Duo therm

[Lyle Johnson]

A friend owns a 1984 Beaver ...having trouble keeping duo therm going...lights up but will shut down after about two minutes...
Could it be the thermo coupler?
Thanks ,Lyle

[Joel Ashley]

Domestic/Duo-Therm made several appliances.  If you're talking about his furnace, it sounds like it might be the sail switch.  If the fan isn't blowing fast enough to make sure the burner plenum doesn't get too hot, the sail switch won't move far enough to keep the system from cutting off the gas within the first couple of minutes.  

But there are other possibilities, since more than one functional requirement has to be satisfied upon startup and during operation (gas flow, air flow, burn air supply, room temp.) or it shuts down.   Does his gas stove have enough pressure to work properly?  If not, it could be his propane regulator -affecting stove, furnace, hot water, and fridge.

I suppose a thermocoupler could be the culprit if it failed to register the flame presence.  Or as mentioned, the switch in the furnace compartment that senses gas flow.  Or even a dirty burner nozzle - a good thermocouple may be picking up an existent but inadequate flame;  how long since he had regular service on the unit?

Forgive me, it's been a good 20 years since I tore apart the furnace on our '84 Pace Arrow.

Joel

[Lyle Johnson]

Well...the unit is quit old but in nice shape for it's age...:--)the fan would be my next look...just replaced the combustion fan in my hydronic befor we left.  Thanks for the info

[Dennis Crawford]

Is this the old type with a pilot light or one of the newer electronic starts?

Dennis

[Tom and Pam Brown]

Not exactly sure what appliance we are talking about.  However when my LP gas detector went out my stone nor my fridge would work until it was replaced.  The detector shut off the flow of gas once replaced all was well.

Not sure if this helps, but it made me feel better to try and help.

[Joel Ashley]

If his LP sensor was shutting down the gas at the tank head, his unit wouldn't light up and burn for 2 minutes.  The only pilot light on our 1984 rig was in the oven.  The fridge, water heater, stove, and furnace all used electric igniters.  

The 2 minute initial burn is indicative of one of the sensors shutting off the furnace gas valve because something wasn't normal or adequate for proper operation.  Low battery voltage, for example, might make the blower fan run too slow so that the sail switch sensed inadequate air movement within the two minutes, and shut things down.  As previously mentioned, a number of other things, from thermocouple to gas pressure, could also initiate shutdown after an initial startup.

Joel

[Lyle Johnson]

The water heater is working on gas...so it seems the gas pressure is fine...it has a pilot lite...the flame when on is blue and seems to have good flame...

[Joel Ashley]

That narrows it down a little.  But check that adequate combustion air is available too, as well as a clear exhaust.  Mud wasps are notorious for crawling in the intake/exhaust ports on the outside wall during periods of coach storage.  Their nests can effectively block fresh air or exhaust ducts.  A flashlight examination through the outside fitting may reveal something.

Otherwise, the thermocouple and sail switch (adequate battery voltage to spin the fan enough) would get my attention, and maybe a removal of the unit and tear down to clean the nozzle and burner box.  He may want to leave that to a pro that can warranty his work.

Joel

[Lyle Johnson]

Ok...I will check the exhaust...good idea...the unit has been sitting a long time.
I will also check where that "sail" switch is and take a look at the condition.
Thanks,
Lyle. (OSU-Dad)...:--)

[Joel Ashley]

The sail switch has to be somewhere inside the ductwork, in or very near the furnace housing itself, where it can be activated by air flow as the fan draws from the return air vent.  The switch itself may be fine, and it is usually more an issue of inadequate battery voltage available to power the fan at full speed.  So make sure the batteries and post clamps are up to snuff and check all the furnace wires for integrity and solid connections.

Sometimes the thermostat itself on the wall is the culprit, but the symptoms dictate checking the most likely causes first.  I wish I could give more details, but its been too many years since I had to work on mine.

Joel
(OSU Grandpa - College of Veterinary Medicine.  . )

Addendum:  as previously mentioned, if the unit hasn't been checked by a trained tech for a number of years, it might be a good idea now.  I know it costs a bit, but the reason I pose the option is that the burner box is lined with thick heat tolerant ceramic that can develop micro-cracks over time.  An older unit like this is very likely to have the problem, which can be dangerous.  The burner flame and heat can seep through the cracked material, presenting a fire hazard.

[Lyle Johnson]

Maybe a good idea....

I'm thinking a new unit would be a better way to go...and possibly cheaper in the long run