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Old 09-25-2018, 06:29 PM   #1
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Heater question

Hello- I have a 1978 Sunline 21RB with a Duo Therm 65920-017 heater (furnace). The heater hasn't been used in years (if ever) and I'm trying to get it working. The pilot lights, but as soon as the gas valve knob is released, it goes out, and turning the knob to "on" produces no flame. I pulled the heater apart and saw no obvious problems (other than evidence of mice inside the flame box), but then again I've never seen the "guts" of this before. Can anyone suggest any checks that could be done to isolate the problem? Thank you.
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Old 09-26-2018, 07:13 AM   #2
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Wow an old push button heater! It more than likely is a bad thermocouple. The pilot should stay lit as long as the heater is in use.
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Old 09-26-2018, 07:47 AM   #3
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Quote:
Originally Posted by mainah View Post
Wow an old push button heater! It more than likely is a bad thermocouple. The pilot should stay lit as long as the heater is in use.
Double wow....!!!

I agree, the classic cause of bad thermocouple is that the flame blows out as soon as the pilot knob is released. When the thermocouple heats up it is supposed to send feedback to the gas valve that the flame is lit and then allows the gas pilot to stay on. If there is no flame feedback, then the flame goes out as soon as the pilot lighting knob/button is released.

See if you can find a thermocouple replacement. If you cannot, let us know and we can try and dig up online info to find one that will work.

Hope this helps

John
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Old 09-28-2018, 02:11 PM   #4
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John and Mainah-
You were right; the thermocouple was bad. Found a similar one at Ace Hardware and replaced it. Now, when I release the gas knob and turn it to "on", there is a burst of flame, then either it goes out entirely or only the pilot remains lit. Bad main gas valve?
Thanks.
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Old 09-28-2018, 06:58 PM   #5
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Try this there is a section for the type of heater you have maybe it will help. questargas.com/ForEmployees/qgcOperationsTraining/Furnaces/DuoTherm_65900%20SERIES.pdf
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Old 10-02-2018, 08:32 AM   #6
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Here is a clickable link for the DuroTherm manual Mainah posted. Thanks Tim!

questargas.com/ForEmployees/qgcOperationsTraining/Furnaces/DuoTherm_65900%20SERIES.pdf

Let us know how you make out. Always learn something on these older ones.
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Old 10-05-2018, 07:09 PM   #7
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Gentlemen:

Making progress but not out of the woods yet. The manual was a big help. Learning from it that the fan had to run before the main gas valve received power (which it wasn't), cleaned a dirty contact in the thermostat which got the fan going. Still no voltage at the gas valve because the fan won't activate the fan switch. The contacts in the switch are fine because when I push the switch by hand, there is voltage at the gas valve. My current question is: does anyone know what the fan switch actuator arm is supposed to look like? The arm is simply a 2" length of strip steel about 1/4" wide, and it doesn't present enough surface area to the airstream to be affected by it. Could there have been a "sail" of some kind attached to it that fell off or rotted away? Thanks again.
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Old 10-06-2018, 07:26 AM   #8
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There should some type of "sail" it seems be there somewhere in the bottom if it fell off. Hope you find it it's heater weather all ready up here below freezing this morning.
Everything depends on seeing airflow once the T stat is on or it won't fire the two basics of keeping the gas valve open is the sail switch and the overtemp sensor, the sail is normally open and the overtemp normally closed at rest. The wind closes the circuit for the sail then opens the gas valve if the fire box gets too hot the over temp switch opens and shuts the works down regardless how hard the wind blows. I would be kind of cautious about fabricating a sail, too big and it might stay closed too easily not allowing enough air flow like in the case of a weak battery. It very well maybe the same switch in a more modern heater maybe compare the part numbers.
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Old 10-06-2018, 08:29 AM   #9
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I have not been able to find a parts diagram on your furnace model. Yet anyway. Need to find the right part number of the sail switch. They make several of them.

Like mainah said, the size of the sail is special to the size of the blower so it is only as big as needed to make the safety switch work. Your battery needs to be charged up when testing this as a greatly discharged battery will not create enough power to spin the fan fast enough to create the "right" amount of wind to trip the switch. They tested how much wind is needed to not have the system overheat.

Here are a few if the Atwood sail switches. These are "NOT" for your furnace, at least I do not think they are, it just shows on a quick hit mow many different ones there are. I stopped looking for more as I'm sure the are more

https://www.ebay.com/itm/Atwood-Hydr...item282979f3ad

https://www.ebay.com/itm/Atwood-Hydr...item28697036a7

https://www.amazon.com/Duo-Therm-Sai.../dp/B00B2AOVFW

Once a part number is found, it gets a lot easier to find one.

Hope this helps

John
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Old 10-06-2018, 03:14 PM   #10
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Success! The sail switch (Cherry Electrical E22) is correct as found; the 12VDC power was wired backwards causing the blower to run in reverse.


I'd be embarrassed to say how much time I spent chasing these problems down, but it's done now. Thanks to you, John B. and Mainah, for your assistance.
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Old 10-06-2018, 04:21 PM   #11
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Congratulations may you have heat! Must be a small port the air comes thru that's just a standard issue micro switch. Yeah it's hard to get off the ground when your prop turns the wrong way.
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Old 10-07-2018, 11:02 PM   #12
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Great!!! May the heat be with you!!

A squirrel cage fan does blow air when running backwards, just it blows a lot more air running the right direction

Thanks for reporting back, your learning will help many others in the future. And I'll add it to check list of what can go wrong...

John
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