HELP -- Furnace Problem --- & It's Cold here

[Dan DeBruin]

I had similar symptons last fall after the summer of no use. I took the intake and exhaust and blew compressed air to clean it all out. There was some kind of junk in there (I think spider residue) because after it worked fine.

Good luck, stay warm.

[Possum]

Originally Posted by harv12

My guess {with the info provided} would be perhaps it's out on a control board limit. If you try to fire furnaces to often with no "proving" switch it should "lock itself out."
You may want to try disconnecting the power from the board; without specifics I can't tell you much, but it should be some type of plastic plug. The breaker may work to do the same thing, but often these boards have built-in capacitance {there's a $10 word} which means that the board could hold power during your breaker on/off cycle.
You may also want to try looking at the pilot/igniter while someone cycles the t-stat on. It may be something is in disarray there; bugs / corrosion...
This is from a guy who owned a company with lot of folks who could destroy an anvil with a toothbrush
Good luck and PM me if you have more specifics... model of furnace etc...

According to the paperwork in the unit, we have a Suburban Gas Furnace and it could be an SF-20, SF-25, SF-30, SF-35 or an SF-42 but because of the size of the cold air return I would think it is an SF-42.

[Possum]

Originally Posted by Dan DeBruin

I had similar symptons last fall after the summer of no use. I took the intake and exhaust and blew compressed air to clean it all out. There was some kind of junk in there (I think spider residue) because after it worked fine.

Good luck, stay warm.

I will try the compressed air fix when we get back home.

[Ontheroadrv]

I'm a RV repair man if it tries to fire but doesn't stay lit the igniter is probably bad. When it heats up it sends small amount of voltage back to the board saying its lit. The other guy saying he turned on and off several times probably heated it up enough for it to send the voltage his is probably going out or maybe really dirty.

[Dunner]

The other guy used it every winter for 6 more years without another problem.

[JimM68]

Originally Posted by Ontheroadrv

I'm a RV repair man if it tries to fire but doesn't stay lit the igniter is probably bad. When it heats up it sends small amount of voltage back to the board saying its lit. The other guy saying he turned on and off several times probably heated it up enough for it to send the voltage his is probably going out or maybe really dirty.

if you can pull the igniter out and clean it with a scotchbrite pad, that will help

[JimM68]

note also that the "sail switch" has to work before they will even try to light.
no airflow = no sail switch = no attempt at ignition.

[wa8yxm]

Some good advice (When they say "Check ____" it is good advice as a rule)

I will give you the process. this is from memory so if I miss something.. Sorry.

Thermostat calls for heat

Control board starts the blowers motor (yes, 2 blowers, one motor)

Air flow closes a sail switch and when this happens:
The control board opens the gas valve and begins sending high voltage to the ignighter points.

Sparks ignight the gas

Now we get a split.. Some furnaces use the same wire for ignigition they use for flame sense, some use two wires.

If you have one wire then after a per-set number of seconds it switches over to flame sense mode and if the sensor sees roughly .48 volts (480 mv) operation continues,

In the 2 wire, when flame is sensed (480 mv on the sense wire) the ignighter shuts down but operation continues...

If flame is not sensed after a set number of seconds the gas valve closes and after a set period of time the blower stops.

IF it is sensed, then when the T-Stat says "Hot enough" the gas valve is closed and after a set period of time (Cool down period) the blower is shut down.

Possible issues:
Bat thermostat
Bad blower motor, low battery voltage, bad connection not blowing enough air
Bad sail switch
Out of propane, low propane pressure, bad solenoid valve
Failure of the ignigiton (often due to excessive spark gap by the way but other things possible)

Failure of the flame sensor

Over temp sensor failed or tripped.

At several steps in the process BAD CONTROL BOARD (more in a line or 3)

And the ever popular "Other"

IF, as in my case, it turns out to be a bad control board, Consider the proper Dinosaur board as a replacement.. I have an attwood, the OEM board lists at like 250. The dino board less than 100, it is better engineered (Protection against what i think killed the original) and the look and feel of that board just screams "Quality" to my trained eyes and hands. Not so much the OEM board.

Oh, and it worked, first time and so far every time.

[Possum]

I was surprised at the quality of the responses I got and thanks everyone for your help. I finally got around to pulling the furnace out to start checking everything and as it turns out I guessed wrong at the size. It is an SF-35 not the 42.

Tried all the possible fixes and finally found (help from someone else with the proper tester) that the electronics board had a hit and miss internally and that explained the almost starting and the nothing happening scenarios. Changed the board and the furnace now works flawlessly.

happy camper again.