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Honeywell Fan Limit control used on a wood/coal fired furnace (C) InspectApedia.comWood Furnace Fan Limit Control Switch
Heat damage warning

  • POST a QUESTION or COMMENT about furnace blower fan limit switch controls when used on a wood-burning furnace or coal-burning furnace, special operating instructions for use during electrical power failure

The fan limit switch used to control the building air circulating blower fan may be damaged by high heat encountered when used on a wood or coal burning stove if the manufacturer's warnings are not strictly heeded.

Watch out: All of the wood and coal stove furnace manufacturers' instruction manuals that we have reviewed warn against overheating, over-firing, stove damage, safety hazards, and loss of warranty coverage from such damage including damage that can occur during an electrical power failure.

That does not mean that you can not use your wood-fired heater during a power failure but it does mean that if you don't follow the manufacturer's instrutions for that situation your wood-fired furnace may be unsafe.

This article series discusses safety questions that arise if you are installing or using a wood burning stove or furnace.

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Wood-burning Furnace Fan Limit Control Warning

fan limit switch bimetallic springOn 2022-02-24 by Andrew - The fan limit cannot take the excessive heat and breaks.

My fan limit switch is installed on a wood furnace. there is an insulated box which surrounds the wood stove. when hot, the fan switch turns on a blower which blows hot air throughout the ductwork of our house.

The system works great, EXCEPT, when the power goes out and there is a fire in the wood furnace.

The fan limit cannot take the excessive heat and breaks. I believe the spring just expands to the point where it loses its temper. Isn't there a non mechanical option for a fan limit?

Or perhaps a fan limit that can take the excessive heat. I usually have to replace one or two every winter, and thesed damn things are getting expensive. Any help or advice is appreciated. Thanks

This Q&A were posted originally at FAN LIMIT SWITCH TROUBLESHOOTING FAQs

See also

FAN LIMIT SWITCH

On 2022-02-24 by Inspectapedia Com Moderator (mod)

@Andrew,

Thank you for an important question.

I'll do some research on alternative fan limit controls and you might also ask the manufacturer of your wood furnace for control suggestions.

Watch out: when the fan limit is unable to turn OFF the heating system at high temperatures, such as in the wood-stove-fire + electrical power loss case that you describe there are serious fire and flue gas safety concerns as well as the risk of a damaged heat exchanger leading to need for costly furnace repair or replacement.

I think what we want is a spring-loaded combustion air damper that is held open but closes under mechanical spring force if the electrical power to the wood furnace is lost. That still not prevent overheat damage but that's a question to take to the manufacturer.

I suspect that most woodstoves either have an equivalent of that control, operated by temperature, or the woodstove manufacturer includes specific instructions for use of the woodstove during a power failure.

Warnings about Fan Limit Switch Damage and Wood Furnace Damage and Unsafe Conditions from Over-Firing or from Operation during a Power Failure

Though we don't have the brand and model of your wood furnace and so can't comment on its controls, it may be helpful for other readers concerned about installing and using a wood furnace and selection and safety features of the wood furnace fan limit control to see this

ROYALL WOOD FURNACE MANUAL [PDF] Royall Furanace, - cited in complete detail below on this page.

The company shows a conventional Honeywell type fan limit control switch like the one we include above in this discussion.

In the Royall Wood/Coal furnace manual did not find a specific discussion of what happens at this wood furnace if there is a power loss, but for this model (your wood furnace may well be different), the company notes that the heater can run under "automatic draft" that is thermostatically controlled OR under "forced draft" (when burning coal) that turns on a combustion fan.

This manual (p. 24) notes that

The 3-speed circulation blower is activated when the temperature sensor of the fan/limit controller indicates that air has been heated to the fan/limit control ON temperature setting. The circulation blower pushes air around the burn chamber, through the heat exchanger area and into the plenum.

One might infer that in this design, if the blower never turns on (because of a loss of electric power) then other mechanical thermostatically-operated controls will close the furnace's combustion air supply damper (and of course in a power loss the powered combustion air fan would also stop), thus allowing the heater to cool down as its fire is diminished by the drop in combustion air.

Watch out: this manufacturer (p. 25) and probably every other one warns:

Power Failure Instructions [for the Royall Wood/Coal Furnace]

Warning: always closely monitor the operation of the Royall Wood / Coal Furnace during a power failure.

To operate your ROYALL Wood/Coal Furnace in the event of a power failure:

1. Close the draft dial on the ash door

2. Remove the air filter.

3. Open all heat registers.

4. Do not load the furnace more than 1/4 full.

5. Keep sliding smoke baffle closed.

Use extreme caution when operating your Royall Wood/Coal Furnace during a power failure. Over-firing can damage the furnace and its components, will void your warranty, and may result in property damage, bodily injury or even death.

In our OPINON you couldn't ask for a more-clear warning about operating a wood furnace during a power failure.

I INFER from those instructions that if you already had built a bigger than 1/4 full wood fire and there is a power loss you need to take steps to remove wood and shut down the heater to avoid danger and damage.

Watch out: your report that you have had heat damage to the fan limit controller switch on your particular wood furnace may mean that you have been over-firing it, damaging the control, and making your heater un-safe.

Nordyne Limit Switch L150F at InspectApedia.com

It might be that other fan limit controls like the snap-disc (or "thermo disk" or "thermo disc") type controls shown above and discussed below would reduce some damage from overheating during a power outage but in our OPINON ANY safety control that is exposed to temperatures outside of its intended design range may be damaged and may be made unsafe.

Do tell us the brand and model of your wood furnace, tell us what your IO manual says about operating your heater during a power failure and about over-firing and its effect on the fan limit control switch.

Our example, the Royall wood / coal furnace is indeed using the Honeywell Fan Limit controller with these specifications

Fan Limit Switch 5" Element for Royall Furnace models:

Specifications - Honeywell Fan Limit Switch

Fan and Limit Control

Starting at our home page for fan limit controllers - FAN LIMIT SWITCH

you'll see warnings about overheating and damage to the helix coil that operates this control.

Wood / Coal Burning Furnace Blower Fan Limit Controls & Instructions

Thermo disks used to control the blower fans on an Ashley Wood burning furnace from United States Stove Company cited & discussed at InspectApedia.com

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