Fan limit switch shuts down when the heat from wood furnace is too high

[Beno]

Hi there,

I have a Caddy wood/electric furnace. The blower/fan is controlled by a Honeywell fan limit switch. The fan limit switch has no reset button on it. Last night I filled up the firebox with wood, to have enough heat for the night, and shortly after that, the blower stoped working. I think that the fan limit switch has a state where it detects too much heat and tries to shut down the furnace. In my case though, the opposite was achieved, the ducts got very hot. I closed the damper but the fire was still going on with very low flames. I didn't know how to force manually the blower to work. Finally I started the electric element of the furnace which turned the blower on and cooled the ducts. Shortly after that, the fan limit switch started to work again (I guess reseted itself).
How can I prevent this to happen in the future?

Thanks

 

[OldStoneHouse]

That's interesting - I've never been able to get mine nearly hot enough to hit the high limit! I thought it was supposed to shut the damper but keep the blower on - at least that's what I get from my manual. Perhaps it wasn't wired properly?

Brad

 

[Beno]

Well, it could be wired wrong because I did it But, I followed the manual and the system worked great last winter, now this was the first time I had the issue. I am not sure if there is (or should be) a connection between the damper and the fan control. The damper is shut in case of a power outage or if the thermostat hits the limit for desired temp. I think that the general rule is not to fill up the firebox with wood (which I did, having some softwood and 1/2 rotten wood).

 

[laynes69]

I have only had our limit hit a couple of times. I'm not sure about the electric portion of the furnace because we have wood only. From what I have read if the electric portion hits the limit, the elements will shut off, but the blowers should remain running no matter what. At that point the electric element has to be reset. I would be worried if the blower shuts down when a limit is met due to the overheating of the unit, especially in a high burn. For us the limit is wired into the damper above the door. If the limit is met that damper closes, but the blower continues to operate so it can cool the furnace. Once the temp drops the damper will open back up and vise versa. Is your furnace keeping up? Maybe if you haven't, speed that blower up to remove the heat faster from the heat exchanger.

 

[Stove Geek]

Hi Beno

maybe you should contact SBI at 418-878-3040 and select the techincal support. We will help you out. It looks as if the wiring is done wrong or maybe the fan limit is busted. If you removed the cover plate off the fan limit box, does the wheel spin? Does it activate the fan at any point? you will see three pins on the wheel. The high limit is at 250F. Is there a pin at 250F? there should be one at around 150-160F and a lower one around maybe 100F...

 

[yooperdave]

Is your furnace keeping up? Maybe if you haven't, speed that blower up to remove the heat faster from the heat exchanger.[/quote]
i think if you wanted to remove more heat from the exchanger, you would slow the blower not speed it up. this way, the sir moving across it has more time to gain more heat. what about the thermal overload in the motor windings???could that be the cullprit?

 

[Beno]

I looked again at the specs of the fan limit controller and is specified that it should be installed on a heating insulating material, and its teperature should be lower that 88C / 190F. In my case, the fan limit controller is installed directly on the hot air duct, right above the furnace. It could be that the temps there get very high, and affect the fan limit controller. After the furnace cools down a bit, the blower starts working again. I didn't change the factory setting for the limits/pins, but I'll check that also. I'll go to Home Depot to look for a heating insulating material, and let you know.

 

[laynes69]

I installed mine in the front of the plenum, the only place I could due to the internal backflow dampers. Your damper should close when the limit is met, and the blower continue to operate. I believe the reason why you are hitting the limit, is the damper sounds as if its staying open even after the limit is met. Sounds like they are backwards, and its not safe. Anytime you increase the blower speeds, the limit control will cool faster than a lower speed. Of course your air will be warmer on a lower speed, but more goes up the chimney. I built a mount for our limit control with a piece of sheetmetal that I bent to mount to the plenum to keep a 1" gap from the limit/control.

 

[Beno]

Thanks laynes69, this is very interesting. I installed our limit control on the left side of the furnace, as specified in the Caddy manual. Is this a problem? Do you think that I wired something incorrectly, if the limit control is not closing the damper? I ussualy prefer to control myself the damper, to optimize the burning process and not the house air temp. After about 40 min, when the firebox is very hot, I close the damper. I'll try to close the damper earlier, to make sure the furnace is not too hot. I'll also build a 1" spacer as you did, to create a gap.

 

[laynes69]

To get the most from the furnace and comfort in the home, let the thermostat do its thing. We load the furnace, open the damper via thermostat on the wall and run it open for about 20-30 minutes. Once the fire is burning good and hot, we set the thermostat on the wall and let it do its thing. We get very little in the chimney, and we stay comfortable. Don't worry about letting the furnace get hot, just don't run it wide open. I had to set my limit control in a different position, I couldn't put it where the manual stated. Where you put it is fine. Just make that spacer and check your wiring and you will be just fine.

 

[laynes69]

You get things figured out Beno?

 

[Beno]

I created 1" gap between the fan limit and the plenum, and also started to fill up the firebox no more than 2/3. I make sure that the damper will not stay too long open. I checked the wiring diagram and found a mistake. The 2 wires from LINE of L6064 which are black and white were connected wrong at the Power Block. White should go to C and black should go to B (it was the opposite before). Now the L6064 seems to work better: it starts the fan at 150F and stops the fan at 110F. Now, L6064 shows around 175F-200F during normal operation, is this what you guys have too? Another thing that bothers me: it seems that the fan blows all the time at the same speed, which should be MED-LOW. I don't know if this is indeed MED-LOW, it should be by connecting the red wire HIGH from L6064 to Power Block D. Do you guys have 2 speeds on your furnace? I think that the lowest speed is only reachable by pressing the summer switch; otherwise the fan should always run at ‘medium low’ speed. I tried the summer switch and the noise made by the fan seems close to the one made when burning wood.

 

[laynes69]

I don't have those temperatures in the plenum of the woodfurnace. Mine stays around 150 unless the damper is open then the temps increase to 175. But then again I'm closer to the heat exchanger and my fan speed is set at med-high. The lower the fan speed, the higher the output temps on the registers. My furnace has 4 speeds that I can choose from. Low, Med-Low, Med-High and High. I don't hear a difference between the high and med-high on the blower, but the register temps are at least 10* warmer. I should try Med-Low, but i'm afraid I wouldn't get good enough flow in our ductwork. Temps would be higher in the ductwork that way, which is one reason why you are hitting the limit and why I can't. From what I have read, even though my register temps aren't higher with the increased airflow its more efficient. Glad to hear things are better.

 

[Beno]

The Caddy diagram shows connection to the med-low, which is D on the Power Block. |I'd like also to try to use med-high instead of med-low (although this may mean more electricity and noise probably) at least for the experiment. Can you please describe how did you make the connection to med-high instead of med-low?

 

[laynes69]

We have a distribution block beside the blower in the rear of the furnace that has the speeds on it. The white is on common and the hot wire goes to whatever speed I want. If your furnace has a blower with belt drive, I believe thats a 2 speed blower. The direct drive blowers are a 4 speed blower. Even with the blower on a higher speed, the furnace is still quiet.

 

[Beno]

The wiring is OK now, when I hit the 250F limit the damper closes and the fan keeps blowing.
I found the real problem that caused the heating: the air filters. They were clogged with dust. I found new filters 12x24x1 at the local HW store and now all works fine: I have 150F temps and the house heats faster. Also less noise in the ducts. Thank you all!

 

[laynes69]

Awesome, thats good news. Is those temps you are seeing at the plenum on the limit/control? Have you changed the blower speed?

 

[Beno]

These are the temps I see on the limit/control. I don't have other device to measure the temps in the plenum. How often should I change the air filters? I payed about $10 for a couple of filters. Is this OK?