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Original Duotherm Thermostat question

August 03, 2018, 01:20:34 pm

Howdy Gents,
As I recondition systems on my 1992 U280, I have developed a problem with the bedroom thermostat.  It works good when fan is selected to low or medium.  When switched to high or auto, I get the low power red light on and it shuts down.  By selecting low or medium again.  I pulled the thermostat out from the other unit up front and plugged it in at bedroom and all is fine.  Works like new.  I have changed out the Integrated Circuit chip on the bad thermostat and it still doesn't work in high.  All transistors, capacitors and resistors check good.  On off switch seems to be working properly and the board shows no damage from shorts etc.  Anybody think of something else to check?  Unit has a differential temperature of 25 degrees on a 88 degree day.  I can exist in low or medium, but now have my teeth into this fix and want to get my HIGH.  Thanks for any responses.

jk

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Re: Original Duotherm Thermostat question

Reply #1 – August 03, 2018, 02:23:44 pm

Have you tried the bad unit up front?  If it does the same thing I would then monitor the power level.  Could be that the units sensing circuit for the power level is going bad or that you do in fact have low power.  If power level is right on the unit's cut off level then it will be OK until the high setting takes that last little bit and trips it.

Just a thought.

Keith

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Re: Original Duotherm Thermostat question

Reply #2 – August 03, 2018, 02:43:46 pm

Good suggestion,  Keith. I can check that in short order.

jk                                   

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Re: Original Duotherm Thermostat question

Reply #3 – August 03, 2018, 04:01:43 pm

No luck with the swap.  I'm going to take another look at the switches.

jk

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Re: Original Duotherm Thermostat question

Reply #4 – August 03, 2018, 11:16:16 pm

On my u320 I had thermostat problems , it was the push buttons.. I went on line to digi key and found a similar switch.. Took it apart and relpaced the silver " tinfoil "that they use to make the switch connect.. Be careful the little springs " coil" will try to fly away..reassembled it. . No more problems....I am not certain but the foil must work as a capacitor because I could not find any contacts.

Dave

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Re: Original Duotherm Thermostat question

Reply #5 – August 04, 2018, 02:52:50 pm

If this works, here is a picture of the thermostat.

jk

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