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Old 11-10-2011, 06:09 PM   #16
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Heat - short cycling


ok so I just opened the top duct on the front of the furnace (panel facing me). I was able to view down inside where the probe sits. There is no clam shell. It simply sits between 2 of the 3 chambers of the heat exchanger.
I don't see any cracks on the exchanger. It really looks in great shape. No signs of wear (24 years old). The old probe was 11.5" long, and the new one is the same.
I will reinstall probe and ensure nothing is touching... So I guess I still don't know what's wrong... Great idea though, thanks 'how'!

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Old 11-10-2011, 10:32 PM   #17
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Heat - short cycling


So while the top of the furnace was open after cleaning the lower main blower filter (metal coils), and upper evaporator coils with Spray Foaming coil cleaner and flushing with water the furnace ran!

But, when I sealed the upper duct back up and forced all the air back through the evap coil the furnace cut out again! I cleaned evap from bottom with spray foam, I will clean again tomorrow from top? Is this the best method. There is no visible dirt, but I guess internally its clogged???

What's the best method to clean the evaporator coil I guess? It's an 'A' coil style... Residential...

Last edited by dk4dk4; 11-10-2011 at 10:43 PM.
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Old 11-11-2011, 12:19 AM   #18
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Heat - short cycling


Common reasons why a gas furnace will fan/limit will trip. The stuff in bold is the stuff I'm not sure that you've checked out.
-faulty fan/limit control. limit temp controls are set too low. (200F is best)
exchanger cracked or probe touching the exchanger.
-The furnace is over gassed. Clocking your meter is the check.
-The fan isn't working properly. Motor capaciter faulty, motor failing, motor running backwards, squirrel cage dirty, cage in backwards,
-Air restriction. dirty filters in furnace, dirty filters in heating outlets, covered or restricted air returns or heating outlets, ducts undersized, ducts shut off, dirty AC coils,

You had mentioned on an earlier post that you were not sure the higher fan speed came on or not. Did that get sorted out? I know you checked for a crack in the wall of the exchanger where the fan/limit was situated but how far down did you check?

Overheating is supposed to be a pretty easy problem to sort out????

Does the furnace limit when you have the duct opened up for the exchanger examination?

Last edited by how; 11-11-2011 at 01:08 AM.
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Old 11-11-2011, 04:16 PM   #19
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Heat - short cycling


Hi 'How',
Thanks for the tips again!! I just finished 'clocking my meter'.. Cool exercise... Here's what I found:
2 revolutions on Gas meter took 29 secs.
3600/29 * 1050 = 130,344 BTU/HR

My furnace input rating is 81000 BTU/HR. Looks as though I'm running hot!

Can someone validate my calculations? I'm in Ontario, Canada (not sure if that matters)...
Can I dial back the gas into the furnace?
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Old 11-11-2011, 04:38 PM   #20
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Heat - short cycling


What does the dial say (size) that you are reading?
Is it metric or imperial?
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Old 11-11-2011, 05:21 PM   #21
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cubic meters, m3, so metric I guess.
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Old 11-11-2011, 05:24 PM   #22
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Heat - short cycling


So I dialed back the main gas shut off valve supplying the furnace... I closed it half way... Now the furnace does not shut off, does not over-heat... I don't want to leave it like that, but it was a test to prove things out...
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Old 11-11-2011, 06:08 PM   #23
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Heat - short cycling


Sounds to me like you have found the problem, your regulator is in need of adjustment.

But your math is off

1 m3 = 35.314 Ft3
2 revs = 70.628 Ft3 for your sample (if you are reading 1 m3/rev)
Heat content for natural gas = 1050 Btus/ft3
70.628/29 sec = 2.43 * 1050 = 2560 btu's/sec
2560 * 60 sec/min = 153,433 btu/min
153,433 * 60 min/hr = 9,205,994 Btu's/hr

So what is the meter graduation that you are reading 10th's, I would bet 100ths of m3

Which would result in 92,056 Btu's/hr

Close to what your furnace input value is.

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Last edited by Jackofall1; 11-11-2011 at 06:31 PM.
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Old 11-11-2011, 06:23 PM   #24
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Heat - short cycling


Definately don't leave your gas cock 1/2 on!
Does the dial you are watching on your meter say .01

I'm in the field so this is a rough metric/imperial estimate at what I think you have but its probably closer to 92,050 BTUs
(3600) times the calerific value of the gas in your area (1050 which is your info) times the size of the dial (.01 which is my guess) divided by the time it took it to rotate once. (14.5 sec.) times 35.31 as the quick multiple for converting Metric to Imp. But I'm still not sure what dial your measuring.

Was there a gas hot water tank / stove/dryer on at the same time you were clocking to mess up your dial rotation speed?
Does your gas come into the furnace area and then go through a reg before going to the furnace?
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Old 11-11-2011, 10:28 PM   #25
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Heat - short cycling


ok I knew my math would be off as this was my first meter clocking...
lets say I'm at 92k btu with 80k btu rating input. My gas valve has an input adjustment and output adjustment. Are we talking a turn or two?? Or do I need a new part?

part is Honeywell vr8204a2001

thanks guys, great work!
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Old 11-12-2011, 12:24 AM   #26
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Heat - short cycling


Now your walking into more dangerous territory. Do you have 2 lb gas coming from the meter with a regulater on that gas pipe in the furnace room to bring it down further?
What is written on the dial that you are clocking your furnace with?

Just turning down the gas valve without a manometer in place to tell me what your pressure is before the gas valve and after...would be riskier than I am willing to go without more information.
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Old 11-12-2011, 02:27 PM   #27
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Heat - short cycling


outside on the meter there are only 2 dials. They both spin left and the left side goes a lot faster then right dial. Both dials are marked m3. That's all that's on the face to read.

no regulator inline before gas valve in furnace. Just a stop valve which is at half spot still.

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Old 11-12-2011, 02:33 PM   #28
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neighbours they have same meter as mine, dials are marked proper. Left is .01 and right is .05 both are in m3.
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Old 11-12-2011, 02:59 PM   #29
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Heat - short cycling


Ok, That eliminates some of my DIY concerns for your particular gas valve adjustment.
Usually this should only be done by some one professionally trained and with a manometer.

I think your gas valve has two adjustment for gas. One is for the main gas and one is smaller and closer to the pilot tube for the pilot.
If this doesn't make sense for what you see when looking at your gas valve then send in a photo of it. Try turning the main gas adjustment no more than one half of a complete revolution counter clockwise and re clock your meter again.
Make sure that any other gas appliance (HW tank, dryer, fireplace etc are on the pilot only position when doing the clocking) are not throwing out your measurements. Report what happens.

Last edited by how; 11-12-2011 at 03:11 PM.
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Old 11-12-2011, 04:53 PM   #30
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Heat - short cycling


quick question after the adjustment do you know how many secs I should clock for 80000 btu.

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