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Old 10-28-2012, 06:12 PM   #1
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Hot furnace

Hi all. Is there a certain temp that our furnaces are supposed to run at? I was checking the temp of the Atwood unit in our 89 Southwind this morning and it was almost 200 degrees! I also noticed that the burner cycles off and on before the thermostat is satisfied, going off on high limit but the coach was almost like 80 degrees, so I know the furnace was drawing in pretty warm air.

I am getting a light haze of smoke inside the coach after the heater runs for a while but cant tell where it's coming from. I took one of the duct knockouts off from the furnace and checked to make sure the HX didnt have any cracks or holes. I didnt really smell anything burning either but it took a long time of running for the smoke to clear up. Thanks
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Old 10-28-2012, 06:22 PM   #2
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Make sure you have a working, less than 5 year old, CO detector! If it was the first time you've run the furnace, the smoke could be from dust burning off the heat exchanger for the first run of the season. If it keeps up, I'd definitely have it checked out. Where did you read 200 degrees? Is the flame the proper size and a blue color?
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Old 10-28-2012, 06:39 PM   #3
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Exhaust temps outside the coach will get 350-400 but what concerns me is the limit switch shutting it off before the thermostat. I agree with bob that the smell could be dust burning off. Furnaces are tough to inspect for mud dauber nests without pulling them but I've seen some very clogged.
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Old 10-28-2012, 06:41 PM   #4
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I just replaced mine in my '87, it was acting up and had bubbled insulation on both sides of the power plug on the ground wire.
Over $500 but priceless piece of mind.

If you have smoke and your smoke detector isn't sounding off, fix it!

Pull your furnace and check it out, the HX is easy to remove. Look for cracks and any foreign stuff.

Don't be a stat this winter.
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Old 10-28-2012, 08:59 PM   #5
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I took the air temp with an infrared gun at one of the registers. Flame looks good, heat exchanger looks fine also. Fan seems to be running at full speed.

This motorhome was not heavily used, so I know the furnace is not wore out. Is the gas flow on these furnaces adjustable?
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Old 10-28-2012, 09:05 PM   #6
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I'm gonna guess no but is it possible to drive with the furnace running? If not, how do most of you in the older less fancy coaches get heat while you travel?
We have no heat strips in the a/c's, dash heat wont be enough in cold weather.

I've been tempted to build some kind of heater and pipe hot water back to it from the engine while driving. I guess it would probably need a recirculating pump to help it along also.
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Old 10-28-2012, 10:47 PM   #7
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Other than adjust the air shutter, the burner is not adjustable unless you get another size orifice on the gas jet, which is not advisable. You can certainly drive with the furnace on except in certain tunnels and other areas that have signs saying turn off LP appliances.
My heater has never blown air anywhere close to that temperature. A bare foot would be burned on the vent grate at that temperature. Something is not right.
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Old 10-28-2012, 11:01 PM   #8
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Originally Posted by BFlinn181 View Post
Other than adjust the air shutter, the burner is not adjustable unless you get another size orifice on the gas jet, which is not advisable. You can certainly drive with the furnace on except in certain tunnels and other areas that have signs saying turn off LP appliances.
My heater has never blown air anywhere close to that temperature. A bare foot would be burned on the vent grate at that temperature. Something is not right.
The pressure regulator on the propane tank is new but I guess I should have the pressure checked.
I've been doing some reading tonight and it looks like I may have some poor ducting issues also, may add another vent off the plenum.
I hate to have to replace a perfectly good furnace, guess I'll call Atwood tomorrow and see what they have to say.
I do have a sticker inside the furnace that says max operating and duct temp 200 degrees but thats too darn hot for my comfort.

Can some others check your output temp and see what you get from yours? Thanks
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Old 10-29-2012, 02:36 PM   #9
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I was on hold with Atwood this morning and finally had to give up. I drove to an rv repair dealer this morning and they said that because it is a 30,000 btu unit, it will blow hot but shouldnt be quite the 200 degrees I was describing.
They suggested adding another vent, which I plan to do this evening.
If I had the money to blow on a new furnace, I would buy one in the 20,000 btu range, 30k is just too big for our coach in this climate.
Same thing with houses, furnaces are greatly oversized and dont need near the capacity they are equiped with. The fellow at the rv repair place said that coach manufactures dont know weather an owner will live in Florida or Alaska, so they have to size all furnaces the same. Makes sense to me.
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Old 10-29-2012, 04:06 PM   #10
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Quote:
Originally Posted by vegas39 View Post
I'm gonna guess no but is it possible to drive with the furnace running? If not, how do most of you in the older less fancy coaches get heat while you travel?
We have no heat strips in the a/c's, dash heat wont be enough in cold weather.

I've been tempted to build some kind of heater and pipe hot water back to it from the engine while driving. I guess it would probably need a recirculating pump to help it along also.
I (and several people I know) travel with the rear most roof vent open just a crack in cold weather, the vacuum it creates pulls a lot of air down the back of the m/h.

In my little 24' class C I can keep the very back of the m/h well above 60 even when it's below 0 outside. It also helps keep condensation and moisture down too.

If that's not enough you can get an auxiliary heater, generically called a "bus heater" or "bunk heater" for under $200, see below. No pump required, there's lots of flow to the heater lines from the water pump. Convection helps with the circulation too.

bus heater | eBay
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Old 10-29-2012, 06:42 PM   #11
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What it sounds like is that your furnace is "limiting"? The blower can't move enough air past the combustion chamber to keep it cooled for whatever reason, so heat builds up, causing the limit switch to turn the gas supply off, giving the blower time to catch up cooling the combustion chamber. The burner then reignites, starting the cycle over again.

I would vote with the dealer who suggested adding an air duct - BUT - I would do that only after confirming the existing ductwork isn't plugged? That would include intake and outlets. Either might be the cause. Adding a duct is pretty easy work actually, and not at all expensive?

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Old 10-29-2012, 07:54 PM   #12
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Pull the access cover on the furnace and see if it works properly. That creates a BIG "duct"
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Old 10-30-2012, 12:08 PM   #13
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My furnace was running into the overtemp limit, found the plastic blower wheel was warped and the motor itself had rough spots in rotation. Also the fan made more noise & vibration. After removing the furnace, the ducting was partially blocked by carpet and the fabric underlay below the floor was sagging and blocking the duct. Replaced the motor and the blower wheel, dressed the opening....
Photos here...
HydroFlame 8500 furnace repair
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Old 11-07-2012, 01:24 PM   #14
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I pulled the furnace out and gave it a good inspection, all was well. The furnace has a label that says 130 degree temp rise, so at the 75 degree indoor temp I was running it at, 200 degrees is fine for air output.

I also fixed a couple duct issues, one was kinked and one was way too long.
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