Suburban Furnace issue

[arhayes]

We came home (full timers) to our 380TH after running some errands to find a noise inside we couldn't figure out. Turned out the furnace was on but the blower was not. It's 80 degrees outside and the bedroom AC was on, but the main unit was set to Auto rather than Cool. It was not running, but apparently thought the furnace should be? I turned the thermostat to "off", but the furnace continued running. I finally pulled the fuse which got it to stop. Anyone else had an issue? I see nothing in the manual covering this. I reinstalled the fuse and the furnace did not come on again, but now I'm a little nervous to reinstall the fuse without us here. Our Momentum has a Suburban furnace. Thanks.

[TucsonJim]

I don't think you have an issue. With the main thermostat set to auto, and the bedroom AC turned on, the coach probably dropped to less than the set point temperature on the main thermostat. So the heater turned on.

I've owned quite a few RVs with different brands of furnaces. On all of them, the furnace fan will continue to blow for quite a while after you turn the unit off. This clears all the hot air out of the combustion chamber and is a safety feature. If you'd let it run another couple of minutes, it would have shut off on it's own. When you put the fuse back in, the combustion chamber was already cool, so it did not turn back on. If it happens again, turn off the furnace and let the fan run it's course without pulling the fuse. This will allow the combustion chamber to cool down safely.

Jim

[arhayes]

Jim- the concern was that the furnace fan - tied to unit #2 - was NOT running. Just the burner was going. The set point for heat was set at 59 and the inside temp was 76. The AC for unit #1 was running. I usually only select either "cool" or "Furn", so not sure how we had it set on "Auto". It may be that the burner had just turned on and the fan had not started when we discovered it. The last time we'd used the furnace was in February since we wintered in Florida.

[Dave and Monica]

Jim- the concern was that the furnace fan - tied to unit #2 - was NOT running. Just the burner was going. The set point for heat was set at 59 and the inside temp was 76. The AC for unit #1 was running. I usually only select either "cool" or "Furn", so not sure how we had it set on "Auto". It may be that the burner had just turned on and the fan had not started when we discovered it. The last time we'd used the furnace was in February since we wintered in Florida.

That shouldn't EVER happen that the furnace burner is fired but no fan running. The furnace control is set-up that the fan has to run to close a sail switch before the gas valve will open and start the ignition sequence. If the furnace fan isn't running you don't have any combustion air being drawn into the furnace either besides recirculation air...very dangerous!

Alan...perhaps a silly question but how did you know that the furnace burner was running...sound? Any chance that it was the water heater burner that you heard?

Dave

[scott42blue]

That shouldn't EVER happen that the furnace burner is fired but no fan running. The furnace control is set-up that the fan has to run to close a sail switch before the gas valve will open and start the ignition sequence. If the furnace fan isn't running you don't have any combustion air being drawn into the furnace either besides recirculation air...very dangerous!

Alan...perhaps a silly question but how did you know that the furnace burner was running...sound? Any chance that it was the water heater burner that you heard?

Dave

Times two on sail switch.
Dave has given excellent advice

[JustinThyme]

I just reviewed the schematic and service manual for the suburban furnace. Before the module that controls the gas valve and igniter even gets power it has to go though a high limit switch and a sail switch. If either of these are open it cant open the gas valve or activate the igniter. When the Tsat calls for heat the W wire is energized which pulls in a time delay relay relay (this is for the blower and comes into play at the end of the cycle) and starts the blower. Once air flow is established the sail switch closes providing power to the burner control module. The module monitors DC voltage and so long as it is above 9.5VDC it will begin its sequence, If not it goes into a standby mode until the voltage is above 9.5VDC. When the sail switch closes the burner control module is energized and begins a 15 sec delay timer for a purge cycle. Once the 15 seconds is up it starts the ignition sequence for 7 seconds by opening the gas valve and energizing the igniter. If the probe does not detect flame within that 7 secs the sequence starts again at the 15 second purge. It does this three times, if for 3 cycles there is no ignition detected the module locks out the burner and illuminates the fault light and the blower will run for 3 minutes then shut down. If it does ignite and all it good normal operation will continue until the tstat is satisfied and the W wire de-energizes. This is where the time delay relay mentioned earlier comes in. Its a 90 second delay on break which will keep the blower going 90 seconds once the gas valve is opened and the flame goes out. At the end of 90 seconds the blower shuts down. The only possible ways it can run is either a severe miswire which was ruled out as it would do this from day one and would not stop by pulling a fuse. The other would be if the sail switch some how or another is stuck closed and the blower motor is kaput which even then it would only run a short period of time before the temp in the burner exceeds the high limit switch which would shut it down until it cooled then repeat the cycle.

The safeties on these are pretty darn good, they have to be just because of the nature of it. The only time I've ever seen any type of furnace catch fire was a result of someone intentionally cheating all the safeties to get it to run. 100% of the time when the safeties fail and no one messes with them the furnace wont run.

The auto on the Tstat is for the AC fan.

[arhayes]

It was furnace. The water heater was in "electric" mode. It sounds like the blower must have been on - maybe I just wasn't identifying the sound, but I opened the basement door and pretty much just heard burner - not fan - BUT did not put my hand over a floor vent to verify. We just heard a noise we initially couldn't identify since we've rarely had to use the furnace this first 9 months on the road.

I reinserted the fuse and it has not come on again. May have just been a one off where the #2 unit temp sensor thought it was below 59 degrees and since it was in Auto mode kicked in the furnace. It couldn't have been running for long because the rig was not very hot and the propane bottle is still pretty full. About the only propane usage we have is when we're moving and it's required to keep the fridge cold.

[Dave and Monica]

Hope it's an isolated incident but I'd be suspicious of the tstat. From what I understand, the 'auto' setting is just for fan selection...you still have to select heat or cool and temperature set point...the furnace is controlled from the primary tstat only, correct? I'd advise you to talk to Customer Service at 574-825-9679 and see what they say.

Dave

[arhayes]

I do have to backtrack a little. We moved today and after getting set up in our new digs I forced the furnace "on" and discovered that the blower WAS also on, so I misled everyone. That fan is quiet, because even in the basement I was mostly hearing the burner. My apologies.

On the Auto setting, there are actually 2 types . Each unit (our Momentum has 3) allows you to choose a lo-med-hi- or auto fan speed. On units 1 & 3 you can select either Fan, Cool or Off. On unit 2 you can select Fan, Auto, Furn or Cool. You do set a cool temp and a Furn temp separately. My understanding is that setting Auto mode would allow the thermostat to call for either AC or Heat.

We'll be in Indiana next month, so if e have another problem between now and then then I'll set up an appointment to have it looked at. I guess I was a little panicked when I figured out it was the furnace and didn't investigate as much as I should have. Still - there was some kind of hiccup and I'll hope there's not another. It aounds like there are enough safety checks in place that the worse that might happen is having a hot rig and no propane.��

[JCR GD]

Soooo are these systems smart enough not to compete? Can zone 2 be set to 80 on Furn. & zone 1 be set to 70 on A/C and they will each be trying to over come the other through the hallway?