Furnace heat is intermittant

MaxAZ · 11-27-2007, 05:40 AM

I spent last weekend at Glamis and the furnace only worked correct 1 night. It is a Suburban SF-30F (Stock # 2301; SN 981704616), about 9 years old. The fan does not seem to come up to full speed as it used to so I suspect that the motor is not strong enough to activate the vane switch. I did notice there was more heat blowing out the exterior exhaust vents than through the interior ducted vents. There's almost no airflow inside at all. Seems odd that I'm getting heat out the outside vents. My problem for testing is the way the unit is mounted. I can't get to the motor or vane switch because that side of the unit is up against an exterior wall so it appears I'll have to remove the whole unit and test it on the bench somehow. I'd like to test the 12V supply first prior to removal and would like some info on the wiring. What's 12V supply, what are the control (thermostat) wires and how do I identify them?

MaxAZ · 11-27-2007, 05:40 AM

I spent last weekend at Glamis and the furnace only worked correct 1 night. It is a Suburban SF-30F (Stock # 2301; SN 981704616), about 9 years old. The fan does not seem to come up to full speed as it used to so I suspect that the motor is not strong enough to activate the vane switch. I did notice there was more heat blowing out the exterior exhaust vents than through the interior ducted vents. There's almost no airflow inside at all. Seems odd that I'm getting heat out the outside vents. My problem for testing is the way the unit is mounted. I can't get to the motor or vane switch because that side of the unit is up against an exterior wall so it appears I'll have to remove the whole unit and test it on the bench somehow. I'd like to test the 12V supply first prior to removal and would like some info on the wiring. What's 12V supply, what are the control (thermostat) wires and how do I identify them?

Workshop · 11-27-2007, 12:10 PM

Max,

If you don't have the manuals, here are two links: SF series install and Parts manual they might help.

You mentioned that the unit worked correctly only one night. Were you operating on battery or 120v when the problem occured? It will be normal to have hot air flow out the furnace exhaust and it will be separate from the interior airflow which is generated by the fan's rpm and the temps on the inside wouldn't be as hot as the outside exhaust because of the heat loss of the ductwork and airflow.

Your question, what is 12v supply, it is the 12v supplied by the battery or the converter when connected to 120v. If you do not have the correct voltage because of a weak battery or converter, the furnance will not operate correctly.

All things being equal, might check the duct work at the casing and make sure they are connected. Your thermostat, mounted on the wall, is basicly a switch that tell the heater when to come on. If you can raise the demand for heat and the unit comes on, lower the arm and it goes off, I would guess the T-stat is operating correcly.

Hope somthing helped.

MaxAZ · 11-27-2007, 05:21 PM

Those manuals should help alot. It shows the wiring diagram that I wanted so I could test the 12V supply. I wish mine had exterior access like the drawing showed with the outside door, it would make troubleshooting a whole lot easier. I'm think I'm stuck pulling mine out and sticking it on the bench to work on it.

MaxAZ · 11-29-2007, 04:58 AM

I've had the furnace out and on the bench for testing. I applied power to the motor and verified that the vane switch does engage so the motor has enough power and RPM to flow air. This thing is so modular it's hard to see how to work on it though. How do you test stuff on these things when you can't even see them?

Workshop · 11-29-2007, 11:07 AM

Now sure if we are suppose to see what we're doing.

Have had the Atwood furnace in my units and lucky enought that I've never had a problem with them. Here are two Atwood sites that are interesting and might help.

Atwood Training Manual, it is a big file so wait for it to open. Covers a lot of Atwood products. This is a Video of the furnace operation and furnace info.

Hope something helped.