't ignite without fan " , posted Tue 21 Oct 18:27
Based on what you described, I still can hardly understand what's going on there. Please show your furnace's year, make,model number and make a clearer statement about the trouble.
I like fixing everything.
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| | Ixnay888
| "Re(2):Help! Furnace won't ignite without fan" , posted Wed 22 Oct 11:28
The thermostat has two separate settings, heat/off/cool, and another on/auto for the fan.
When the AC kicks on (COOL-AUTO), everything works fine, cold air through the whole house.
When the heat kicks on (HEAT-AUTO), you can hear a lot of clicking and the furnace doesn't ignite, no heat through the house. Like I said you can kinda smell like a plasticy smell.
When the heat kicks on with the fan running (HEAT-ON), everything works fine, the furnace ignites fine and heat through the house.
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| | Ixnay888
| "Re(3):Help! Furnace won't ignite without fan" , posted Wed 22 Oct 11:30
Hopefully you can understand the problem from that. I believe its a 1991 furnace install. I'll have the make and model when I get home from work.
Thanks for the help!
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| | Freon
| "Re(4):Help! Furnace won't ignite without fan" , posted Wed 22 Oct 16:08
I assume you meant to say when you set the thermostat to FAN-ON and then call for heat, everything works.
The clicking you hear when the heat does not work most likely is the gas valve open and closing because the flame sensor is trying and not seeing flame. If I am correct, then what is troublesome is why does it work when the fan is on. Possibly, and I emphasize possibly. there is a small crack in the heat exchanger that allows air to affect the flame and thereby the flame sensing. I would watch the main burner flames carefully for any signs of something moving part of the flame erratically. The main burner flames should all be consistent without any signs of a 'wind' blowing them.
Another possibility is the fan running affects the voltage just enough to allow the flame sensing circuit to work. I would definitely find the flame sensing probe and clean it with emery cloth and check all electrical connections. If things work, then the heat exchanger crack is less likely. Again, this is all speculation.
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| | Ixnay888
| "Re(5):Help! Furnace won't ignite without fan" , posted Thu 23 Oct 09:37
The way I described the thermostat is correct, sorry for not being clear enough!
There are two setting controls on the thermostat, each with a selection to choose: #1 HEAT or OFF or COOL and #2 OFF or AUTO. Normally when you are supposed to run the heat in your house, you have #1 set to HEAT and #2 AUTO. When the Heat kicks on, the Fan automatically kicks on.
When my heat tries to kick on with this setting I can see the pilot light but it just keeps clicking, about every 2 seconds. The furnace does not ignite at all. Because it doesn't ignite, the fan won't kick.
However, if I have the controls set to #1 HEAT #2 ON, meaning the fan is constantly running, and the heat kicks on when the house is below wanted temperature - the furnace clicks a few times, but is able to ignite after a few moments. The fan running seems to help the furnace ignite.
Because it's a 1991 install (I don't have the part/model yet) I'm assuming a furnace fan or something is no longer working, but I don't know what...
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