Limit switch doesn't function on oil furnace

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Testing and adjusting your fan limit switch in your gas or oil-fired furnace can prevent costly repairs to your blower and heat exchanger.

The purpose of a fan limit switch is to automatically control the furnace blower allowing it to run for a predetermined time to allow the heat exchanger to cool down after the thermostat temperature has been achieved. The limit switch also prevents the furnace blower from distributing cold air through the home until the oil or gas burner has warmed the heat exchanger and supply air plenum. In other words the fan limit switch is a time delay device that starts the fan about 30 seconds after the call for heat and allows it to run about 30 to 90 seconds after the set point has been reached.

On older units the switch is an analog combination fan and limit control while newer units may use electronic controls but may also have a sensor connected to the control board.

Limit-Switch Location

The fan limit control switch is...

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The Auxiliary Hi-Temp Limit Switch

The CB-3500 and CB-5000 furnaces are equipped with an auxiliary L-290 high temperature limit switch which

is mounted on a bracket at the front of the combustion chamber as shown in Figure 7B. This switch senses the

build-up of heat within the combustion chamber beyond the fan limit control setting and is designed to protect the

furnace from damage due to overheating.

Understanding the Function of the Auxiliary L-290 Auto-Reset Hi-Temp Limit Switch

If the temperature of the combustion chamber has exceeded the range of the fan limit control, the auxiliary

L-290 auto-reset hi-temp limit switch opens and shuts off power to the primary control on the burner, and the

burner shuts down. After the combustion chamber has sufficiently cooled, the L-290 switch will automatically

reset, and allow the burner to re-start.

The L-290 hi-temp limit switch will open and shut down the burner if...

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I have a 3 year old Carrier gas furnace (Heat only; No A/C) model 59SP5A. When maintaining temperature, it functions fine, no error codes. But when it has to heat the apartment up a bit, it overheats, and shuts down with Code 3-3 (presumably because it had to stay on too long). It cools itself off with the 4 minute house blower, and then re-ignites, runs, and eventually gets the place up to temp. Part of me wants to leave it alone; but I am curious if I am risking damage or shortened lifetime by ignoring?

I figure it is the limit switch, not the flame rollout (because it auto-resets). And because the furnace can function normally, I assume the limit switch is functioning. The air filter is clean, (it's the fine-grade honeywell filter from Home depot). There is no obstruction on the air return, and all register vents are wide open with relatively short ducting. What should I look at next? From reading posts, it looks like measuring the temp rise would be good. ...

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hello i have a modine PA50A control code 01 trying to get a wiring diagram … read more

Airheatman

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is 5 zone and it heating just fine. All the circulators are working and for the most part it operates just fine except every couple of hours the valve opens up and pumps water down the drain. As I say… read more

Phil

Mechanical Engineer

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we have an Aprilaire 8466 Thermostat less than 1 year old. My wife discharged static electricity when attempting to change the temperature and the thermostat went blank. Changed the batteries, hit the… read more

Airheatman

Heating & Air Conditioning Contractor

6 years college

1,836 satisfied customers

my 286 board is giving me 12 flashes...thermostat calls for heat...just cool air … read...

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If you've ever listened to a forced-air heating system on a cold day, you might have wondered why the burner begins to roar a few minutes before the blower fan comes on. This doesn't mean you are sending hot air up the chimney; it means the limit switch in the furnace is working.

Forced-Air Heating Systems

These heating systems burn fuel such as oil or natural gas, and have large blower fans to send air throughout the house in a system of supply ducts. Instead of heating the air that circulates, a burner warms a metal heat exchanger that warms the air inside it. The warmed air passing through the heat exchanger is what circulates through the house.

Limit Switch Functions

If the blower came on at the same time as the burner, it would pump cold air through the house at first. The limit switch includes a sensor in the heat exchanger to determine when the air is warm enough to circulate. Once the air reaches a preset temperature, the limit switch turns on...

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What is a Fan Limit Switch?
A furnace has two switches called a fan limit switch and a high limit switch, these switches can be separate or in one package called a combination switch.

High limit switch
A high limit control switch will turn off the gas to a furnace if the temperature becomes too high. For example if the blower motor stopped running then no air would move through the unit. The flames would raise the temperature inside the unit and have no place to go getting hotter and hotter. As a safety the high limit switch turns the gas off when the temperature becomes too high.


Fan control limit switch
The fan switch reads the temperature inside a furnace and once the temperature becomes warm enough turns the blower motor on sending warm air into a home. This is used so that cold air is not blown in to a house waiting for the gas flames to heat up enough before the blower turns on.

Depending on the furnace these...

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If you're having trouble with your furnace and you suspect it's the furnace limit switch, read on. We'll give you some ideas of how to troubleshoot the switch on your own. Narrowing down the likely problems can save you some money when you call in a professional HVAC installation and service company.

A Word of Warning
Gas heaters and furnaces do have automatic shut off switches and safety devices built in to avoid gas leaks. That said, they are not fail safe. If you notice an odor of gas in your house, don't attempt to troubleshoot the furnace. Avoid turning on lights and don't try to shut off the gas supply to the furnace. Leave your home, leave the door open for ventilation and call the gas company or the local fire department. Don't re-enter your home until the gas company or the fire department has informed you it's safe to do so.

What is a Limit Switch?
The limit switch is the device that tells when to turn the hot air blower on and off. The limit...

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Fan limit switch troubleshooting diagnosis & repair:

Check this fan limit switch control if your furnace fan won't start (won't turn on) or won't stop (runs continuously). Guide to troubleshooting heating system furnace controls, limit controls, and fan controls.

This article describes in detail the testing and diagnosis of problems with warm air heating furnace combination controls, also commonly called the "fan limit switch" on warm air heating systems. As a working example we examine the Honeywell combination fan and limit control type L4064B.

We also discuss the diagnosis & repair of common furnace operating troubles such as a blower assembly fan that runs continuously - it may be a simple switch setting. The photo at the top of this page shows all of the controls and wiring terminals in a Honeywell combination...

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