Saturday, November 17, 2018

Furnace Fan short-cycling

This is the fan limit switch at the cabin. Honeywell L4064B. Moving the two lower pointers farther apart should reduce short cycling.

I spent the night before opening-day of firearm season at the cabin.

The furnace was short-cycling.

Today I put in a new thermostat. I did not think that was the cause but it was easy and it needed replacing because it was a mercury switch thermostat.

Honeywell makes a $25 unit that is an exact match for the one that came off the wall.

The fan is still short-cycling.

Potential causes include:
  • Clogged ductwork (think Red Squirrels here, folks)
  • Clogged air filter
  • Not enough ducting (the ducts in 2/3 of the rooms were shut off so this might be a player)
  • Excessively cold return air (don't know where return air ducts are, perhaps in the shutoff rooms that are not heated or the unheated crawl-space beneath the cabin)
  • Defective temperature limit switches
  • Improper limits set on the limit switches. This is a possibility. I will move the Fan Shutoff to the low limit and see if this helps.
  • Holes rusted in ductwork and/or drain features plugged. (Suggested by an email friend who often chooses to be anonymous.)
Source of troubleshooting punchlist 

I need to find a manual for this unit so I can figure out where the air filter is.
Some people wouldn't bother to get the furnace straightened out, but the cabin has been a source of emergency housing when assorted family members have found themselves without housing.


  1. My vote is on the red squirrels.

  2. My vote would be to install a self-regulating wood stove.

  3. Mine short-cycles when the flame sensor gets sooty from the flame it is constantly exposed to. About twice a season I have to take it out and clean it with steel wool (not sandpaper).

    Don't know if yours has one of those sensors, but thought I would add to your checklist.

  4. I agree with Scott, that's the first thing I check when it is short-cycling. The next thing I check is a clog in the filter or the duct work. The exhaust on my high-efficiency unit seems to be a haven for spiders. The webs create just enough blockage to trip some sensor and cause the short-cycling.


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