Wood and Oil furnace blower problem

[Alk]

Hi guys... I'm new to the board and hoping for some enlightenment here. I have centralized heat. The wood furnace is connected to the oil furnace via the duct work. They are next to each other in the basement. There are two thermostats upstairs to control the heat. The wood (Clayton) furnace air intake and blower is control by one of the thermostats. What is happening is when the wood furnace has been used the blower will often continue to run even though the fire has long been out and there is no ambient heat in the plenum. However, the oil furnace runs the blower perfectly, shutting on and off as it should.
I have replaced the fan limiter switch twice with no improvement. I've ordered another switch, this time a Honeywell, hoping the other two switches (non name brand) were somehow inferior. A Hvac has determined the problem is with the switch but has had no success eliminating the problem. I've now taken over... lol. Any suggestions, questions or answers would be appreciated. TIA... AL

 

[brenndatomu]

If the firebox is cold, and the fan/limit switch has been replaced, about the only other option would be a wiring short.

 

[Alk]

If the firebox is cold, and the fan/limit switch has been replaced, about the only other option would be a wiring short.

Thank you. I then wonder if there would be a short in the thermostat wiring, which would likely be an easy fix by either by replacement or switching wires...

 

[begreen]

It's simple to test. Remove one of the thermostat wires that connects at the furnace switch. That will create an open circuit.

 

[Alk]

It's simple to test. Remove one of the thermostat wires that connects at the furnace switch. That will create an open circuit.

I'll try that. Where is the furnace switch? And, with two thermostats I'm not sure how I would know which wires to disconnect. If the circuit is open what will happen? Sorry, I'm not too familiar with this subject but willing to learn...

 

[begreen]

I'll try that. Where is the furnace switch? And, with two thermostats I'm not sure how I would know which wires to disconnect. If the circuit is open what will happen? Sorry, I'm not too familiar with this subject but willing to learn...

I was thinking this would be where the thermostat wire connects to the switch on the wood furnace, but I was assuming that switch is separate from the fan control switch on the oil furnace. Now, llooking at a Clayton schematic, I see no thermostat connection. Only a snap switch. What switch did the hvac person replace? It could be the wrong one.

Does your system have two zones with each thermostat controlling a separate zone? Post some pictures for verification.

 

[begreen]

This is the schematic I have for the Clayton 1660

 

[Alk]

The limiter switch is on the Clayton housing and I assume controls the blower for both units...I may be wrong. It definitely controls the Clayton, as it came that way. The replacement switch must be the correct one (matching numbers with the original) but not reliable as far as it seems...or? The Clayton thermostat controls both the wood intake air and blower and works perfectly controlling the air intake flapper. Now you have me wondering if the Bryant oil furnace runs off a different limiter switch even though the switch appears to be operating the blower on and off based on its separate thermostat setting. I don't understand why the blower keeps running when the plenum is cold, but that only happens occasionally and only when it had been activated by the wood heat. Weird!

 

[Alk]

I've unintentionally left out one critical element. Now and then, when the blower is running and with the Clayton firing away, the blower will start banging around in its housing. The motor mount is flexible, so no apparent damage is being done. I've taken the cover off and watched it. The way I stop the thudding is by pushing in the button on the limiter to 'manual' from auto, and this works every time. Sorry, I forgot to mention.

 

[brenndatomu]

What model Clayton is this? There are lots of different ones, and the wiring varies drastically.
I assume we are talking about the large blower that blows heat into the house here, not the little combustion blower on the front? That's the one that the tstat controls...the large duct blower is only controlled by the fan/limit switch...is that the one that was replaced?

 

[Alk]

The Clayton is a 7.1, and yes, we are talking about the blower that is actually housed in the Bryant oil furnace for heat distribution. When the blower is thudding about, as it just was, its seems as if it is trying to re-start but not getting enough current to do so. Again, when I push the limiter to manual it operates fine. Sorry to be so repetitive...

 

[Alk]

The Clayton is a 7.1, and yes, we are talking about the blower that is actually housed in the Bryant oil furnace for heat distribution. When the blower is thudding about, as it just was, its seems as if it is trying to re-start but not getting enough current to do so. Again, when I push the limiter to manual it operates fine. Sorry to be so repetitive...

The limiter that controls the blower is the one that was replaced.

 

[brenndatomu]

we are talking about the blower that is actually housed in the Bryant oil furnace for heat distribution.

So the Clayton is set up in series then...only one blower for both furnaces...

 

[brenndatomu]

The Clayton thermostat controls both the wood intake air and blower and works perfectly controlling the air intake flapper. Now you have me wondering if the Bryant oil furnace runs off a different limiter switch even though the switch appears to be operating the blower on and off based on its separate thermostat setting.

The thermostat only controls the intake damper...NOT the blower....the fan/limit switch (FLS) does that. (unless some cowboy HVAC tech wired up something crazy)
Yes, each furnace has to have its own FLS.

 

[Alk]

I wonder if they are wired together to the one fls?There is another fls on the clayton housing but it isn't hooked up to anything. Hvac sez it's burned out as the dial is at the top. Maybe at one time a hvac dude may have wired both to the one fls? If so, it was a long time ago and years before this oddity.

 

[Alk]

So the Clayton is set up in series then...only one blower for both furnaces...

Yes...

 

[brenndatomu]

I wonder if they are wired together to the one fls?There is another fls on the clayton housing but it isn't hooked up to anything. Hvac sez it's burned out as the dial is at the top. Maybe at one time a hvac dude may have wired both to the one fls? If so, it was a long time ago and years before this oddity.

I don't know how it could be run from one FLS.
Something has to sense when there is heat in the Clayton supply plenum...is there a snap switch maybe?
We need some pics of this setup...

 

[Alk]

Ill try to get my camera to hook up with this computer... thanks

 

[Alk]

so far no luck uploading photo... ok, so the tstat only controls the air intake, thanks for the clarification. The plenum is one unit and both furnaces feed into it. So when the oil is on the Clayton's housing, where the fls is, heats up.

 

[brenndatomu]

They share a plenum?! Oh man, I gotta see this...sure hope you can get the pics thing figured out!
Have a smart phone?

 

[brenndatomu]

And if that's the case, then there probably is only one FLS, and it makes absolutely ZERO sense that the blower would work fine on oil, but not wood...

 

[Alk]

I do, but it sucks real bad. Internet bad here too. The Clayton housing is connected to the oil housing by a 1' x 1.5' x 2' long plenum at floor level. There are two plenums. One leaves the Clayton at the top of its housing, the other out the bottom to the oil where it continues to other parts of the house. It's worked great for a couple dozen years until this.

 

[Alk]

I'll try my wife's phone, much better...

 

[Alk]

And if that's the case, then there probably is only one FLS, and it makes absolutely ZERO sense that the blower would work fine on oil, but not wood...

Absolutely!

 

[Alk]

FLS