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Old 11-13-2011, 12:19 AM   #31
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Heat - short cycling


16.7 seconds for one spin of the .01 dial if the calorivic value for the gas in your area is 1050. One of the many reasons why we use manometers as a basis for gas valve adjustments is every .1 of a second you are out from 16.7 will vary the readings by about 500 BTU's.

I would do another clocking of the valve before you adjust it.
Let the furnace run for 5 minutes to stabilize before testing.
Make sure no other gas appliances are running.
(3600 times 1050 times .01) divided by the number of seconds it takes the .01 dial to rotate once. Multiplyed by 35.31
This only applies to your furnace for the numbers that you have supplied.

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Old 11-13-2011, 12:17 PM   #32
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Heat - short cycling


problem with that is if the gas cock is full open the furnace only runs for 2.5 mins before triggering limit. I am going to adjust today. I will full open cock valve, adjust valve half turn, and start clocking from there. Let it stabilize like you said and tune in small amounts. I will let you know 'how it goes'...
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Old 11-13-2011, 02:53 PM   #33
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Heat - short cycling


I don't think your gas valve is over gassed enough to account for your furnace limiting out in 2 1/2 minutes with the fan running constantly.
Your clocking is not high enough to account for it all. Your manual gas cock restriction to the furnace is compensating for either a faulty fan/limit control, a yet as discovered exchanger crack that is affecting the fan/limit controls, failing motor capaciter, something that is limiting air duct volume or one of the other causes listed in my 2011/11/11 6:19 posting.
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Old 11-13-2011, 03:00 PM   #34
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Heat - short cycling


How,

I have finally resolved the issue! I was able to dial in the pressure to the valve and clock the meter. The furnace now runs within spec and no longer over heats!

My Theory:
In the middle of July I had my parents bring in the HVAC guy (who installed system) to shut off the low speed on the main blower. It was costing a lot to run the motor 24/7. I believe he's tampered with the gas valve to create a service call in the fall when the house switches from A/C to Heat. I had to dial back the valve 2.5 turns counter clockwise (it looked bottomed out)!! This furnace has run flawless for 24years, for it to be out that much sounds fishy! The only change was having the original guy who installed it come in and make adjustments to the blower wiring. When he was called to investigate issue in the fall he wasn't interested in fixing the problem, but wanted to install a whole new system!

If I could buy you a beer I would! I really appreciate your expertise and advice!
Till next time!

Doug.

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