Btu Buddy 70: Smoking Gas Package Unit
When Bob arrived, he talked to the customer and then turned the unit off at the disconnect switch at the unit. He then went inside and turned the thermostat up to call for heat. He then went outside to the unit and started it with the disconnect switch. Sure enough, a light black smoke was coming out of the vent. Bob removed the burner compartment panel and looked at the flame and it didn’t look all that bad. He was scratching his head when Btu Buddy showed up.
Btu Buddy asked, “What is the problem, Bob?”
Bob said, “I have never seen a gas appliance smoke like this, and it really smells bad.”
Btu Buddy then asked, “What would cause a gas flame to smoke?”
Bob said, “Maybe too much fuel, or not enough air. The vent seems to be working and I can hear the vent fan turning, so it must be good. Maybe there is too much pressure on the gas manifold causing too much gas for the amount of combustion air.”
Btu Buddy then said, “Why don’t you take off the vent cover and see if there could be a blockage at the fan outlet?”
Bob removed the cover and took a light and looked inside and said, “Here is the problem. The fan wheel is all torn up. I don’t see how it could move any air, but something is moving the air.”
Bob removed the fan and wheel and looked it over. He then said, “I don’t see how that worked at all. There is no sign of the rest of the fan wheel. I wonder where it went. I think I need to talk to the customer and see if she knows anything.”
Bob talked to the customer and the customer said, “We had problems with this unit last spring, just at the end of the heating season. The contractor is no longer in business. The contractor told me that the heat exchanger had a hole in it and changed it out in warranty. It only cost us for the labor. He didn’t mention what could have caused it.”
She told Bob to make the repair.
Bob was making the repair when Btu Buddy suggested, “I think the contractor last year found the defective fan wheel when he installed the heat exchanger and just threw the missing pieces away and didn’t mention it. It was going to cost even more, so he just didn’t do his job because he didn’t want to go back to the customer for more money than his estimate. He must have noticed that the vent smelled bad, but assumed it was venting good enough. Probably the old fan wheel was what caused the heat exchanger to fail. If the unit had operated for long, the heat exchanger would have failed again and then the customer would have really been mad.”
Bob decided to see if he could change just the fan wheel and tried to remove it from the fan motor shaft. Then he discovered the hex head nut holding the fan wheel on the shaft was rusted out inside and an Allen wrench would not get a grip to turn it. He then said to Btu Buddy, “I am going to have to change the motor and the fan wheel - even more expense.”
Bob went to the supply house with the old motor and fan wheel where the counter man said, “You are doing the right thing. I have noticed that nobody can remove these old fan wheels from the shaft because of the rusty Allen screw.”
Bob had the new fan assembly and went back to the job to install it.
Btu Buddy said to Bob, “Shine your flashlight into the heat exchanger at the top where the fan mounts and at the bottom where the burners are and look closely for signs of soot deposits.”
Bob used his light and examined the heat exchanger from both ends and said, “There are no signs of soot. We are good to go forward.”
Btu Buddy then said, “Thank goodness the homeowner got right on this, before it had enough running time to accumulate soot.”
Bob asked Btu Buddy, “Why was there so much rust in the fan section?”
Btu Buddy explained, “The unit is about 11 years old. Every time it starts up, it gets a blast of moist air from combustion and that air is slightly caustic, about like a soft drink. After years of exposure, it just accumulates rust. I have never seen one that is rusted out like this one. Notice, the new fan wheel is coated with a galvanized zinc coating. The original one should have been coated the same way. It is possible that it did not get a good thick coat of galvanizing for proper protection.”
Bob then asked a crucial question, “How come combustion was allowed to keep going when there was not enough airflow? It seems like it should have shut the unit down.”
Btu Buddy then said, “Let me start by saying, there are three methods of proving airflow in a forced combustion system:
1. Some systems have a pressure switch across the blower that proves airflow.
2. Some systems have a “sail switch” that moves to make a set of contacts to verify combustion.
3. Some combustion motors have an “end switch” built into the motor to assure the combustion fan is turning. This assumes that the fan will move air when the motor is turning.
All of these systems open the gas valve when airflow is established. I guess your vote would be for either number 1 or 2, after today’s experience.”
Bob said, “Yes, this system sure didn’t function as expected. Who would have expected a fan wheel to completely come apart? It seems like it would have made a noise.”
Btu Buddy said, “The last technician did not do a professional job. You know better than to leave a job like this when it still had problems.”
Publication date: 01/19/2009