Pilot goes out when combustion fan goes off

[mikeflom]

I have a 14 year old Rudd hot air gas furnace in a basement with a standing pilot/thermocouple. It uses a Honeywell gas valve. It has a funnel over the pilot, connected to a pilot blower fan (is this a combustion blower) via a tube which is connected to a metal flue that is vented through the chimney. The chimneys were last cleaned about 5 years ago. There is an identical furnace sidewise mounted in the attic which has never exhibited this problem.

The pilot started to go on the basement furnace out a couple of years ago, more and more frequently, but intermittently. Lately, it occurs about 5 out of every 7 or 8 days when the heat is on, even if there is no significant wind.

I replaced a pitted and eroded thermocouple, but no apparent effect on problem. . I had an HVAC technician in who after 2 hours of diagnosis still isn=E2=80= =99t sure what the problem is, but here=E2=80=99s what we observed and did.

- Adjusted the thermocouple so that it was farther into the pilot flame. No effect.

- Replaced the thermocouple =E2=80=93 twice. No effect.

- Tightened the gas line from the valve to the pilot. No effect.

- Observed that the pilot flame goes down by about 1/3 to =C2=BD when the blower motor comes on, but only if the funnel over the pilot is installed. o No observable effect if the funnel is removed.

- Observed that the pilot going out can be replicated but not consistently, as follows: o Pilot Blower fan shuts off (due to thermostat going off or manually switching off furnace). Happens frequently but intermittently, e.g.

5/10 times. =EF=82=A7 Like a backdraft is occurring or something else is interrupting g= as flow to the pilot. o Banging on cover of main fan motor. Happened once. o Doing nothing. There were no fans running, and pilot just went out.

- Replaced the thermocouple (twice). Pilot keeps going out.

- According to the tech, there are no lights or other diagnostics indicating any problems, specifically with the pilot blower.

- Removing the funnel over the pilot seems to fix the problem. However, the funnel and tube are clean, unblocked and don=E2=80=99t show any soot or other deposits.

- He tested removing the external vent from the blower motor but only once, the pilot stayed on when the pilot blower fan was shut off.

His conclusion after talking to a senior technician is that there is probably a problem with the pilot blower motor, which =E2=80=9Csounds funny= =E2=80=9D to him. I don=E2=80=99t know what it is supposed to sound like, but it=E2=80=99s no= t grinding, clinking, etc. He says this is a $400 part (plus labor), including the housing.

He left the pilot funnel off, said I should have the flue cleaned just to eliminate that as an issue, which I will. He says if that=E2=80=99s not the problem the next thing they will have to = do is replace the blower motor.

Do you agree with this diagnosis? Is it safe to leave the pilot funnel removed with the furnace operating?

Thanks much

[Zyp]

You have a damaged or ruptured fire box [heat exchanger.] Call your local HVAC guy and have him pull the blower housing and inspect the heat exchanger from there. If he can't find anything, there is a new product on the market that can be "sprayed" into the firebox chamber and detected using a halon [freon] detector in the area I just mentioned.

The problem occurs after the heater has been on for a bit, and the rupture opens, letting in blower air. When the inducer stops, the blower creates a positive pressure in the heat exchanger and blows the pilot out.

Generally, on a furance that has air conditioning, and a standing pilot, there exists excessive condensation around the pilot area and, as a result, the heat exchanger fails [rusts through]. This is on older furnaces. Newer furances will not have this problem because there is not a burning pilot.

Replacing the furance will reduce your utility cost of operation saving you money. Probably paying for the change out in less than 10 years depending on the cost of fuel in your area.

[Don Young]

The pilot started to go on the basement furnace out a couple of years ago, more and more frequently, but intermittently. Lately, it occurs about 5 out of every 7 or 8 days when the heat is on, even if there is no significant wind.

I replaced a pitted and eroded thermocouple, but no apparent effect on problem. . I had an HVAC technician in who after 2 hours of diagnosis still isn't sure what the problem is, but here's what we observed and did.

- Adjusted the thermocouple so that it was farther into the pilot flame. No effect.

- Replaced the thermocouple - twice. No effect.

- Tightened the gas line from the valve to the pilot. No effect.

- Observed that the pilot flame goes down by about 1/3 to ½ when the blower motor comes on, but only if the funnel over the pilot is installed. o No observable effect if the funnel is removed.

- Observed that the pilot going out can be replicated but not consistently, as follows: o Pilot Blower fan shuts off (due to thermostat going off or manually switching off furnace). Happens frequently but intermittently, e.g.

5/10 times. ? Like a backdraft is occurring or something else is interrupting gas flow to the pilot. o Banging on cover of main fan motor. Happened once. o Doing nothing. There were no fans running, and pilot just went out.

- Replaced the thermocouple (twice). Pilot keeps going out.

- According to the tech, there are no lights or other diagnostics indicating any problems, specifically with the pilot blower.

- Removing the funnel over the pilot seems to fix the problem. However, the funnel and tube are clean, unblocked and don't show any soot or other deposits.

- He tested removing the external vent from the blower motor but only once, the pilot stayed on when the pilot blower fan was shut off.

His conclusion after talking to a senior technician is that there is probably a problem with the pilot blower motor, which "sounds funny" to him. I don't know what it is supposed to sound like, but it's not grinding, clinking, etc. He says this is a $400 part (plus labor), including the housing.

He left the pilot funnel off, said I should have the flue cleaned just to eliminate that as an issue, which I will. He says if that's not the problem the next thing they will have to do is replace the blower motor.

Do you agree with this diagnosis? Is it safe to leave the pilot funnel removed with the furnace operating?

Thanks much Before you get too far, check that there is adequate airflow into the furnace space and that you do not have something causing negative air pressure and/or a slight downdraft. Sounds to me like and airflow problem.

Don Young

[MLD]

[mikeflom]

All: Thanks everyone for your responses. FYI 1. The furnace is in a large undisturbed basement with no drafts, only one door (almost always closed), closed windows, and nothing near or covering the vents in the furnace housing. 2. The pilot orifice appears clean.

Zyp You are correct, the furnaces are also used for air conditioning. There is a condensate pump, not sure if it is working. There is no external visible sign of rust near the pilot or anywhere else on the furnace, except on two of the three bolt heads securing the inducer blower to the heat exchanger.

Below are some other symptoms and additional tests I performed after reading your response. Do these symptoms confirm your diagnosis? If so I'll skip the $180 flue cleaning and have the heat exchanger inspected.

- I hear a "puff" sound when the power is cut (and inducer blower and main blower turns off).

- When I put a lighted butane lighter in three gas jets I tested (even ones several inchces from the pilot) and cut the power, the flame on the lighter goes out instantly with the "puff" sound. This occurs if the furnace has been heated only for 10+seconds. Also, when covering the hole on the inducer blower where the funnel tube above the pilot normally connects, there is no significant sensation of back pressure coming from the inducer when the power is cut.

Thanks for your kind input Mike

PS There are no signs of external rust in the pilot/gas jet area (except two of the three draft inducer/blower screw heads are rusted) or on the external surfaces of the furnace. However, there is rust on the ground below the condensate pump which was replaced a few years ago, and rust is possible in the water coming out of the condensate pump (there's a clear plastic flexible tube but its discolored brown from age so it's hard to tell what's brown and what's rust).