Suburban Furnace

[Pig Rig IV]

I have a suburban furance in my 84 eagle which will run for a while then just blows cold air,  if shut off and back on the hot air will start to blowing again,  usually for about 20minutes?  any suggestion? Thanks Jeff

[Eagle]

Sounds normal to me.

[ryoo]

What I would check is the air switch.  With the unit put a shop vac. to the exhaust (upper) tube and let it run for a moment.  Sometimes the air switch sticks. That should fix your problem.

[pvcces]

Jeff, when you turn on the thermostat, that turns on the fan. If the fan air flow is sufficient to trip the sail switch, it is supposed to turn on the gas valve and the ignitor. When this happens, you should be able to hear the clank of the gas valve turning on, and depending on conditions, you might be able to hear the snapping sound of the ignitor.

If the thermocouple in the burner does not sense the right amount of heat, the board will time out and shut off the ignitor and the gas valve. It will NOT shut off the fan, so the furnace will just blow cold air.

We have seen the symptom that you describe. In our case, the furnace would always light again after the fan stopped, but not before.

Low voltage at the furnace will cause boards to malfunction and to fail. Low voltage does not prevent the fan from running. Measure with a DVM. Voltage should be 12 volts or very nearly that for normal ignition.

If you decide that the furnace needs a new board, I recommend that you do not use the manufaturer's replacement. Dinosaur boards are much more reliable and cost no more than OEM. Current models include a fan shutdown feature for a little more money.

Try this link: www.dinosaurelectronics.com/

Good luck with your furnace.

Tom

[Hartley]

How cold is it outside?

Propane will not vaporize as well when the tanks are exposed to freezing weather.
Small tanks are more prone to this than large tanks. If the tanks are frosting up they may be
getting too cold to have enough output to keep the furnace running and the furnace controls
will shut down because it thinks it's out of gas and will cycle trying to ignite until enough pressure
is available to light up again.....

Just a thought...

[NJT5047]

While not a Suburban, my Hydroflame burns until the thermostat stops calling for heat.  It will warm the interior faster than the surface the thermostat is mounted on which  causes the flame to go out, but the fan will continue to run.  However, the flame will reignite without turning the unit off..may reignite while the fan is still blowing.  As the temp stabilizes, it will cycle normally.   
It doesn't sound like your heater is working correctly to me.  Sounds like an overheat or, as others have suggested, the sail switch isn't working correctly and the unit's timing out.
Insects will get into the exhaust and cause all sorts of odd symptoms.
I'd be contacting Suburban and verifying, beyond doubt, whether the heater's working correctly.   Dinking around with it may prove hazardous.    Replacing the board is easy enough...but, a board may cost 1/3 of the cost of a heater.
The age of the heater may help decide what to do with it. 
 
Good Luck, JR