| DUO THERM furnace limit switch [message #65081] |
Fri, 20 November 2009 16:29  |
vernonite
Messages: 5
Registered: June 2009
Location: United States
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My Duo Therm furnace (1978) comes on and runs for about a minute and then flame stops, fan continues (forever) no more heat.
I removed the limit switch and tested it with thermometer prob taped to Therm-o-disc and blowing on it with a hair drier, it opens at about 209 degrees and closes at 179 degrees (specs say 200 off, 170 on, close enough).
When I tested the furnace with limit switch jumped it runs but gets very hot. When air from vents reached 160 degrees I got concerned and shut it down (what is normal temperature of vent air?).
The fan is working good, lots of air from the 3 vents. Why does it get so hot as to trip the limit switch? or does the limit switch open and close during normal heat cycle? If is does then I must have a control module problem as it never recycles unless I turn the thermostat off and on.
If the limit switch is only a safty feature then it seems I must be getting too much flame to cause the overheating. Is there some way to adjust the flame?
John Wolever
Camp Verde, AZ
928-567-3805
[Updated on: Fri, 20 November 2009 16:44] Report message to a moderator |
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| Re: DUO THERM furnace limit switch [message #65096 is a reply to message #65081] |
Fri, 20 November 2009 18:10  |
idrob
Messages: 645
Registered: January 2005
Location: Central Idaho
Karma: 0
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| vernonite wrote on Fri, 20 November 2009 14:29 |
My Duo Therm furnace (1978) comes on and runs for about a minute and then flame stops, fan continues (forever) no more heat.
I removed the limit switch and tested it with thermometer prob taped to Therm-o-disc and blowing on it with a hair drier, it opens at about 209 degrees and closes at 179 degrees (specs say 200 off, 170 on, close enough).
When I tested the furnace with limit switch jumped it runs but gets very hot. When air from vents reached 160 degrees I got concerned and shut it down (what is normal temperature of vent air?).
The fan is working good, lots of air from the 3 vents. Why does it get so hot as to trip the limit switch? or does the limit switch open and close during normal heat cycle? If is does then I must have a control module problem as it never recycles unless I turn the thermostat off and on.
If the limit switch is only a safty feature then it seems I must be getting too much flame to cause the overheating. Is there some way to adjust the flame?
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A generic answer to your questions, as many furnaces work the same.
The limit switch is very often a problem when the unit heats, then will not come back on. The disk thermostat will click back closed, yet the contacts are pitted enough so power will not flow through. A full cool down often makes them work again for one or two cycles, then they stop conducting again. A replacement of the thermo switch is the repair.
Many furnaces really do not have enough outlets and then the temp runs very high and the limit switch must become the control switch. If you have a 30,000 btuh furnace with only 3 ducts, each must cary 10,000 buth, which is a lot for a 3" duct. In addition, although the fan may seem to be running fine, a 30 year old fan motor may have slowed down just enough to appear to be delivering a good air flow, but in fact is not.
It is not uncommon for the limit switch to become the de facto control system, even in new coaches which have too few ducts or too long duct runs.
In general, there is no flame adjustment possible, other than more or less air to make it burn a nice blue flame, with no yellow tipping. The orifice in the main burner is factory set at a particular propane burn rate, and there are no ways to reduce that.
Very hot air is not uncommon from RV furnaces, especially when run against the limit switch. They are marginally designed and everything must work just right to have them work well. As they age, they can get grumpy fast, due to limited tolerances in the original design and installation.
These are some opinions and some facts based on 30 + years of RV's, about 20 in all, plus repairs on friends units. I hope it is of some help to you.
Rob Allen
former owner of '76 x-PB
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| Re: DUO THERM furnace limit switch [message #65144 is a reply to message #65096] |
Sat, 21 November 2009 00:27 |
vernonite
Messages: 5
Registered: June 2009
Location: United States
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Thanks for the information, if I understand correctly its ok for the limit switch to turn off, but ignition should automatically start again after it cools down and limit switch turns back on.
I will try to find a replacement Therm-o-disc.
John Wolever
Camp Verde, AZ
928-567-3805
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| Re: DUO THERM furnace limit switch [message #65161 is a reply to message #65081] |
Sat, 21 November 2009 07:18 |
fred v
Messages: 999
Registered: April 2006
Location: pensacola, fl.
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if the flame go out and won't come back on it is going into "lock out". that means it is trying to re-light but doesn't for some reason and the module locks it out. when you first turn it on the blower should come on for 20 seconds then it lights off.
mine is a Suburban but they are very similar. mine would light off as soon as the blower came on and it would go into lockout on occasion. i replaced the module board and it worked fine for awhile. now mine cycles on the limit switch. it should not do that all the time.
pull the furnace and look for dirt dobbers in the combustion blower. if you find them put a screen on the outside vents.
there is a good manual at:
http://bdub.net/manuals/index.html
replacement module boards:
http://www.dinosaurelectronics.com/UIB_Chart.htm
Fred V
'77 Royale RB 455
P'cola, Fl
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| Re: DUO THERM furnace limit switch [message #65168 is a reply to message #65144] |
Sat, 21 November 2009 07:56 |
idrob
Messages: 645
Registered: January 2005
Location: Central Idaho
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| vernonite wrote on Fri, 20 November 2009 22:27 |
Thanks for the information, if I understand correctly its ok for the limit switch to turn off, but ignition should automatically start again after it cools down and limit switch turns back on.
I will try to find a replacement Therm-o-disc.
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Yes, that is exactly what should happen. Again, these click on thermo disk switches are notorious for this issue of poor contacts.
Rob Allen
former owner of '76 x-PB
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| Re: DUO THERM furnace limit switch [message #65898 is a reply to message #65168] |
Sat, 28 November 2009 23:24 |
vernonite
Messages: 5
Registered: June 2009
Location: United States
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After my previous furnace test I forgot to turn the thermostat off. The next morning I discovered the furnace fan had been running all night. The next time I tried furnace (a few days later) the fan made a squealing noise at startup and gas would not ignite even with limit switch jumped. I suspect the fan is not able to close the sail switch. I am thinking the sealed 31 year old fan bearings are preventing adequate fan speed. My earlier test allowing the 160 degree vent temperature probably did not help. Accessing the fan motor and sail switch will require removal of furnace.
Now I am thinking of installing a catalytic heater. It would be more effecient and not use up battery running fans. I am looking at a vented automatic ignition with thermostat version from Arnie Lind (www.ventedcatheater.com).
John Wolever
Camp Verde, AZ
928-567-3805
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| Re: [GMCnet] DUO THERM furnace limit switch [message #65906 is a reply to message #65898] |
Sun, 29 November 2009 06:19 |
Charles
Messages: 455
Registered: January 2004
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I have found a heat strip in my airconditioner adequate down to around 30
degrees
as long as the 30 degrees wasn't maintained for more than 4 or 5 hours.
Have you tried banging on the unit whit a hammer while the fan was running?
Sometimes the sail switch sticks a little. Vibration of blow with hammer
vibrates it loose.
Charles
----- Original Message -----
From: "John Wolever" <jwolever@msn.com>
To: <gmclist@temp.gmcnet.org>
Sent: Saturday, November 28, 2009 11:24 PM
Subject: Re: [GMCnet] DUO THERM furnace limit switch
>
>
> After my previous furnace test I forgot to turn the thermostat off. The
> next morning I discovered the furnace fan had been running all night. The
> next time I tried furnace (a few days later) the fan made a squealing
> noise at startup and gas would not ignite even with limit switch jumped. I
> suspect the fan is not able to close the sail switch. I am thinking the
> sealed 31 year old fan bearings are preventing adequate fan speed. My
> earlier test allowing the 160 degree vent temperature probably did not
> help. Accessing the fan motor and sail switch will require removal of
> furnace.
>
> Now I am thinking of installing a catalytic heater. It would be more
> effecient and not use up battery running fans. I am looking at a vented
> automatic ignition with thermostat version from Arnie Lind
> (www.ventedcatheater.com).
> --
> John Wolever
> Camp Verde, AZ
> 928-567-3805
> _______________________________________________
> GMCnet mailing list
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Charles Wersal
Duncanville, Texas
26 foot 1975 Glenbrook
Pandora's Box
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| Re: DUO THERM furnace limit switch [message #65927 is a reply to message #65898] |
Sun, 29 November 2009 11:15 |
idrob
Messages: 645
Registered: January 2005
Location: Central Idaho
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| vernonite wrote on Sat, 28 November 2009 21:24 |
After my previous furnace test I forgot to turn the thermostat off. The next morning I discovered the furnace fan had been running all night. The next time I tried furnace (a few days later) the fan made a squealing noise at startup and gas would not ignite even with limit switch jumped. I suspect the fan is not able to close the sail switch. I am thinking the sealed 31 year old fan bearings are preventing adequate fan speed. My earlier test allowing the 160 degree vent temperature probably did not help. Accessing the fan motor and sail switch will require removal of furnace.
Now I am thinking of installing a catalytic heater. It would be more effecient and not use up battery running fans. I am looking at a vented automatic ignition with thermostat version from Arnie Lind (www.ventedcatheater.com).
|
It does sound like your old furnace is just worn out in many ways and it time for a replacement. The Cat heater you found is one of the better ones, especially in that it is vented, but realize three things about it:
1. It is only 5200 btuh, and your original furnace will have a useable output of about 24,000 btuh. So, it is much lower output. That 5200 btuh is about the same as a plug in electric heater, and may or may not be enough for your needs.
2. The Cat heaters have a limited life due to contamination of the surface materials, especially due to dust.
3. The cost is probably similar to a new furnace.
Decisions, decisions, I made a hundred of them today, most probably wrong...grin.
Rob Allen
former owner of '76 x-PB
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| Re: [GMCnet] DUO THERM furnace limit switch [message #66104 is a reply to message #65906] |
Mon, 30 November 2009 12:55 |
vernonite
Messages: 5
Registered: June 2009
Location: United States
Karma: 0
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Junior Member |
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After I removed the furnace I noticed some debris like burned fiberglass possibly interfering with sail switch. It's probably a good thing that it forced me to deal with the old furnace. I am sure more problems were lurking.
I am not planning to camp without electric when its less than about 50 degrees. Hopefully a 5200 BTU Catalytic heater will suffice. I sure can use the extra space given up by the old furnace and less concern with battery drain.
John Wolever
Camp Verde, AZ
928-567-3805
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| Re: [GMCnet] DUO THERM furnace limit switch [message #68175 is a reply to message #68163] |
Wed, 23 December 2009 13:26 |
GMCWiperMan
Messages: 1248
Registered: December 2007
Karma: 1
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Senior Member |
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Robert, I don't know anything about the specific "fan limit control
switch" you're referring to, but NO or NC thermostatic switches with
just about any trip point you want are readily available from
electronic supply houses like Mouser.
http://www.mouser.com/catalog/catalogUSD/640/2340.pdf
Ken H.
Americus, GA
'76 X-Birchaven
www.gmcwipersetc.com
On Wed, Dec 23, 2009 at 12:44 PM, Robert <rbc1965@yahoo.com> wrote:
>
>
> The Therm-o-Disc company sells ONLY to contractors & NOT to the public. I tried to get a replacement furnace fan limit control switch. Good Luck finding a replacement. Your gona need it.
> If you find a company that handles that type of part you should past it on here. I would appreciate it.
> Robert.
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