Fan & Limit Control Switch FAQs

Index to diagnostic Q&A on furnace heater fan limit switch controls

Fan Limit Switch Cycles Blower On / Off Rapidly or Intermittently FAQs

Tip: See the diagnosis and repair advice at FURNACE FAN CYCLES DURING HEAT 

Limit fan switch keeps turning on and off

My limit fan switch run an goes off in the furnance I think it's a bad limit fan switch it comes on an goes off (Jan 4, 2016) Donnell

Reply:

Donnell,

In FAN LIMIT SWITCH TROUBLESHOOTING (where this question was first posted) you'll find two related articles you should review

• FURNACE FAN CYCLES AFTER HEAT
• FURNACE FAN CYCLES DURING HEAT

Goodman Janitrol 1992 gas furnace cycles on and off rapidly

Gas burners cycles on 10 seconds...off for 20 seconds repeatly.

The blower has stayed on with out shutting off because it never reaches the t-stat set temperture.

Is it the limit or gas valve.Also,can the pilot light on this furnace be converted to a electronic ignition. It is a goodman-janitrol made in 1992 (Jan 13, 2013) JohnBoy

Reply:

Watch out: Very rapid on-off cycling of your gas burner sounds like a control problem and an unsafe heating system.

Shut it off while waiting for the service tech.

The problem maybe trivial - such a loose wire, but repeated rapid on-off cycling can damage the equipment.

My furnace blower is short cycling on and off

My blower is short cycling with the burner staying on. When I watch the fan limit switch (Honeywell Disc type), it rotates up as the burner heats up the supply plenum (is that what it is called?) and the fan kicks on (on setting is at ~150).

As the fan starts moving air, the switch quickly drops below 50F and the fan turns off.

The box is hotter than 100F at this time. If I put the fan in manual, I get warm air out of the ducts but the limit switch remains below 50. Is there a way to calibrate the switch? Even at the lowest off setting, it cycles off. I have ordered a new switch but I am looking for a way to keep the house warm until it arrives.

I am reluctant to leave the house with the fan on Manual and the burner going. (Jan 20, 2016) Paul F said:

Reply:

Paul

I would watch the limit switch - it may be that the plenum is overheating - perhaps due to low air flow rate perhaps due to a dirty blower or dirty air filter or inadequate return air

My furnace blower fan is not turning on and off at proper times, erratic blower fan operation on heating furnace

Hello, the problem I'm having is ,I changed my thermostat yesterday thinking it was my problem.

When I activated the new stat and called for heat the gas valve activated and the heat turned on ,but the blower fan did not , eventually the fan did start maybe 60-90 seconds after ignition, the unit ran for 5-10 mins then shut off fan continued to run then it eventually shut off.

I waited about 15-20 mins knowing that my stat was still calling for heat and not satisfied but the unit did not turn back on I appreciate any help. 2016/03/24 Cory

Reply:

Cory,

As the thermostat successfully called for heat, it's not likely to be the problem.

It is normal for the blower fan to not start until the plenum above the heat exchanger at the furnace is sufficiently warm - that's to avoid blowing cool air on the building occupants.

Watch the fan limit switch as the burner runs: you should see the disk turning until it reaches the FAN ON setting. On some systems that might take a minute.

It is also normal for the fan to keep running a bit after the furnace turns off, extracting the remaining heat and avoiding damage to the heat exchanger (from overheating).

But if the blower and furnace are turning off before the thermostat is satisfied, that is the room temp is below the set temp on the thermostat, then something else is wrong.

The most common cause, I suspect, is a furnace that is overheating, perhaps because of a dirty air filter, dirty blower fan, or crimped air duct or inadequate supply air. But that wouldn't necessary cause inconsistent or "erratic" blower on-off operation. For that I'd look for a failing electrical component: a control, relay, door switch, even wire.

Check those out and keep us posted.

Reader follow-up: ruling out a dirty air filter as a a cause for blower fan cycling off and then back on

Cory said:
Thanks for the fast reply,Today I was working on the problem a little more,I checked the air filter and noticed that it was dirty so I removed it completely for the sake of getting the unit to run properly I will be replacing , I assumed the unit was cutting out on limit because of the lack of air flow the unit.

But my trouble continued unfortunately. I called for heat and the unit operated properly the burner started first followed by the blower ,the unit ran properly for 10-15 mins so I lowered the stat to make sure the unit cut out which it did , I waited several more Minutes to call for heat again and unfortunately it wouldn't come back on.

I will take your advice and look for damaged air duct or blocked blower fan to see if this is hurting my situation .Thanks again for your help

Reply:

Cory from the sequence you described, if a furnace starts normally and runs 10-15 minutes normally, then won't re-start, most likely the problem is not in the air duct system.

If this is oil heat I suspect the burner needs cleaning, service, adjustment.

Reader follow-up: gas furnace blower fan erratic behavior traced to loose wire

Cory said:
The system is gas ,the system is actually very simple it consists of gas valve 24 transformer combination switch and blower motor.

Ok so I went over everything again and I ended up finding a faulty connection with my wire going to my gas valve,it is a push in and lock stlye connection so to the naked eye it looked like it was in correctly ,you live you learn I guess thanks for all the help it is appreciated.