Short cycling during cool down

nd_alex

I have searched the internet for this problem and have not even seen it described.



I have a Coleman 8665D766 down draft mobile home furnace in my garage. Late last spring, the main blower motor died, and the furnace went through a couple of high-temp shutdowns before I noticed. This is an older furnace that has no electronics in it, and no adjustable controls. It is entirely controlled by temp limit switches.



I have since replaced the main blower and the furnace works again. However, it now short cycles during it's cool-down period:



1. Thermostat calls for heat.

2. Burners ignite, combustion blower starts.

3. Main blower starts after combustion chamber heats up.

4. Thermostat reaches desired temp.

5. Burners terminate, combustion blower stops.

6. Main blower continues for a minute or two.

7. Main blower stops for about 5 seconds.

8. Main blower starts again for just long enough to spin up to full speed (2 seconds?)

9. Steps 7 and 8 continue until combustion chamber is cooled off.



I am guessing that one of the limit switches was damaged during the high-temp shutoff periods last spring. I cannot find a manual for this furnace, but there is a wiring schematic on one of the covers that I can reproduce and post if needed.



Thanks in advance for any clues anyone can offer!

JStar

Furnace

Downflow furnaces are notorious for temperature related problems. Sounds like the heat exchanger is still warm enough to turn the fan on through the limit switch.



It could also be a bad limit. They tend to wear out after being overused in short cycle situations.

Empire_2

Snap disc fan?

If is a snap disc fan control which is most likely on the left side of the blower, Then replace with same or an adjustable.  Most problems are due to the heat left in the HX.  A time controlled fan control would be of benefit.  Time on/off control would alleviate the problem.  Post some pic if you can,.  Throw in the wiring diagram too.





Mike T.