Flames come on and cut off 5 times before staying on.


  #1  
Old 09-14-12, 04:00 PM
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Flames come on and cut off 5 times before staying on.

I wonder if this is because I am starting the furnace cold.

I gave my furnace a test run by raising the thermostat temperature up. The pilot comes on, then the flames start and remain on for about a half minumte. But they cut off and come on about 5 times for a half minute before staying on continuously, then the blower fan comes on and it seems normal. I have run the system for about 5 min before shutting it down with the thermostat.

I tried it once after shutting down and restarting the furnace quickly and the flames stayed on. So maybe the furnace is detecting a cold start and needs to heat up the heat exchanger.

Do I have a problem? I do not remember the flames coming on and off in the past.

The furnace is an older Bryant gas furnace, it vents through a flue to a chimney. I believe the filters are clean, I did not check the chimney. The blower is not making any strange noises.
 

Last edited by geo8rge; 09-14-12 at 04:42 PM.
  #2  
Old 09-14-12, 05:11 PM
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You most likely have a dirty flame sensor. Does the furnace have a pilot which stays on all the time?
 
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Old 09-14-12, 05:49 PM
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So---

I presume you have a Bryant furnace with a pilot light that's lit each time the thermostat calls for heat?

If so, the problem is characteristic of one condition: the pilot burner and pilot burner orifice are dirty and need to be cleaned.

Once the pilot burner is lit, the pilot flame needs to be big enough to heat up a bi metal heat sensor on the pilot light/switch. When that switch is made, it turns on the main burner. When the flame is too small, the main burner keeps turning on and off as you describe.

You need to take out the screws holding the pilot burner in place and gently bend the pilot tubing out so you can get a 3/16" wrench on the brass ferrule that holds the pilot tubing in the pilot burner.

loosen the ferrule until the pilot tubing comes out. You'll find a bell shaped pilot orifice that needs to be cleaned using the wire from a wire brush and blowing on it. Use a wire brush to clean the pilot burner and blow any dust out.

Reassemble and you should be back in business.

You should also remove the burners while you have the pilot out and use a wire brush to clean any accumulated rust and dust off the burners.

You get three gold stars for testing the furnace before it gets cold.

Cleaning the pilot burner and burners is annual maintenance that should be done each year.
 
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Old 09-14-12, 05:58 PM
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As usual, SeattlePioneer is right. If this is a standing pilot furnace, as it would seem, follow S/P's instructions & you should cure your problem. I'll second his award of the gold stars.

Hey S/P, shouldn't that be a 7/16" wrench or does this thing have a tiny pilot tube?
 
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Old 09-15-12, 11:55 AM
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Hello Grady,


7/16" is the correct size to fit on the brass ferrule that holds the pilot tubing into the pilot switch.


Thanks for the correction Grady, I should 'a checked rather than trying to remember from the last heating season.
 
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Old 09-16-12, 08:52 AM
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No problem S/P. As many times as you have corrected me ...
 
  #7  
Old 09-17-12, 05:28 PM
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Thanks to everyone for the advice. I will see if I think I can do it myself.
 
  #8  
Old 09-20-12, 07:16 AM
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Questions on the Carrier 3 wire pilot light assembly

I have a Bryant furnace with a Carrier 3 wire pilot LH 680 005.

I cleaned the burners, the combustion chamber, and the orifices that feed the gas to the burner as was suggested. I unplugged the pilot assembly and plugged it back in. I rotated the pilot assembly on the gas tube and tapped it a bit, but did not remove it. After I did this the furnace blower comes on the first ignition and seems to operate better than before (I can hear the gas moving faster from the burners).

Questions on the 3 wire pilot light assembly:

Should the pilot assembly be able to rotate freely on the tube that feeds the gas, or should I tighten the brass nut on the bottom where the tube enters?

While the pilot flame is blue and splits toward the thermocouple and the furnace, just like in the pictures I see on the internet, the pilot assembly appears to have rust on all the parts exposed to the flame.

1) Should the pilot assembly be replaced?
2) My local HVAC supplier sells a replacement part made in Taiwan, should I hunt around the internet for a genuine OEM Carrier/Bryant part? I also see Robertshaw replacement parts, with a pretty brass finish. Any thoughts if one is better than the other?
3) Is removing and replacing the old assembly something that should only be done by a professional?

Again thanks for the advice.

Note to amateurs: cleaning the burners was not too much different than cleaning the burners on a gas oven. I removed 2 screws on a sheet metal shield, removed the shield, and the burners came out easily. I cleaned them with a wire brush and vacuumed out the pile of ash in the bottom of the furnace. I should invest in a pipe cleaner for the inside The pilot light is connected to a tube that feeds gas, so I am a bit more reluctant to mess with that. I have called professional service people ($225 in New York City) at least 3 times for various things and none has offered to clean the burners or replace the old rusted pilot, even though I would have paid for that if they mentioned it.

A previous post by someone with a similar problem that found just playing with the 3-wire pilot assembly fixed the problem. My guess is debris is blocking the thermocouple so it has to get hotter to hit the switch, taping on it removes the debris.


Bryant Gas Furnace - ignites, goes out, ignites, goes out -- 3 wire pilot burner

Read more: http://www.doityourself.com/forum/ga...#ixzz271KUYpGw
 
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Old 09-20-12, 07:42 AM
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The brass compression tubing needs to be made gas tight to the pilot switch. Use the 7/16" wrench to tighten the nut.

Don't tighten it excessively --- just enough to prevent gas from leaking out of the fitting. You can use a 50% dishwashing liquid/water solution and a paintbrush to see if the fitting is gas tight.

If it's loose as you describe you will be leaking gas which will reduce the gas available to fire the pilot switch properly.

I wouldn't replace the pilot switch because of the surface rust you describe. I've only replaced pilot sweitches when they fail to operate properly. I've never had an occasion where rust suggested a defect that required replacement --- and I've seen 1000s of these furnaces.

Manufacturers recommend that homeowners do pretty much nothing on a furnace except replace the filters. But cleaning or replacing the pilot switch is usually a relatively easy task.

I have no experience with imported off brand parts. For DIYers, I would recommend using OEM parts to minimize any difficulties in replacing parts.

But frankly, I don't see a reason to replace the pilot switch based on your report of some surface rust.
 
  #10  
Old 09-22-12, 07:40 AM
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Once again thanks to all for the advice, it was very helpful.

Once again thanks to all for the advice, it was very helpful.
 
 

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