furnace fan

trailrider wrote:

dougrainer wrote:

mchero wrote:
The Hi limit switch on my Atwood furnace is normally CLOSED, opens when overtemp condition.
From the Atwood service manual;
The limit switch is an in line safety device which protects
the furnace from any over heating conditions. The
contacts in the limit switch open at a given temperature
setting, shutting off power to the valve

Once the burner shuts off, the fan continues to run to cool the chamber down. Once the Limit closes, the fan will shut off. This can happen from normal operation when the burner chamber gets too hot, but the wall tstat has been satisfied. The wall tstat if still calling for heat, the Limit will open if the chamber gets too hot and will then close again once the chamber is cooled down and the burner will then come back on. If you have reached the tstat setting , the odds are the Limit is now OPEN from the chamber heat and will run the fan until the chamber cools down enough to close the limit switch. Doug

Again, we don't even know the make and model of the furnace in question. BUT: The Atwood Service Manual description of high temp limit switch function is correct for any Atwood or Suburban furnace made in the last 20 or 30 years. The high temp limit switch has NOTHING to do with the fan operation in these furnaces. That is controlled by the time delay relay on earlier furnaces or a relay on the circuit board on newer furnaces.

Some really old furnaces had a bi metal switch that allowed the fan to run long enough to cool down the furnace.

You are correct about not posting about brand/ model. BUT, the Hi limit DOES affect the fan operation. IF the Hi Limit has been tripped(open), then after burner shut down and tstat interior temp has been met, the FAN WILL run until the chamber cools down enough to close the Hi Limit switch. Once that happens, the fan shuts OFF. The logic in the Ignition module controls this. Doug

mchero wrote:
**snip**
I would like to thank Doug as well as the others for their contributions to these rv forums.
**snip**

I keep a notepad in my desk drawer. When Doug or Old_Biscuit speak, I write it down. I include title, topic, date posted, by whom, and final solution, if disclosed. I'm patiently waiting for this solution.

There is a wealth of knowledge on this forum but it can be difficult sometimes to root it out.

Thank You to all that contribute.

Trailrider

Both Doug and myself are very aware
that the OP did not state that his furnace was an Atwood.

ZRuss, the OP joined this forum back 2004. Perhaps, someday he will return to this post and let us all know what model furnace he has.

Atwood or not, interesting information being passed around. I have done my own work on my Discovery and many other rv's. Not as much as Doug and some others.

Again

I would like to thank Doug as well as the others for their contributions to these rv forums.

Waiting for the day Zruss posts the make of his furnace and of most importance, the outcome.

dougrainer wrote:

mchero wrote:
The Hi limit switch on my Atwood furnace is normally CLOSED, opens when overtemp condition.
From the Atwood service manual;
The limit switch is an in line safety device which protects
the furnace from any over heating conditions. The
contacts in the limit switch open at a given temperature
setting, shutting off power to the valve

Once the burner shuts off, the fan continues to run to cool the chamber down. Once the Limit closes, the fan will shut off. This can happen from normal operation when the burner chamber gets too hot, but the wall tstat has been satisfied. The wall tstat if still calling for heat, the Limit will open if the chamber gets too hot and will then close again once the chamber is cooled down and the burner will then come back on. If you have reached the tstat setting , the odds are the Limit is now OPEN from the chamber heat and will run the fan until the chamber cools down enough to close the limit switch. Doug

Again, we don't even know the make and model of the furnace in question. BUT: The Atwood Service Manual description of high temp limit switch function is correct for any Atwood or Suburban furnace made in the last 20 or 30 years. The high temp limit switch has NOTHING to do with the fan operation in these furnaces. That is controlled by the time delay relay on earlier furnaces or a relay on the circuit board on newer furnaces.

Some really old furnaces had a bi metal switch that allowed the fan to run long enough to cool down the furnace.

Thanks Doug! Good to know!

mchero wrote:
The Hi limit switch on my Atwood furnace is normally CLOSED, opens when overtemp condition.
From the Atwood service manual;
The limit switch is an in line safety device which protects
the furnace from any over heating conditions. The
contacts in the limit switch open at a given temperature
setting, shutting off power to the valve

Once the burner shuts off, the fan continues to run to cool the chamber down. Once the Limit closes, the fan will shut off. This can happen from normal operation when the burner chamber gets too hot, but the wall tstat has been satisfied. The wall tstat if still calling for heat, the Limit will open if the chamber gets too hot and will then close again once the chamber is cooled down and the burner will then come back on. If you have reached the tstat setting , the odds are the Limit is now OPEN from the chamber heat and will run the fan until the chamber cools down enough to close the limit switch. Doug

Many posters indicate OP has an Atwood furnace. I have not seen where the OP has stated make and model of his furnace.

mchero wrote:
The Hi limit switch on my Atwood furnace is normally CLOSED, opens when overtemp condition.
From the Atwood service manual;
The limit switch is an in line safety device which protects
the furnace from any over heating conditions. The
contacts in the limit switch open at a given temperature
setting, shutting off power to the valve

And most likely it only turns of the burner, and lets the fan to keep running to cool down the heat exchanger.

Dusty

The Hi limit switch on my Atwood furnace is normally CLOSED, opens when overtemp condition.
From the Atwood service manual;
The limit switch is an in line safety device which protects
the furnace from any over heating conditions. The
contacts in the limit switch open at a given temperature
setting, shutting off power to the valve

What the problem is is a laundry list.. What the SOLUTION list (During storage) is much shorter.
Option 1: Remove outside cover and if it's an Atwood furnace on top of the blower housing is what looks like a switch (it is a circuit breaker) Switch it OFF. Of course now the furnace is non-op till you turn it on.

Option 2: disconnect battery .. Physically lift the negative lead off the battery. (This also disables charging)


The laundry list OEM atwood board.
Bad T-Stat likely won't do it. same for T-stat wirings
Bad relay on control board
Bad other on control board
Short on control board wiring.

NOTE: There are two different patterns if lighting fails. Atwood shuts off fully and goes through the entire restart process.. Dinosaur board does NOT kill the blower.. SO if you have a dino board

Add the T-Stat and the T-Stat wires to the suspect list.