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davelinde
01-07-2010, 02:29 PM
We are here in sunny but cold FL and using the furnace a lot more than in the past... of course when we need it the most it quits on us.

Initially I'd wake up in the morning to cold (yes we had propane) but if I cycled the thermostat off then on the furnace fired immediately. I should have taken this as sign of more ominous things to come.

Finally one morning it quit, nada, nothing, would not start up. So we cranked up the electric heaters we'd been using for supplemental heat anyway and I figured I'd look at it later that day. When later came I cycled the thermostat from off to on and the heat started again. Odd we thought. The next day we went through the same dance - heat stops in the night, won't start in the morning, starts again fine 12 hours later...

The third time around this block, not so lucky. The furnace just quit. The control board would make a slight hum when the thermostat went on but nothing else happened. Based on internet wisdom I checked all connections for being loose/corroded and wiggled all wires to see if I could convince the beast to work. Based on additional internet wisdom (backed by the Suburban manuals and schematics) I eliminated a few possible issues and decided this was over my head and called a service tech.

The tech agreed with all my diagnosis and proceeded to replace the control board - no change. Next he concluded my fan motor must be bad, but he directly applied 12V to the fan and it spun fine. Next suspicion was the thermostat (even though we could hear the control board hum/not-hum when the stat called for heat or not). So he jumpered out the thermostat - no change.

At this point he called the Suburban number and described the symptoms - they gave him the answer immediately. Any arm-chair techs here want to guess?

I'll put the answer in my next post.

caissiel
01-07-2010, 02:36 PM
The draw was to high for the Propane tanks and the overflow just shut off would be my 1st guess.

jmgratz
01-07-2010, 02:37 PM
The Propane regular or supply line was bad?

davelinde
01-07-2010, 02:42 PM
so here's the answer to the problem.

It was the sail switch, a mechanical switch inside the blower that senses airflow and allows the furnace to fire ONLY if airflow has closed it. The tech (and conventional wisdom on the internet) concluded that if the fan never started the sail switch was not involved - and maybe for some furnaces this is true. However, on the Suburban furnace in my 2009 the controlled startup is smarter. Before spinning the fan the controller checks to see if the sail switch is OPEN, eg no airflow detected. My sail switch was stuck CLOSED and the controller would not start the fan with the switch closed. Conversely it will not continue the startup sequence if the switch does not transition from open to closed once the blower has run a little.

Actually, this makes sense - if the switch is stuck closed it no longer provides the protection it was intended to give. The fix was easy - just take the crud out of the switch that made it stick and re-install.

I pass this along to maybe save others a cold night and a service call charge. I also learned a few things about the furnace. The control board looks like it's buried out of reach, but - in fact - there are two easily reached screws that hold the bracket that holds the board. The entire assemble slides out for easy servicing. I also assumed the only way to get at the sail switch was to remove the blower... not true. The blower shroud comes off pretty easily and the switch is attached to the shroud. Actually... a look at the exploded view in the manuals would have told me all this - but when you are cold and annoyed and confused you don't always read manuals.

Hope this helps someone!

caissiel
01-07-2010, 02:48 PM
Well done, but why did it restart after refusing to restart when cold. Must be that in colder air it stuck and was freed in warmer air????

davelinde
01-07-2010, 03:06 PM
Well done, but why did it restart after refusing to restart when cold. Must be that in colder air it stuck and was freed in warmer air????

Here's my guess that fits all my symptoms.

The switch closes with airflow then opens with gravity when there's no airflow to close it. The switch acquired some "crud" somehow which was making it stickier each time it cycled. During the night, with no motion in the FW the furnace would go from on to off and the switch would stick. When it got cold again the thermostat would kick on but the control board would sense the switch fault (closed when it should be open) and lock out the firing sequence. Later when we walked around the switch would open from a slight shake but the firing sequence was still locked out. When I cycle from on to off and back to on at the thermostat the firing sequence was re-attempted and worked (at least once).

As the switch got stickier it took longer and longer for something to jar it open again to the point where it finally had a "hard" failure.

This kind of failure could be really annoying since I can imagine corrosion or crud or something making it very intermittant. I was VERY happy the switch was clearly stuck closed when we pulled it out. Other ways to get at this could be to tap on the unit to free the switch (tried that in my case didn't work), or more cleverly I could have turned the unit off and read continuity through the switch without disassembling it (now that would have been smart of me). Or, I could pull a lead off the switch before attempting to fire to see if forcing it to open cleared my problem.

I do not know how common this is - but lots of t-shooting guides indicate a sail switch as a first thing to check. The problem is the guides seem to assume the problem will be a switch that's stuck open... obviously they can stick closed too.

TXBobcat
01-07-2010, 03:28 PM
How about a picture to see what your discussing..

Some of us old goats are visual and need all the help we can get..

Good info.. Thanks for the information.

BC

caissiel
01-07-2010, 03:37 PM
Here in the north a lot of times things just freezes and gets hard to move like linkage, greased bearings, and I would also guess that crud could also harden.
At times we see it freeze and only thaws in the spring Ha ha ha ha ha.

davelinde
01-07-2010, 03:42 PM
http://www.rverscorner.com/articles/furnace1.html#sail

has a sail switch picture.

http://bryantrv.com/docs.html#furnace

has a bunch of manuals, you can find some exploded view drawings in there.

btw - this is accessed from outside. There are 4 screws holding the access panel in place and 6 more screws holding the exhaust vents to the access panel. You can get the whole thing off if you take out the 4 screws and just pull straight back, or just take out all 10 screw and it comes off in parts to reveal the guts.