It was cold last week when I was up in the mountains. The coldest day was 8° F and the furnace worked fine. But a few nights later in slightly warmer temperatures it failed to start. The blower would come on, I could hear the gas light for a few seconds and then the gas would stop.

This happened on two successive nights. Once at about 5 am and once at 1 am. I fired up the generator and ran a space heater to get some heat.

My stovetop burner was looking pretty weak when this happened. So maybe I wasn't getting enough gas pressure to the furnace? I don't recall if this happened on the first or second 30 lb propane tank but it seemed to work good for the first few days which would have been the first tank.

My hunting partner says it's the thermocouple but I can't find a thermocouple on the parts diagram.

I have an Atwood 8500 furnace.

 

Thermocoupler would be my first thoughts. I had that happen to my furnace at home. I took some fine sand paper to it and put it back on. Been fine ever since.


Running low on propane could be a problem as well. But I have never had an issue with temps in that range using propane.


Did you check the furnance itself?

 

I think what you are looking for is an electrode. Off the blower assembly.

 

You said the stove flame looks weak and the furnace runs for a few seconds and shuts off.

Swap the two tanks. If the furnace works normally with the tanks swapped, replace the pigtail between the side that didn't work and the regulator. BTDT.

 

This evening I found an interesting tidbit about the propane regulators that automatically switch from one tank (the empty one) to the full one. The BTU output _drops_ when it switches to the secondary tank! It may not be every regulator that does this but I found documentation for two different brands that show the lower output on the secondary tank.

 

Board not getting a ground back. Most likely board has failed. Furnace will not light at all with pressure below 10" water column. Easiest way to tell gas pressure issue is to simply light the cooktop. If there is good flame there, pressure is adequate to light furnace. Regulator auto switching will not cause this.

Board relies on ground through flame to detect ignition, if it does not see flame within a couple of seconds it shuts down as a safety precaution so there is no danger of propane build-up. All model furnaces both Atwood and Suburban are the same in this regards.

Any dealer can check the board for you (most will at no cost) and sell you a new board if necessary.

Steve

 

Old trick in the crowd I run with is to first disconnect all the push-on electric leads and then put them back on to their respective post just to eliminate resistance from corrosion as an issue.

Am with glsuratt that the comment regarding fuel pressure might play into the equation, but my vote is to do the quick and easy test of re-seating all the electric leads.

 

The fact that you have ignition at all means you have power all the way through the ignitor circuit and are past the board and everything ahead of it including all the terminals. You also know the air sense circuit at the blower is good as is the high limit control. The ignitor lead is the same one that provided the flame sense to the board. A breakdown on that lead is very unusual. However, if you did not have ignition at all, Jay's suggestion would be the first thing to try as you would immediately know you did not have a complete circuit somewhere.

Most of the time you will get ignition all the way down to an 8" water column which is easily detected by simply looking at the gas flame on the cooktop as it will be very weak. and well below what the manufacturer says. But if your tanks are very low on propane and you get shrinkage during a cold snap, you can get erratic pressure. To further check this issue, light all the cooktop burners and operate the water heater on propane. Can you do it?

If all the furnace parts on the OP's furnace are original, the board is well into its life expectancy and is still the most common cause of this problem.

Steve

 

I've got a few parts on order including a board from Dinosaur Electronics. The parts aren't horribly expensive (except for the board) so I'm going to throw parts at the problem.

Testing this could be challenging. It only failed to light during two nights when it got cold, after running OK for at least 5 days.

I didn't buy a new propane regulator. I need to look at what I have first before I buy another one. I found one that has a higher BTU output than most. How will the components in the trailer react to getting more propane?

 

Having the ability to deliver more propane i.e. BTUs per hours will have no bearing on your appliances and is of no value. The higher output regulators came about with the advent of larger furnaces, refrigerators, and water heaters as they had larger orifices and thus used more propane. In your case there is zero advantage and zero disadvantage other than perhaps cost.

Steve

Steve

 

The propane grill I connect to the side of the trailer seemed to be underwhelmed by the amount of propane it got. That's one reason I was thinking a higher BTU regulator might help.

 

You really need to be reading pressures to answer these questions, although I will tell propane supply is very seldom a problem anywhere except at the gas vale at the appliance and if it were, the first thing to do would be adjusting the regulator you have, but you must have a manometer to do that.

Most RVs handle an outside grill without a problem and most of the RVs with regulators capable of delivering more propane seldom require them. However, you will not harm anything by going larger. It is like going with a larger service panel at your house. If you don't need more amperage what purpose would it serve?

Steve

 

If the propane stove you are using ALSO has a regulator you may be getting reduced flow because of that.... I think I read somewhere you need to remove the regulator from the gas grill before connecting to the RV gas system.

 

Yep, I did that. The regulator/valve was removed and replaced with just a valve.

 

Hum, did those folks remove the redundant regulators in all their other appliances?

Steve