Fan & Limit Switch FAQs-4

Fan limit switch accuracy & temperature response

Fan Limit Switch Temperature Response & Accuracy FAQs

Can I replace a traditional bi-metallic spring fan/limit control with a newer, more-accurate controller for my furnace?

Your explanation of limit switch was, and is, the best on the internet. Your thoroughness is unparalleled.

I recently bought a mid-century modern home with a 550 gal oil tank in the garage and a hallmark furnace and needless to say have been searching for resources.

I had 4 wires going from the thermostat hoping I wasn’t gonna have to tear out walls.

Well, I tore out the 50 year old walls and ran 2 18/8 down (for the future needs).

Here’s my question, I want to know if I can get something “better” than this mechanical limit switch. I like that it’s mechanical for safety purposes but I want more control than metal expanding.

Love your site and I can pitch in on the software side if help is needed. - Anonymous by private email 2018/02/07

Reply: probably not, for safety and functional reasons

at FAN LIMIT SWITCH - topic home, I have added details to distinguish between the traditional bi-metallic spring helical probe used by Honeywell and White Rodgers and newer snap-switches that are used by some furnace manufacturers.

You'll see that the principal difference between the disc-type snap switches and the older controls is that the older ones are adjustable.

But it's also the case that a furnace might be unsafe if you swap in a different control that does not place the temperature sensor where the furnace manufacturer wants it.

And I have not found solutions that include the adjustable features of the traditional control.

I, too, like the supposed precision of the newer controls but the decision isn't that clear. Aside from giving up adjustability, you'd probably need several, certainly at least two, snap-disc type sensor/controls, and they'd need to both be located essentially at the same sensing point.

You might also be voiding warranty.

Those old helix type bimetallic spring devices have been used in many thousands of heaters over many decades. They perform well and by offering a large sensing area they might be more resistant to problems from dust and debris than snap-discs.

Where the older spring type controls gave me trouble (when I worked on heating equipment back in the dark ages) was as stack-relays on oil fired equipment. Using the same approach but inserting the probe into the flue of an oil fired boiler or furnace meant exposing the probe to sooting. Those controls were finicky and gave trouble on occasion.

Sure there may be dust in the supply plenum too if a furnace is being run without filters or is otherwise moving a lot of building dust. But that dust is not as troublesome as oil burner soot in a flue.

In sum, I would be very cautious about swapping in a different type of safety control than what was specified by the manufacturer of your heating furnace.

How fast or responsive should the dial turn on a fan limit switch?

How fast or responsive should the dial turn on a fan limit switch? (Jan 2, 2015) Anonymous

Reply:

Anon

What an interesting question. I've never timed the dial turn, but I can say that when the furnace burner turns on you should be able to see the dial rotating slowly as the plenum heats up.

The movement is plainly visible but with a *cold* furnace the movement may not begin until the plenum temperature reaches the low end of the limit switch sensor range.

The time to get from fan on to fan-off varies widely as (except for cases where the burner and plenum temperature reach the fan OFF upper limit) that depends on when the thermostat is satisfied.

What to do if the furnace fan/limit switch that isn't working

I have a fan/limit switch that isn't working (shuts off the fan AND the burner at the high temp, causing furnace to overheat and then shut down).

I have one like the black honeywell shown above, but its beige instead. Like this:

but without the hard stop in the middle of the dial.

My question is, what the heck do I replace it with? I can deduce that a Honeywell 4064:

With the same length (11.5"), but which model? It's hard to even tell the difference other than the insert length.

Any advice? Thanks for the help!
Cheers,
Jon

Reply:

Jon

I agree it's confusing. The simple approach is to take your old limit switch to your heating supplier and let them help you find a compatible replacement.

On the Honeywell page whose link you cited there are many control numbers but the variations are typically these:

1. bulk pack (multiple controls in a package) versus individual controls

2. Control sensor length (must be the same as your old one)

3. Limit switch with or without manual FAN ON switch

Some controls may also come with different operating temperature ranges or defaults - that's what you also want to match to your old control

The old model Honeywell limit switch you have installed (per your other photo link) MAY be mounted on your furnace in a location at which the new controls with a larger base won't fit - that's why you want to take the old one to the supplier - to be certain the new one

- fits in the old space

- has the right sensor length

- the same operating temperature range (most of these are the same)

- your old control did not have a manual fan ON switch so you don't need that feature.

What to do about a Very Hot furnace blower housing

My Intertherm E3 electric furnace and blower function normally.

However when both the blower and elements are off the blower housing becomes to hot to touch.

When I cycle the blower the housing cools down but immediately heats back up once the fan is off. I can visually see that the elements are off when this occurs. Any suggestions? Thanks. (Mar 31, 2014) Darren

Moderator Reply -

Watch out: Darren I'm not expert on this equipment but what you describe doesn't sound safe. I'd think that when the blower is OFF the heating elements should be OFF for safety and to avoid damaging the equipment. There may be a problem with the limit switch or with wiring.

I'd contact your heating service company or the company - and I'd turn the thing off until this is resolved.

Intertherm Heating and Cooling
Customer Service
8000 Phoenix Parkway
O'Fallon, MO 63368
www.intertherm.net

Where is the Furnace Fan Limit Control Located? - FAQs

Where is the fan limit switch on a Miller Oil Furnace?

We have a Miller oil furnace, and I'm trying to locate the limit switch. Model Beckett AF9 On 2017-11-27 by Kathy

by (mod) - fan limit control on your Miller oil-fired furnace

Kathy,

The fan limit control on your Miller oil-fired furnace will be just about in the location shown in the photos and sketches in the

article FAN LIMIT SWITCH GUIDE.

But take a look at the different types of fan limit controls, since there are several types that you'll want to be able to recognize.

In concept, the fan limit switch will always be located where its temperature sensing probe can be inserted into the supply air plenum - that's the temperature that it's monitoring.

You'll need to lift up and then out the upper front cover to see the furnace burner and controls.

Or you can use the little "picture" icon to the right of the Comment button to enter a comment and include a photo of your furnace with cover on and with its cover removed and I can probably circle and point to the limit switch.

The "model" you cite is for an oil burner, not for the limit switch.

The Miller Furnace company does not provide readily-accessible installation and operation manuals for their heating equipment at their website - at least not that I can find, but if you don't have the manual for your furnace you can find one at our list
MILLER International Furnace Company - https://inspectapedia.com/heat/HVAC-Manuals-Air-Conditioners-Boiler-Furnaces-J-O.php#MIller

OR you can surely contact the company to ask for one

• Miller Heating and Cooling
  Customer Service
  8000 Phoenix Parkway
  O'Fallon, MO 63368 USA
  http://millerac.com/Products/Furnaces/OilFurnaces.aspx
  
  We also provide Miller / Nordyne furnace manuals at MILLER International Furnace Company
  
  Also see NORDYNE Furnaces
  

Fan Limit Switch FAN-ON / AUTO Button Operation FAQs

Tip:

See also BROKEN FAN-ON / AUTO BUTTON REPAIR FAQs

Is PUSH - ON or PUSH-OFF always true or the manual fan controller ?

Editor, a question: You write to push the manual switch in for continuous blower use but in the next paragraph you say to pull it out. Huh? See below:

PUSH the white button IN to force the fan to ON or MANUAL or always-running mode.

PULL the white button OUT to put the fan back into AUTO mode - turning on and off under control of the switch and furnace temperature.

On systems where we have installed high quality air filtration to address an indoor air quality issue, and where the fan is rated for continuous duty, we may pull this switch out to keep the fan on continuously. - On 2017-04-13 by Mark I

Reply by (mod) -

Thank you Mark. I agree that there might be some fan limit switches that do not work the way I describe.

The widely-used Honeywell Fan Limit Control used as an example in the above article includes an instruction sheet for the fan limit switch showing the same information as is actually embossed on the switch body: the marking tells us PUSH ON, PULL AUTO.

In some of our photos of this control at FAN LIMIT SWITCH (click to enlarge) you'll see PUSH MAN, PULL AUTO - which have the same functions as "ON" and "AUTO"

You can see this more clearly if you click to enlarge the illustration. https://inspectapedia.com/heat/Fan_Limit_Switch610-DFcs.jpg

This means that if you push that switch IN the fan will run continuously while if you pull the switch OUT the fan will run automatically under control of the room thermostat and furnace. In the AUTO mode the fan will not start, in a response to a call for heat, until the plenum has warmed up and the fan limit switch has rotated to the set ON temperature.

I appreciate your comments and would much appreciate knowing the fan limit control brand and model that works in the opposite direction so that we can add that information for other readers.

I'll edit the material above to add your note.

Some other brands/models of fan limit controls like the White Rodgers fan limit control FD51 shown below use a sliding toggle switch rather than a Push/Pull switch to control the fan operation.

Tip: Details about the manual FAN ON or FAN AUTO ON switch right at the furnace or air handler fan limit control switch are

at BLOWER FAN CONTINUOUS OPERATION.

Also see FAN ON AUTO MAN THERMOSTAT SWITCH for manual FAN-ON switch found on many room thermostats

Also see BLOWER FAN CONTINUOUS OPERATION FAQs 

FAN-ON Button on my limit switch doesn't work

The fan manual button dose not work - (Nov 23, 2014) Anonymous

Reply:

Anon

If the manual fan switch never worked I suspect a wiring error.

If the switch used to work then it needs replacement. Check first for a loose electrical connection.

Also see BROKEN FAN-ON / AUTO BUTTON REPAIR FAQs