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Rick
Rick, HVAC Supervisor
Category: HVAC
Satisfied Customers: 21532
Experience:  40+ yrs. experience as a licensed oil & gas technician.
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I have a model no cdhs110 furnace and besides having to replace

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I have a model no cdhs110 furnace and besides having to replace the thermocouple numerous times last season it has been a dependable furnace. I purchased and installed myself prob over 20yrs ago. So my problem is I just replaced the thermocouple again yest this time bought a quality one from plumb supply hse. Was told ones from box stores are garbage. Anyway relit pioot last night and tested good stayed lit. Cycled the furnace on tuis morn with stat set at 74, hse temp was like 69 I believe, burners kicked on boower went on heated till reached 71, shut off and pilot went out . Not sure what to do next.

Rick :

Welcome, my name is XXXXX XXXXX I will do my best to help you with your issue.

Rick :

Do you have a strong pilot?

Customer:

Yes

Rick :

Then you may have a faulty gas control valve. Aside from a weak pilot that would get blown out when the burners shut down a faulty gas valve on a unit of this age is the most likely culprit

Rick :

There is one other rather unlikely possibility

Rick :

If there is a terminal block between the thermo and the gas valve or limit terminals on the gas valve then a tripped limit could also cause this

Rick :

Actually the limit wouldn't be connected to this type of terminal. It more likely a spill switch or an overtemp sensor

Customer:

Thats what I was thinking also but didnt want to waste time or money changing if there was something else simple I was missing. Cause really not like a lot of other moving parts. And I know the wall thermostat works cause when air cond runs it works fine. Plus the pilot would not go out if it wasnthe stat. So I eill try replacing gas control box.

Rick :

Just check for the terminals I mentioned in my last comment

Rick :

before you spring for a new valve

Rick :

If these "safety sensor" terminals are on the gas valve they would be entirely separate from the 24 volt terminals that power the valve

Rick :

2 terminals with leads connected to a temp sensor

Rick :

Not a common config but it is a possibility

Rick :

Do you have any more questions?

Customer:

Im looking at furnace now to check sensors

Rick :

OK

Customer:

Ok so I cleaned off one sensor on upper plenum area and relit pilot and am running furnace to see how long will run and if runs the cycle if pilot stsys lit this time

Rick :

That doesn't sound like what I was referring to. I'm not talking about the limit switch/control. The limit would shut down the burner but not the pilot. I'm talking about a sensor with leads connected to the gas valve (not the 24 volt circuit) or a terminal strip attached to the thermocouple.

Customer:

So far still running. Yes is a sensor two wires from wherethermocouple threads into to thetwo term sensor attaches to frt chamber area. Only other wires from gas valve go to connections by where thermostat wires all hook up at transformer

Rick :

That could be the problem. Try jumping the terminals

Rick :

You could have a bad sensor or your unit may be getting too hot

Customer:

Ok furnace just ran the cycle and shut down and the pilot stayed lit.

Rick :

So looks like we figured it out. Any more questions?

Customer:

Will keep an eye on . If any more trouble will try those sensorss first. Thanks

Rick :

You're welcome!

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