Limit Switch Failure?

I am trying to determine if the limit switch is bad on my gas forced air furnace, or is it something else. Here's a typical cycle:

1. Thermostat sends signal to furnace for heat.

2. Burner and blower kick on simultaneously, every time. Air blows cold for the first few minutes of operation, then blows normal/warm.

3. When desired temp is reached, thermostat sends signal to furnace to shut off.

4. Burner and blower shut off simultaneously. After 10-15 seconds of being off, just the blower kicks on and runs for about a minute or so. Sometimes, but rarely, the furnace will shut down normally, burner first, then blower a minute or so later.

There is no obstructions in the plenum, cold air return is not blocked, and the limit switch and sensor are not dirty or dusty.

Furnace is an 18 year old Comfortmaker. Is the limit switch the problem?

 

It sounds like the thermostat is configured for electric heat instead of gas heat and you have a faulty fan limit, but I would be interested in hearing other opinions.



What thermostat do you have (make and model)?

Is the evaporator coil clean?

Do you have a furnace model number?

When was the last time this furnace was cleaned and inspected?

 

Thermostat is a LUX PSP511A - used for gas, electric and oil furnaces.
Evaporator is clean.
Furnace is a Comfortmaker GUF050A008
Cleaned and inspected about 2 years ago.

Limit switch is a Honeywell L4064A2782.

 

https://pinnaclemil.propertysolution...e74e499791.pdf

Is the Gas/Electric switch behind the stat set for Gas?

 

The switch on the back of the stat was set to electric. I moved it to gas, and the furnace now operates in what I would consider - normal operation. The problem I stated above has been resolved!

I must thank you for this quick and cost-free solution! I appreciate your input.

I have lived in this condo for 3+ years, and for those 3 years, I have wondered why the furnace operated the way it did. The previous owners at the time told me it was a new stat. They obviously installed it with the wrong setting. Since the furnace provided heat, I never really questioned the operation until this winter. I never thought it was the thermostat.

I'm glad I posted this question. I have also printed out the PDF you sent me, future reference. Thank you for the PDF and the advice!

 

People LOVE to replace thermostats for pretty much any old reason. It seems like an easy task to do.

But it's frequently more complex than people suppose, and people rarely read and understand the instructions that are packed with the thermostat.

It's quite common for people to replace a thermostat that was working fine, then not be able to get the furnace to work. It's also common for people to replace a thermostat and to have the furnace not work properly, perhaps for years because it wasn't installed properly.

This is one of life's little ironies with which furnace repairmen become familiar.

 

I have a Limit Switch problem I Think anyway- I own a York 90 Furnace inspected for use in Feb 98, not to many problems, but now the furnace comes on when it calls for heat, but then the burner will keep buring but the Fan will not kick on, (Blower Fan) Model P3URD14N09501-It is set up for Propane. Currently I have the Fan switch set to run full time rather than Auto.
Things I have done-Had furnace man come out- it worked fine while he was here. He says its either the Fan motor or the Control board. ? Its not the motor or it would not run set on "Fan"

I replaced the Limit Switch- still have problem

I checked the Filter it has some use but not real dirty

Any suggestions would be greatly appreciated, its cold as hell here in Michigan.

 

This morning I tried to start up my Furnace, with the fan switch on, and blowing, I now get a code 3- Pressure Switch-I am checking out the Pressure switch. I tried jumping the two contacts but nothing happened. Any help will be greatly appreciated. Bill

 

I installed a new Pressue Switch, and reconnected a hose to the trap that was loose, wish I had done that prior to the new Pressure switch, but none the less the Furnace is now working.

 

95% of the time a pressure switch opens for a reason. So replacing the pressure switch has a 5% chance of solving the problem you have, I would estimate.