Carrier 58SS060-CC burner won't stay lit

[claudebarnhart]

Symptoms: Thermo calls for heat; inducer fan comes on; pilot lights; approx 60 sec burner lights; burner goes out in 4-6 sec (pilot stays lit); burner relights in approx 6-10 sec; sequence repeats; burner stays lit on relight 7th or 8th time; circulating fan starts approx 60-80 sec; sometimes during heat cycle the burner will go out, and instantly relight (circulating and inducer fans uninterrupted).

I have replaced the 3 wire pilot assembly, the lower control board, the pressure switch, and the inducer fan control board (relay contacts were burnt) and the symptoms persist. I have cut all power to the unit several times to see if timers would reset, to no avail.

In checking voltage to the gas valve, with R&W jumpered to bypass the thermostat, I note 24 VAC to gas valve term #4 and 0 VAC at term #1 (initially); after several seconds (while the inducer fan is running) 24 VAC is applied to term #1 and shortly thereafter the burners light. A few seconds after the burners light voltage disappears from term #1, burner goes out; and the same cycle repeats. After several such cycles 24 VAC does not disappear from term #1, burners light, fan starts and ops okay. Something is interrupting 24 VAC to the #1 term on the gas valve, apparently causing the burners to go out, once lighted -- but, cures itself after several cycles. ...

Barney, in Memphis

[Admin]

Tricky problem.  I'm thinking it may be a pressure switch issue.  Normally the pilot would go out if the switch opened, but it may be happening so fast that only the burner is interrupted.  As soon as the pilot lights, put a jumper accross the pressure switch and see if that solves the issue.

I have seen plugged or cracked secondary heat exchangers cause that problem.

Flame failure would be the obvious cause, but you replaced the sensor, which I believe it's part of the pilot assembly.  How does the pilot look?  Is it a good size and blue, hitting the pilot assembly sensor?  Does the pilot move at all?  Check the pilot pressure, it may need to be raised to increase the pilot size.  There may not be enough heat on the pilot sensor, biut after a few failed attempts it's getting hot enough to keep the burner on.

[claudebarnhart]

The pilot flame looks okay - I turned it up a bit, no result.

I've noted 24 VAC on all three terminals (Common, NC, NO) of the pressure SW once the thermo is calling for heat; no interruptions of voltage as main burner goes out. 

Vexing!!!!!

[claudebarnhart]

Perhaps a solution!  I'll observe for 24 hours and get back.  Solution involves draft pulling pilot flame away from flame sensor..............and countering same.

[Admin]

It does sound like a flame rectification problem.  Could a ground connection to the board be missing, or a broken ground at the 120v line connection?

Take a volt reading from the neutral wire at the furnace to the gas piping, then from the ground wire to gas piping, then from the neutral to ground.  

Can you tell me if the pilot goes out when you pull a 24v wire off the vent switch or any other 24v safety?

[claudebarnhart]

Some background: Gas furnace is in the basement; connects into 10" stainless chimney flue installed in 2003 when Memphis windstorm destroyed original chimney; chimney height from gas furnace is 36-40' depending on my estimate; big draft from this chimney.

My theory has been that maybe the draft was drawing the pilot flame away from the bi-metallic heat strip at the pilot, and that the heat strip wasn't staying hot enough, long enough.

Rube Goldberg, MacGyver, and I put our heads together and placed a sheet metal plate in front of the pilot assembly thinking to block a high draft from blowing across the pilot flame.  So far, this contraption has worked; and the burners light correctly every time that the thermo calls for heat, and they stay lit.

I took pics, and will try to post them later.

Barney, in Memphis

[claudebarnhart]

Here's trying to upload some pictures of a before and after of installing the draft blocker in front of the pilot flame assembly->

[Admin]

Is there any other appliances vented up the 10" chimney?  You should install a properly sized chimney liner to solve the downdraft problem.

Assuming the furnace is the only appliance using the chimney and the vent connector is a single wall vent connector at a 5' rise, a 4" chimney liner @ 40' with a 20% reduction, would have a capacity of 132,000 BTU's.  Your furnace is 60,000 BTU's so that would be the best fit.  With chimney liners we lose 20% of chimney capacity and another 10% loss if there is a tee fitting to vent a second appliance.

Let me know if there is another appliance using the chimney, like a water heater.  We need to know how many BTU's the input rating is on the other appliances, if any.  If there is a second appliance you would most likely need to use a 5" chimney liner.

Using an oversized chimney can be a problem since it allows the flue gases to cool excessively, thereby slowing it down.  For safety reasons, a chimney liner is a good idea.  It is a requirement in the gas code here in Ontario.

[Admin]

Sorry, I had locked this topic on accident...