Intermittent Furnace Issue

Question

It's an Atwood 8516-III manufactured in 1994.

The furnace runs fine with normal noise and air flow but sometimes the fan will run but the thing won't ignite. This is intermittent - may be two weeks and many firings between failures. Once it fails, it sometimes works after three or four tries but last time wouldn't light after many attempts. Working fine now.

I figured the sail switch and limiter are good because it works almost all the time. Is that a correct assumption? To fix it I installed a Dinosaur board. Of course, it worked great until the old problem resurfaced.

I've cleaned all the connections I can get to and don't see any obstructions anywhere (have blown out with air).

This morning I had what I think is an unrelated problem - thermostat failure. Flipped the switch and nothing happened. Checked for current - was good. Swapped out the thermostat with my other one and it works fine. Is it possible a bad thermostat could be the cause of the original problem (don't think so)? It's an after market Luxpro.

Anyhow, thanks for any input.
jor

artum_snowbird · Answer

In cases like this, I would start with the sail switch.  If I was troubleshooting, I would run wires out of the unit from both sides of the sail switch, and if you get a failure, try connecting the wires and seeing if it works to prove that it is or is not the sail switch.  It is a tiny contact that could be pitted after many years of use.

enblethen · Answer

Locate and blow out the sail switch. Possible that a small piece of debris is causing issue.
Next check the adjustments on the ignitor and flame sensor.
Inspect the ignitor/flame sensor for a small crack in the porcelain.

joraz · Answer

OK, that's two sail switch votes but is it possible to get to the sail switch without pulling the motor? I see it on the drawing but don't see it looking from the outside. Wish I were home to do this. Come to think of it I guess I am home.
jor

enblethen · Answer

Yes, it is on the top of the fan housing in the back.
To use a can of air with the straw would be very easy.
To get to the wiring, maybe a little harder, especially for those of us with fat fingers.
The switch it self is outside the fan housing with a small arm sticking through a hole in the fan housing.

dutch_12078 · Answer

When we bought our small Class C years ago, we had a similar problem with an intermittent furnace. On inspection, I found that the "sail" had a coating of frying grease and dirt on it causing it to fail sometimes when the battery voltage was a bit low, but not low enough to make the converter/charger kick up to the next level. Cleaning the sail with dish soap solved the problem.

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