Unreliable Furnace

Bob&Pat · 10-14-2006, 02:09 PM

At home the motorhome furnace, Suburban SF-42, worked fine off of the 110 for about three days prior to leaving.

However while "dry camping" we experienced the following;

Wednesday, October 11, 2006, at 11:00pm it was 72degrees in the motorhome. I set the Coleman/Mach thermostat for 66degrees, 1:30am furnace fan came on, it did not ignite and the blower shut off.

Went to sleep anyway it was cozy warm in the bed, it was 65degrees in the motorhome.

4:00am woke up it was 60degrees, shut the furnace switch on the thermostat off and back on again and the furnace fan came on and the furnace ignited and we had the heat working off of the inverter and batteries. The furnace ran for one hour and twenty-five minutes until 5:25am and shut off when the temperature was 67degrees.

6:10am the temperature was 65degrees and of course the thermostat was set at 66degrees
but the heat did not come on. Shut the furnace switch on the thermostat off and back on again three times, all that happened was the fan would come on the furnace would not ignite and the fan would stop.

Went back to sleep in the warm bed.

8:00am the temperature was 60degrees and the thermostat was set at 66degrees. Shut the furnace switch on the thermostat off and back on again and the furnace fan came on and the furnace ignited and we had the heat working of the inverter and batteries. It was 27degrees outside the motorhome.

Started the generator and the furnace worked fine just as it should of off the generator power from 8:30am to 10:30am.

Later that afternoon the motorhome was parked and the furnace came on and ignited working of the inverter and batteries once and when it called for heat a second time the blower came on the furnace did not ignite.

Upon arriving home plugged into the 110 at the house and set the thermostat to 60degrees and the heat stayed a constant 60degrees in the motorhome while the outside temperature dipped into the mid 30's.

Furnace seems to work just fine off the 110 and the generator but is not reliable on the inverter and batteries.

Just hate it when things work sometimes and sometimes not, sure enough if you take it in for service it will work just fine for the service technicians.

Any ideas?

Bob&Pat · 10-14-2006, 02:09 PM

At home the motorhome furnace, Suburban SF-42, worked fine off of the 110 for about three days prior to leaving.

However while "dry camping" we experienced the following;

Wednesday, October 11, 2006, at 11:00pm it was 72degrees in the motorhome. I set the Coleman/Mach thermostat for 66degrees, 1:30am furnace fan came on, it did not ignite and the blower shut off.

Went to sleep anyway it was cozy warm in the bed, it was 65degrees in the motorhome.

4:00am woke up it was 60degrees, shut the furnace switch on the thermostat off and back on again and the furnace fan came on and the furnace ignited and we had the heat working off of the inverter and batteries. The furnace ran for one hour and twenty-five minutes until 5:25am and shut off when the temperature was 67degrees.

6:10am the temperature was 65degrees and of course the thermostat was set at 66degrees
but the heat did not come on. Shut the furnace switch on the thermostat off and back on again three times, all that happened was the fan would come on the furnace would not ignite and the fan would stop.

Went back to sleep in the warm bed.

8:00am the temperature was 60degrees and the thermostat was set at 66degrees. Shut the furnace switch on the thermostat off and back on again and the furnace fan came on and the furnace ignited and we had the heat working of the inverter and batteries. It was 27degrees outside the motorhome.

Started the generator and the furnace worked fine just as it should of off the generator power from 8:30am to 10:30am.

Later that afternoon the motorhome was parked and the furnace came on and ignited working of the inverter and batteries once and when it called for heat a second time the blower came on the furnace did not ignite.

Upon arriving home plugged into the 110 at the house and set the thermostat to 60degrees and the heat stayed a constant 60degrees in the motorhome while the outside temperature dipped into the mid 30's.

Furnace seems to work just fine off the 110 and the generator but is not reliable on the inverter and batteries.

Just hate it when things work sometimes and sometimes not, sure enough if you take it in for service it will work just fine for the service technicians.

Any ideas?

Clay L · 10-14-2006, 02:53 PM

Not sure about yours but my furnace doen't require 110 to operate. I believe that is true for most.
Maybe the inverter is drawing the batteries down powering other things that could be running on propane.

smlranger · 10-14-2006, 03:37 PM

Did you by chance note the coach battery voltage during these episodes? I don't know the limit, but if the battery voltage falls to a level that won't spin the fan enough to operate the sail switch, the fan may run but the burner will not ignite. If all worked fine when you started the genset (and when you are connected to shore power), I would suspect a battery issue.

DavidMc · 10-14-2006, 04:43 PM

Suburban did make a dual-voltage propane burning furnace for RVs. Those units also did not have computer boards to control them, just simple electro-mechanical devices.

My 1971 trailer originally had such a furnace which worked through 2004 when I finally replaced it because of a crack in the burn chamber (issue of possible fumes into the trailer).

I agree that the problem may well lie in the voltage levels of the batteries. I second the recommendation to get voltage readings at the batteries (or at the furnace if practical).
Battery voltages drop when current is being drawn (and will rebound slightly when the current draw stops). Battery voltages also tend lower when the temperatures are lower.

A quick web-search on the model number yielded pages that indicate it is 12-volt DC only. Thus it was competing with the inverter for power from the batteries (and not depending upon or using the 110-volt from the inverter).

It also appears that your model is a High-Amperage unit, requiring about 11.5 amps to operate. Multiply that 11.5 times the number of hours you would expect your furnace to run: that will give you x-number-of Amp-Hours. Compare that figure with the amp-hour rating of your battery bank (with the objective of not using more than half of the capacity).

Beabop · 10-14-2006, 05:04 PM

Sounds like the batteries are drawing down. If our DC gets low the fridge flashes low DC. I remember on a trailer we had that only had one battery we used to run the engine awhile every am to keep it up for the furnace.

Old Snipe · 10-14-2006, 05:21 PM

Your furnace has a sail switch in it, basically a switch with a little sail on it that blows closed by the blower fan upon start up and when the fan speed reaches at least 75% of full speed. When it closes, it sends 12 volts to the circuit board and the furnace will function, ignite and produce heat.

The problem is when your batteries run down, they can't supply enough voltage for the blower to turn to the 75% speed, so the furnace doesn't ignite. However, when you are plugged in or on generator power, the batteries are supplemented by the coaches battery charger and full voltage is supplied to the furnace, enabling the blower to run full speed.

As an aside, the blower starts first to blow any residual fumes from the burner before it lights off. It contiunes to run several minutes after the burner shuts off to again vent the burner chamber. It runs this way as a safety feature.

To resolve this problem, have your batteries checked, see if they charge to full voltage, check the cells to see if they are all okay and there isn't a bad cell or two. Unfortunatley, if it's really cold and the furnace runs alot off the batteries, they still can run down. You then should look into adding additional batteries, or changing over to 6V batteries that have higher amp capacity.

Best Regards!

Bob&Pat · 10-15-2006, 04:35 AM

Thanks everyone for your replies and help, looks like I need to check out the batteries. That was my guess but wasn't sure so I posted my experience before taking it in for service.

The coach was delivered October, 2002 that makes the batteries four years old.

Clay L · 10-15-2006, 05:53 AM

I still am not sure why you are running the inverter for the furnace. The furnace will run off the batteries if they are charged.

Bob&Pat · 10-15-2006, 06:09 AM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Clay L:
I still am not sure why you are running the inverter for the furnace. The furnace will run off the batteries if they are charged. </div></BLOCKQUOTE>

I assumed the furnace igniter and fan was 110 and ran off of the inverter.
I thought the batteries fed the inverter and the furnace ran off of the 110.

So when dry camping you shut of the inverter?

John Hilley · 10-15-2006, 06:26 AM

If you're not watching TV or using an AC appliance, in cold weather it is best to shut it off. In standby mode it doesn't draw much current, but you need to conserve all battery power for the furnace. You might also check the condition of your coach battery, it looks like you are close to 5 years on it. You might think of replacing it with higher capacity batteries. I live in North Dakota with similar weather. We use a catalytic heater until we go to bed and then set the furnace to 62 degrees. Another thing is to use as little light as possible as the lights consume a lot of power. We also use XM Radio for entertainment to avoid using the TV and inverter.

Bob&Pat · 10-15-2006, 12:20 PM

Thanks again to everyone for the information.

Internet forums are great we learn so many things and get answers quickly.

I read my owners manual for the Inverter and sure enough it says to turn it "Off" when not in use. In the past it was always left on and of course I turned off the battery disconnect when the motorhome was in storage.

Actually we probably only need to use the inverter during quiet times when we watch tv and need to shut down the generator. That would not be very often.