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Fan limit control switch interior components (C) InspectApedia.com KenFan & Limit Switch FAQs 6
Q&A Furnace fan limit control Diagnosis & Repair

Furnace fan limit safety control Q&A set #5:

This article series explains how to install, wire, test, set or repair the fan limit control switch used on heating furnaces - the same blower that may be used for both heating and cooling.

Page top illustration: the interior controls and standard settings for a typical furnace or forced-warm-air fan imit control switch.

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HVAC Fan Limit Control FAQs set #5

Trane XR80 Furnace Wiring Diagram at InspectApedia.comThese questions and answers were posted originally at FAN LIMIT SWITCH - topic home, so you want to start reading there.

Or check out FAN LIMIT SWITCH TROUBLESHOOTING.

Photo: the interior of a typical fan limit control switch found in the air handler of furnaces - forced warm air heating systems, discussed by Ken Martin's question below

[Click to enlarge any image]

On 2019-03-06 by (mod) - air blower gets stuck and continues to run without stopping

Mark

In the ARTICLE INDEX above check out the article on BLOWER WON"T STOP to see the common causes and cures.

On 2019-03-06 by Mark

Lately I get times when the furnace runs it's full cycle with no problems and then after 4-5 normal cycles, the air blower gets stuck and continues to run without stopping even after the burners are off.

I've tried resetting the thermostat, replacing batteries, tapping the limit switch, adjusting the fan control, and/or turning the power off and back on, all with limited success. Any idea what might be causing these inconsistencies?

On 2019-03-05 by (mod) - furnace is overheating and shutting off

It sounds to me as if there is either been wiring damage or damage to the switch itself. The fan limit should prevent overheating and overheat damage to the furnace by shutting the system down if the plenum chamber is good too high.

Assuming that you have the correct temperature settings on the fan limit switch that should work.

There are of course other reasons that the system would overheat such as a dirty air filter or blower or reduced air flow through the heat exchanger for any reason. More examples are found at

FURNACE FAN STOPS EARLY

On 2019-03-04 by Wayne

Around the first of January the oil furnace overheated because the burner was running but the blower didn't come on.

I replaced the limit switch with the same Honeywell L4064 and it worked fine for two months.

Today it overheated again. I shut off the furnace at the thermostat and turned down the setting below room temperature and turned the furnace switch back on and

after the furnace had cooled down a bit the blower came on and short cycled a few times and shut off when it was cool. I turned the thermostat back up and the burner lit right up and a few minutes later the blower did just as it should.

I did not touch the limit switch at all. Should I replace the limit switch again or is there possibly some other problem. thanks

On 2019-03-02 by (mod) - What causes the unit to shut off before the thermostat setting ?

Anon

A thorough answer to your question about why the fan stops early when the thermostat is still calling for heat can be read at

FURNACE FAN STOPS EARLY

On 2019-02-28 by Anonymous

What causes the unit to shut off before the thermostat setting

On 2019-02-28 by (mod) - What causes the unit to shut off before the thermostat setting ?

Anon

A thorough answer to your question about why the fan stops early when the thermostat is still calling for heat can be read at

FURNACE FAN STOPS EARLY

On 2019-02-28 by Anonymous

What causes the unit to shut off before the thermostat setting

On 2019-02-24 by (mod) - How do you set the limit control buttons

Ricky

What is the brand and model of limit control?

Most limit controllers have a dial and slider settings, not buttons

On 2019-02-24 by Ricky

How do you set the limit control buttons on a limit switch and what position do you set them?

On 2019-02-13 by Anonymous

Unfortunately I'll have to send it tomorrow.

On 2019-02-13 by (mod) -

Whats the brand and model?

Can i see a photo of the installation?

On 2019-02-13 by Steve V.

So I replaced the gas valve with a new one that has an ECO connection. The valve came with a jumper cable.

Do I need to use that on the two terminals since my furnace is not ECO equipped? As of right now the valve is installed...no leaks but pilot will not stay on when I release the pilot light button.

On 2019-02-11 by (mod) - is the pilot working correctly?

Yes if the pilot stays on that usually means the thermocouple is sensing temperature and telling the gas valve "Pilot is on".

About replacing the gas valve - it's basic plumbing connections but

Watch out: take care to soap solution test all you piping joints - a leak can blow up the house.

Keep me posted.

On 2019-02-11 16:02:53.104609 by Steve V.

Great information, I appreciate it. I am just homeowner on a tight budget so before I call in the pros I was hoping to figure this out on my own. It looks to be a very basic furnace. The Thermocouple is in the flame and I did replace that yesterday.

Since the pilot is on and not going out I'm assuming the assembly is ok? (or could there still be a problem there)? I have ordered a new gas valve and will install it tomorrow. If that does not solve the problem where should I look next?

On 2019-02-11 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

On 2019-02-11 by Steve V. - My pilot light stays on but the burner will not kick on

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.


...

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FAN LIMIT SWITCH FAQs-6 at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING FURNACES

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