Fan limit switch shuts down the furnace: Q&A

Avoid damage that may make the heater unsafe

Ducane furnace shuts down at high heat limit after 5 minutes - I jumped the switch - maybe damaged something?

My Ducane furnace is shutting down after 5 minutes of operation on high heat limit. Jumped out the switch and it ran longer.

Didn't let it run to long though it may damage something On 2016-10-26 by David

by (mod)

Watch out: You're right, David: if the system is overheating it can crack the heat exchanger, risking fatal carbon monoxide poisoning.

And jumpering the switch risks setting your building on fire.

And yes you may have damaged the switch makint it unsafe.

 

Bryant air conditioner, Goodman furnace - I'm getting sick - SAFETY WARNING

I'm on rent my landlord has the fan blowing in the summer. The air conditioner doesnt work. Im getting dust and the smell of gas in the house and heat through thr cold air vents as well as vents heat normally comes through.

The air unit is a Bryant and the furnace is a goodman.

Something is terribliy wrong here cause im getting nauseated, my throat swells closed and i have to seek medical attn. Im afraid i wont wake one day because of the way the furnace is set up. On 2016-07-07 by Nancy Pierce -

by (mod)

Nancy this is an issue to review first with your landlord, your rental contract, and with the landlord's legal obligations - that may vary depending on where you live.

Watch out: If you believe there is an immediate life/safety issue you can and should get out of the building and call your emergency services.

 

Furnace overheats, stops before thermostat is satisfied

My furnace overheats and stops without reaching the desired temperature as set-up in the thermostat.

Technician for the company that I have a maintenance agreement with said that the cause is the plenum being undersized.

I can't believe his diagnosis. Should I get a second opinion or could you tell me the real cause of the problem? Thanks. (Jan 23, 2014) Rusty Agus

Reply: undersized plenum might cause furnace overheating

It's a fair question and the explanation is definitely possible, Rusty.

BUT if the system is not new, if it was not behaving this way before, then one would not expect the problem to be plenum size.

Also here's a crude rule of thumb tip: compare the cross section of both the return air plenum and supply air plenum to the cross section of the furnace heat exchanger. If they match then that's good.

Finally, I would look for some other clues that would explain the problem, such as

- reduced air flow due to a dirty air filter

- reduced air flow due to dirt on the blades of the squirrel cage fan in the air handler

- reduced air flow due to squashed, disconnected, or damaged ductwork

Will the fan limit switch prevent the burner from igniting?

Will the limit switch on the blower unit keep the furnace burner from lighting, or cause the burner to light and go back out? - On 2017-12-13 by Jim

by (mod) - Possibly: The limit switch on the blower should indeed turn the heating burner off if the temperature exceeds the upper or safe limit.

The limit switch on the blower should indeed turn the heating burner off if the temperature exceeds the upper or safe limit.

But if the furnace is not in a very hot or overheated condition, the fan limit control should not prevent the burner from igniting. So most likely your problem is elsewhere.

If your furnace uses an oil burner see the diagnosis and repair procedure at OIL BURNER WON'T RUN 

If yours is a gas furnace, see the diagnostic procedure at GAS FURNACE WON'T RUN

Can I override the furnace fan limit control switch?

Can you over ride a combination bower limiter switch On 2016-01-15 by Dee from michigan

by (mod) - NO that is very dangerous

Can, yes in that it's physically possible. Should you ever override a fan limit swithch?

Watch out: the answer is No, subverting this safety control can destroy a furnace or can kill building occupants.

Furthermore, fussing with the switch may bend its temperature sensing spring, making it inccurate and also equally dangerous.

My heater won't run unless I put the FAN ON button

So I have a wall heater and I would have to push they button on the limit switch to make the heater turn on so I've replaced the limit switch and it's still doing the same thing

I have cleaned it all out as well cause I thought it would be clogged causing this problem but no still doing the same thing I turn the heater to auto nothing I turn the heater on nothing On 2018-01-14 by Oscar

by (mod) - When you push the white button on your fan limit switch you are simply turning on the blower

Oscar

When you push the white button on your fan limit switch you are simply turning on the blower or air handler, not the furnace itself.

There is another problem, most likely with the burner that is not operating. I would check the obvious things like fuel supply and if that looks ok, I'd call for repair.

Gas Furnace pilot light stays on but the burner will not kick on - is it the fan limit switch?

My pilot light stays on but the burner will not kick on.

Will the fan limit control switch prevent this from occurring or could it be the gas valve? The furnace is 37 years old. Has been off for 6 months. When I turned it on it ran good for 2 days and then nothing. On 2019-02-11 by Steve V. -

by (mod) - Role of the Fan Limit Switch in furnace operation

Thanks for a great question Steve - really I'm not being patronizing. You're not the first to be confused about the relation of fan limit switch to burner operation.

No, the fan limit switch won't keep the burner from turning on in the first place at the start of a call for heat.

Role of the Fan Limit Switch:

Here's the sequence for a gas fired forced air heating furnace:

Thermostat calls for heat

Pilot is lit or on some systems an igniter operates to ignite the flame

The gas valve opens if the pilot is lit AND the thermocouple on a pilot type system signals to the gas valve that the pilot is on)
[Aside: so you can see that if the pilot is off or there is a bad thermocouple the gas valve won't open, or on a pilotless system if the igniter doesn't ignite a pilot flame the gas valve still won't open]

The gas valve opens and the burner ignites

The air in the supply plenum above the burner gets up to temperature - say 100F.

The limit control monitors the plenum temperature and turns on the furnace blower fan at - say 100F.

The burner and fan continue to operate until the call for heat is satisfied at the thermostat.

The thermostat turns off the call for heat

The burner shuts down

The blower runs another minute or so to extract remaining heat.

As an added safety feature, the Fan Limit Switch WILL turn off the burner - close the gas valve - IF the temperature in the supply air plenum gets TOO HOT - say close to 200F.

That's to detect an unsafe condition in which perhaps the blower is not working (or air flow is blocked) and unsafe temperatures are being reached in the supply plenum (risking cracks, damage, and ultimately fatal carbon monoxide leaks into house air).

Summarizing:

The Fan Limit Switch will

1. keep the blower off until the supply plenum reaches the FAN ON temp

2. keep the blower on at the end of the call for heat to cool down the heat exchanger

3. turn the burner OFF if temperature in the supply plenum reaches an unsafe - too high - temperature

What to check if the gas valve won't open on the furnace

Therefore after all that excitement and hopping about the furnace room, I add that if your gas valve won't open now and the burners don't run

- check for fuel supply

- check for a bad thermocouple

- check for a mis-adjusted pilot that doesn't have the thermocouple in the flame

- check for spider webs or debris in the pilot assembly

the gas valve is my last candidate for replacement but it too could be the culprit