Furnace overheating

[davidsongreg]

I have a suburban sf 25 furnace that is overheating and throwing the limit switch which causes the burner to go off until it cools. It cycles on for about 2 minutes and on for 2 minutes. I replaced the limit switch, replaced regulator, checked amperage, made sure fan wheels are clear, all furnace outlets and hoses clear, and added two more vents to get more air out. Does anybody have any other suggestions? Thanks

[davidjb]

I am not an expert by any means but perhaps there is some type of blockage in your vents or furnace outputs..sorry reread your post and i see you checked for blockage

[davidsongreg]

Yep. Did Check for blockage. Thanks

[TimVWulp]

If you can not find any anything mechanically wrong with the furnace. Fan motor return air blocked floor dampers closed you my have a problem with the propane gas regulator supplying to much gas pressure. This condition will allow to much gas to enter the combustion heat ex changer causing it to overheat. You should have this serviced!!! If you are truly overheating the furnace you may destroy it instead of a simple fix.

[Witch Doctor]

Originally Posted by TimVWulp

If you can not find any anything mechanically wrong with the furnace. Fan motor return air blocked floor dampers closed you my have a problem with the propane gas regulator supplying to much gas pressure. This condition will allow to much gas to enter the combustion heat ex changer causing it to overheat. You should have this serviced!!! If you are truly overheating the furnace you may destroy it instead of a simple fix.

X's 2 make sure you have enough R/A, also how many 4" ducts are attached to the furnace? You can also just have a bad limit. Put a little hole in the plenum and stick a thermometer inside a cooking one will work. see what temp it cuts out at and let me know. Do not touch any metal inside with the probe VERY IMPORTANT.

[davidsongreg]

Return air is clear, all the vents are open. I have 4 four inch vents and 2 two inch vents. I did have 3 four inch vents, but have added others trying to solve problem

[davidsongreg]

Also, heater was working fine up until this summer.
I did put in new regulator. I just took it to an rv mechanic who tested regulator and was the one who suggested extra venting

[Flybob]

Is the intake side clear?

[davidsongreg]

Yep. I took it out and pulled unit out of metal box to make sure

[davidsongreg]

I did replace the limit switch. Yesterday while mechanic was working on the unit, he had a heat gun type thermometer and it was cutting off at about 175

[TimVWulp]

175 you are overheating the furnace. There are only two things that will cause this, low air flow or to much gas flow into the combustion chamber heat ex changer. You replaced the the propane gas regulator on the trailer propane tanks and pulled the furnace out of the trailer furnace compartment. When the furnace was out did you make sure the return air box was open to the fan? The other remote possibility is that some paper or what ever got sucked into the blower fan wheel reducing air flow. If this happens the fan will have a vibration to it as the material will unbalance the blower wheel and restrict air flow. hope this helps

[davidsongreg]

Thanks. I'll try pulling it out again and checking it over. I know nothing is on the fan because I looked to make sure both fans were not clogged up with dirt or something

[Mojojojo]

Could be a bad circuit board?

[Witch Doctor]

Originally Posted by Mojojojo

Could be a bad circuit board?

This is true, it could also be a bad stat. OP can jumper it out with the two wires and see if it will stay on. It can also be that the motor is not blowing enough air over the heat exchanger. He will need to find the amp draw of the motor and match it to the rating of the motor at FLA (full loaded amps) other then that call the service guy back out. All so at the same time jumper out the air flow switch. You need to work backwards from the power source which would be the stat. He has more then enough cfm with all the duct work he stated. I leaning towards that circuit board myself also.

[davidsongreg]

Thanks. I just pulled it out again and ask air flow and fans appear clear. With the front cover off, I can feel ( from a few inches away), the right side if the chamber is getting wicked hour before cycling off. This also happens when camper plugged in. I checked amperage, and it's within limits.
Would a bad circuit board cause it to get too hot?

[davidsongreg]

Sorry about the typos. I only have my phone with me. Getting wicked hot, not hour

[B and B]

I had an issue were the actual duct had a hole in it and was leaking. I replaced the bad duct which happened to feed the vent closet the thermostat. The furnace then cycled off correctly. I am really thinking you either have too high gas pressure. Should be 11 inches of water column. Or maybe the gas orifice has been damaged and is allowing too much gas. The furnace should run without cutting out. when you regulator was replaced is it a two stage regulator?

[davidsongreg]

Yes, replaced with two stage. It's hard to believe orifice could get damaged, but maybe replacing that should be my next step. I actually pulled that at one point and didn't see any visual damage

[Witch Doctor]

Originally Posted by davidsongreg

Sorry about the typos. I only have my phone with me. Getting wicked hot, not hour

Not quite sure what you mean wicked hot? Your heat exchanger should be about the same across the cells. You might have a blockage not having even gas flow or heat. If your side with the limit is on the wicket side it could be your problem. Your heat output should be around 140 degrees not 175. If you have a air compressor blow out your heat exchanger. I truly doubt it is your regulator. If you have a temp gun measure each tube they should be within 5 to 10 degrees across other the that you have a blockage. Sorry but this is all guessing without seeing it and doing it. If you take the front cover off and turn it on will it still go off on your limit?

[davidsongreg]

Yes, the limit switch is on the side that seems to be getting very hot, and it still limits out with front cover off. I'll try blowing out the exchanger and see if that helps.