Furnace problem!!!!
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From: Roanoke, VA
Furnace problem!!!!
If I have my furnace not turned up high, let's say 70 degrees it will run fine and cycle on and off. If I turn it up high (75 or higher) it will run fine then after running for a while it will start blowing cold air.<br>Any ideas as to what it could be?<br>Last year it was doing the same thing, so I took out and looked at it and the squirell cage had a mud daubers sp.? nest in it, so cleaned out and thought all was fine.
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Re:Furnace problem!!!!
Sure sounds like my Duotherm out in my '88 Kit... Or else its ugly twin - I've been fighting ours since it was new - when I went to a RV salvage yard, they said I'd get more reliable heat by starting a campfire in the middle of the floor - they said they have so many problems with Duotherms they won't even sell them used...<br><br>Good luck - I've had mine apart so many times I can do it blindfolded in my sleep - new solid state module, new thermostat - had the gas valve all apart - NOTHING provided more than temporary operation...
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From: Ft. Oglethorpe, GA
Re:Furnace problem!!!!
Sounds like a high limit or airflow problem to me. If your fan is running slow it will allow air to stack up and the high limit can pop. Also, if you have restricted supply or return ducts that can cause the high limit to pop also. Pretty easy to check if you can get access to a thermometer with a probe (or an infrared thermometer) and a meter. Not real sure about the proper temp range on a RV unit but it should be stamped on the high limit device. Homes are usually in the 150 - 190 range. (190 is a bit rare though). Set your meter to check continuity and turn the furnace on. Watch for the high limit to open and see what temp it did so at. Compare that to the rated temp and it will tell you if the switch is bad or if their is an airflow problem.<br><br>Good luck
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Re:Furnace problem!!!!
Been ALL thru mine umpteen times - all switches check out fine with and ohmmeter, including the sail switch - replaced the thermostat with a neat digital type from Walmart many other users swear by - still intermittant and unreliable operation.
Currently mine will try to light 3 times, and the pilot WILL come on - but the main burner won't come online, so the pilot goes out... Acts like a thermocouple problem, but these things use a weird thermocouple setup I don't understand, and I don't know how to test it - it's just a wire lead that goes back from the burner area to the control circuit board - I have 2 of those boards, and both operate the same way... My sail switch is fine, easy to see and check for proper operation, and it does... The limit switches are easy to check as well, with a little applied heat and an ohmmeter...
Currently mine will try to light 3 times, and the pilot WILL come on - but the main burner won't come online, so the pilot goes out... Acts like a thermocouple problem, but these things use a weird thermocouple setup I don't understand, and I don't know how to test it - it's just a wire lead that goes back from the burner area to the control circuit board - I have 2 of those boards, and both operate the same way... My sail switch is fine, easy to see and check for proper operation, and it does... The limit switches are easy to check as well, with a little applied heat and an ohmmeter...
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From: Ft. Oglethorpe, GA
Re:Furnace problem!!!!
Gary, I've seen some of your posts so I know you are technically proficient (way beyond me when you start talking about air volume etc :-)). Sounds like you have checked everything. Main burner not coming on sounds kind of like incorrect gas pressure. Had a manometer on it? I think I read that most rv furnaces run around 11"?<br><br>Also, that board sounds like a board honeywell used to use on big gas heaters. Flame proving wire could only be checked with a honeywell tester. Real pain.<br><br>BG: sounds like you found the high limit. I think they are pretty cheap if you can't test it.
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Re:Furnace problem!!!!
YUP - I made my own "custom" manometer out of some clear plastic hose and a jar of water - set my gas pressure carefully, then tried a bit plus and minus - no help... :
:<br><br>Big problem, is no availability of parts, and what parts ARE available are horrendously high in comparison to the value of the whole furnace assembly - the spare circuit board cost me $125, and the next suspect part, the burner assembly/gas valve is about the same price...<br><br>I'm about ready for the campfire in the middle of the floor bit... 
:<br><br>Big problem, is no availability of parts, and what parts ARE available are horrendously high in comparison to the value of the whole furnace assembly - the spare circuit board cost me $125, and the next suspect part, the burner assembly/gas valve is about the same price...<br><br>I'm about ready for the campfire in the middle of the floor bit...
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Joined: May 2002
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From: Chicago Metro Area
Re:Furnace problem!!!!
Try using a jumper (bypass) on the high limit switch, see if that works . Then you could at least eliminate that component. To me it sounds like a high limit switch. <br>Just my $.02
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