Fan & Limit Controllers

Fan and limit controls, limit switches and fan timers for forced-air furnaces.

The L4064B fan and limit controller turns the blower on and off by plenum temperature and shuts the burner down on high limit. It is stocked in 5, 8 and 11-1/2 in insertion lengths; match the length to the original so the element sits in the air stream. L4029E limit controls are fixed-setpoint high limits, including manual-reset models that must be reset by hand after they trip.

ST9120U and ST9103A electronic fan timers replace the fan control function on furnaces with electronic ignition, and DiversiTech roll-out switches protect against flame roll-out at the burner.

Related: relays, transformers.

Popular part numbers: Resideo L4064B2228, Resideo L4064B2210, Resideo L4029E1029, Resideo ST9120U1011.

Frequently asked questions

What does a fan limit switch do?

It starts the blower once the plenum air is warm, stops it after the burner shuts off and the air cools down, and shuts the burner off if the plenum overheats.

How do I choose a replacement fan limit control?

Match the model number on the old control, the insertion length (5, 8 or 11-1/2 in on the L4064B) and the limit setting. On L4029E high limits, also match the fixed setpoint and whether it is manual or automatic reset.

What does manual reset mean?

A manual reset limit has to be reset by hand after it trips. If it keeps tripping, the cause is usually airflow (a dirty filter, a blocked return or a weak blower), so have the system checked rather than replacing the limit again.