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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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7970 C856 fan cycles when burner is firing. A service tech

Customer Question

Coleman 7970 C856 fan cycles when burner is firing. A service tech replaced the limit switch. Worked for a short time and started to cycle again. Tech said the filters were dirty and the fan could not pull enough to operate the limit switch properly. Filters were not bad but changed anyway. The blower works properly with covers removed. Install covers and the fan starts cycling again. Furnace is on propane and is in a mobile home. What could be causing the problem. It does have the ac coils below the burner.
JA: Did you check the furnace's filters?
Customer: Read the post. Replaced with new.
JA: Do you plan on doing the work yourself?
Customer: Yes
JA: Anything else we should know to help you best?
Customer: No
Submitted: 3 months ago.
Category: HVAC
Expert:  airheatman replied 3 months ago.

Do you see any other numbers on furnace like DGA077 or DGAA?

Customer: replied 3 months ago.
No. Installed in 1993 rated as 70000 btu
Expert:  airheatman replied 3 months ago.

Has he determined the limit is cutting burners off and keeping fan running? Or are you saying burners jeep going but blower is cycling on and off?

Customer: replied 3 months ago.
Burner fires correctly. The blower cycles. Installing the top cover and everything works as it should. Both covers installed and the blower starts cycling again
Expert:  airheatman replied 3 months ago.

I can't find a manual on this one.I am going to open question to see if one of my colleagues may

Expert:  BillyHvac replied 3 months ago.


this appears backwards.

replacing the limit would be if the burners were cycling off and on and the blower continued running. (blower that blows air through registers not out chimney).

Also putting doors off and on would affect the limit not the blower...

are you sure the burners are not what keeps cycling and the main blower for registers continues to run?

Customer: replied 3 months ago.
The blower cycles during burner fire. Not sure if there two switches bit the one replaced was the lower limit switch.
Expert:  BillyHvac replied 3 months ago.

thats our toe stumper.....limits do only one thing...turn the burner off.

fan switches cycle blower on/off. But limits are not related to blower at all

Customer: replied 3 months ago.
The last time I talked to the tech he recommended dumping water on the ac coils because they were probably dirty and therefore the blower cannot pull enough air across the switch to make it operate correctly.
Customer: replied 3 months ago.
Actually the receipt says the fan switch was replaced
Expert:  BillyHvac replied 3 months ago.

you see that would be a limit issue.

If the main circulating fan shuts off then the problem is fan/blower related.

To me it sounds like your burner is turning off by overheat...this would be a limit issue.

Which a plugged ac coil is very common....can take panels off to the ac coil?