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About Joe
Expertise
Answer questions pertaining to Residential and Commercial Air Conditioning and Heat Pump systems. Mechanical and Electrical troubleshooting of these units.Be as detailed as possible when describing problem.Packaged unit vs split systems.Give age of eqiuptment.And what type of heat,Heat Pump,Oil, Gas ,or Electric.
Experience
35 years experience, diversified HVAC/R . Installation and Troubleshooting, Mechanical and Electrical, in following areas, Refrigeration( Walk in and Reach in Coolers and Freezers )Commercial Roof Top and Packaged Heating/Cooling ( Natural Gas,Propane, Electric, and Heat Pumps ) Computer Room Air Conditoning Systems,Commercial Chilled Water Air Handlers and Recipercating Compressors. Residential Packaged and Split Systems Air Conditoning and Heat Pumps.Warm Air Oil and Natural and Propane Gas Heating systems.
Education/Credentials
Graduated from Private Technical School in 1975.Presently an HVAC/Refrigeration Contractor. Have Unlimited Heating/Piping and Cooling Contractor License.Limited Sheetmetal Contractors License.NATE Certifield. York Certifield Heat Pumps in 1981. Served 33 years combination Active Duty Air Force ,
Air Force Reserves and Air National Guard in the HVAC/Refrigeration Shop.Starting as a Apprentice then Technician troubleshooting/installing HVAC/Refrigeration Equiptment Worldwide. Retiring in 2003 as a Mechanical Superintendent.
Awards and Honors
I got many atta boy awards over the years
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You are here: Experts > Home/Garden > Home Appliances > Heating, Air Conditioning, Fridge, HVAC > furnace gas valve keeps shutting itself off
Expert: Joe - 11/2/2009
Question
QUESTION: Hello, I have read your post on a similar issue I have. Your post has the subject "furnace keeps goes on and off" on 2/11/09.
Basically, the gas valve would turn on at the beginning of the cycle for about 15 seconds then it shut off by itself. I have tried your suggestion where you said "Remove tstat from subbase and jumper R to W and start heat ,if unit shuts down good chance tripping on a limit.May need to jumper R to W To G. but try R to W and make sure blower fan runs."
I ran the voltage meter on the gas valve and I could see the power (~24v) come on during the gas valve was opening. Then the main circuit board just shut down the power while the blower was still running. The meter read 0 volt after that.
Please help.
ANSWER: Jak,
you don't mention if the main gas valve relights again or if it shuts down and stays off ?? you can also remove the stat and stat wires from the system by jumpering R,W,G at the circuit board.If you still have the problem and you are sure all your safeties are not tripping , ( you can try checking your safties.should have 0 volts across the contacts when closed while unit is running.) to me it would be the board. ( you do not mention the age of your equipment ) Difficult to troubleshoot via Internet. Boards can be bought on line ( goggle hvac parts ) Also remove circuit board from unit ( if not to difficult ) and check reverse side to see if there are any burnt hot points. Make sure board has good constant 24-26 volt ac feeding it from your transfomer.Anything higher or lower may cause issues.When you lose the 24 volt at the gas valve ,check to make sure you still have the 24 volt feeding the board.Make sure all low voltage wires are secured ( tighten electrical connections )Make sure you have good air flow thru your heat exchanger, filters cleaned all supply registers open ,good return, if you have Central A/C that your indoor coil is clean. not moving enough air acroos the HX will certainly trip safties,
---------- FOLLOW-UP ----------
QUESTION: Hi Joe
Thank you for your quick reply. The main gas valve does not relight again and the blower is blowing cold air for about 20 mins before it stops. How do I check for my safties (I don't understand your term "safties are not tripping"?
My unit is about 12 years old (I think as I bought this house 5 years ago.) I will look up the model and troubleshoot the circuit board when I get home this evening and let you know the result.
Answer
Jak,
look at the wiring diagram ,usually attached to the inside of the blower door. usually safties are normally closed . High limit control, roll out switch etc. gas pressure to low or high. you can ohm out the safties by removing the wires hooked up to them should have a audio sound when they are closed ( if using a digital and having a audio mode ) if you have an old style meter. the needle should move to the opposite side. guessing that your safty controls are ok on start up as you state the gas valve works for 15 sec.So you would have to check them each within the 15 second window, by going to volt scale and putting leads on each safty terminal. if the safty does not trip you should read 0 volt.if it does usually you will read 24 volt.
In closing find your safty controls via the wiring diagram and check each on inital start up ( should read o volt ) leaving leads on control see if it trips ( probably 24 volt at meter ) you only have to wait a short time .If it does not move on to another safty control.How soon does your blower fan start after youe gas valve ignite?? if it's too long a period it may be tripping out on a high limit before the fan starts.What turns on the fan a time delay relay??? or the old sytle type combo fan control, high limit imersion type?? like i said hard to troubleshoot via internet
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