Question about Heating & Cooling
Replaced the thermostat and thermocouple
Posted by Anonymous on
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A Furnace Flame Sensor is a safety device that reads the temperature and ... easy fix, but the furnace is ... ? 5:53
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Oct 19, 2004 - 25 posts - 9 authors
After a certain amount of time of not running, the heater will refuse to kick back on. ... 2 flashes, then a 2 second pause = pressure switch is open. What do .... On LP gas furnaces, the spark starts and the gas valve opens. ... Buffing them up with a little steel wool (5 minute job) is a good annual PM thing to do.
Posted on Dec 16, 2015
There is a vacuum sensor on most units tied from the exhaust gas fan to the sensor. Does the unit try to start? If the unit's exhaust fails to pull the switch it will turn off after about 30-45 sec after starting. "IT" thinks it's not exhausting and stops to keep from overheating. Do you have this exhaust fan and rubber hose to sensor? Can you see if that fan is turning on start?
Posted on Nov 10, 2007
If you have a volt ohm meter take the two leads off the gas valve and ohm out the connection should have resistance for the coil. You said you have 12 volts from the thermostat white and red in the heating mode you should have minimum 26 volts. If you want to check the transformer should have 28 volts. Almost sounding like the transformer is getting weak. You can get parts for a 20 year old unit at any HVAC parts house. Try Washer Specialties here in Wichita Kansas they are on the web. Lowes even carries Transformers. Has to be rated 120 volt primary side secondary side 24 volts and 40 VAC.
Posted on Nov 24, 2008
SOURCE: thermocouple replacement
Maybe you need to approach it differently. Why do you assume you have a bad thermocouple each time? If its because the whole flame goes out and you have to relite it, a lot of things can cause that.
1 ohm out a new thermocouple and record the ohms reading (read tween the center LEAD looking area on the end going into the gas valve and the outer shell (copper colored cover) If its a MV type controls it will have 2 wire ends. With the flame running now check the MV-DC on the same area, as the resistance. Yes you will need 2 hands extra for holding the flame on. It wont stay on without being hooked up. Typically you need 400MV for a thermocouple and 600 is the stahdardf output on a generator with wire leads depending on which you have. If your below these ranges You need to blow out the piulot area with compressed air and retry it. You should get an increase in the milli volts after cleaning because now the flame will hit the pilot or thermocouple or generator as designed and dust was causing it to be lazy and not get enough voltage back to the gas valve to stay on.
Now this is assuming you have the correct pilot orfice and its not too small like with an LP orfice installed on a NG system. The pressures are differant and the hole is way smaller on LP.
This is a really common problem with multi changeouts of thermocouples. Dusty Pilot burners. y policy has always been to blow it out first after observing the type and kind of flame. Then reexamine it after cleaning it out.
If this helps you please rate me as high as you can. If I have not given you enough information please post back so I can better aquaint myself with you problem. Good Luck!
Posted on Mar 23, 2009
if you have replaced the flame sensor and it will not stay lit, the draft safety switch may be opening. It works on positive and negative pressure created by the inducer fan motor.You may have a restriction in your heat exchanger or if is a high efficiency furnace, it may not have the drain tube installed properly with a trap. The trap works just like the trap under your sink and if water isnt in the trap, the pressures can be effected enough to open the draft switch(some call it an air pressure switch)
Posted on Jan 07, 2010
In the path of the flame on the last burner that starts, there will be a small rod. That rod is the flame sensor. Take a scrub pad and polish it. Also check the electrical connection that goes back to the circuit board. Disconnect any ground wire from the circuit board to furnace cabinet, sand it and replace it.
Posted on Mar 14, 2010
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