ACHRNEWS

Btu Buddy 56: A Smelly Gas Furnace

November 19, 2007

Bob is a service technician who is well trained and nationally certified. However, he sometimes suffers from the same confusion that all technicians occasionally do - the facts that he gathers may or may not point to the obvious cause of the problem or the best solution. But Bob has something that no one else has. He recalls his long-time HVACR mentor and imagines him accompanying him as “Btu Buddy,” someone who reminds him to take time to stop and think before rushing to judgment, helping keep him on the right track, even with facts that are confusing.

Bob got a call from the dispatcher to go to a house where the residents smelled some kind of fumes when the furnace was running. Bob told the dispatcher to call them back and tell them to turn the furnace off until he could get there, within the hour. The weather was not very cold, so the furnace was only needed in the mornings.

Bob arrived and talked to the homeowner who described the situation. The homeowner said, “The furnace is under the house, and when it is turned on, I can smell fumes that do not seem normal. The furnace is old, but is supposed to be a very high quality furnace.”

Bob got his flashlight and went under the house. The furnace was a horizontal furnace that was mounted on a sturdy block foundation. The vent pipe ran about 8 feet to a chimney to the top of the two-story house. The furnace looked old, but in very good shape. He examined the flue connector that ran from the furnace to the chimney and discovered the problem; it was rusted to the point that it was falling apart. He took measurements from the furnace to the chimney.

Figure 1. The flue pipe is attached to the box called the draft diverter. When the diverter is removed, you can see down into the heat exchanger. (Figures are from Practical Heating Technology by William Johnson, published by Thomson Delmar Learning.) (Click on the image for an enlarged view.)

Bob then removed the doors to the furnace and used a mirror to examine the heat exchanger. It looked good. He removed the draft diverter where the flue pipe was connected (Figure 1) so that he could see down into the heat exchanger. It all looked good.

Bob told the homeowner what the problem was and started to leave for the shop to pick up materials to replace the vent pipe when Btu Buddy appeared and asked, “What are you going to do, Bob?”

Bob said, “Oh no, I must be about to do something wrong. You always seem to appear at those kinds of times. I am going to the shop for a vent pipe for this furnace.”

Btu Buddy asked, “What kind of pipe are you going to get?”

Bob said, “One just like the one that is on the furnace now. It has lasted for many years.”

Btu Buddy then said, “The one that is on the furnace is a single-wall vent pipe, and that is the reason that it rusted out. That is really not what you should replace it with. You should replace it with a double-wall vent (Figure 2).”

Figure 2. (A) Single-wall vent pipe. (B) Double-wall vent pipe. (Click on the image for an enlarged view.)

Bob then said, “I was just going to replace it with what had been used.”

Btu Buddy then said, “The current code calls for double-wall pipe for a reason. The reason is that single-wall pipe will fail in this application. It would take a long time to fail, but it will fail.”

Bob asked, “What is the big difference?”

Btu Buddy explained, “The flue gases leaving the furnace contain a lot of moisture. If the flue gases cool down below the dew point temperature of the flue, some moisture will condense in the flue pipe. The temperature under the house is cool, probably 50° or 60°F, well below the dew point temperature of the flue gases. The moisture is slightly acid, about like a soft drink, and will corrode the pipe to the point that it will fail, just like the flue at this job. Condensate is what caused it to fail. Also, the flue gases will be cool and slow to start rising up the chimney. You want the flue gases to rise as quickly as practical to prevent flue gas from spilling out under the house. Double-wall pipe will keep the flue gases warm enough to get them to the chimney and they will rise up and out to the atmosphere.”

Bob then said, “I have worked with double-wall pipe in the past, and it is hard to work with. Everything must be just right.”

Figure 3. This illustrates the air space between the inner wall and the outer wall of double-wall vent pipe. (Click on the image for an enlarged view.)

Btu Buddy went on to explain, “Double-wall pipe has a dead air space between the inner and the outer pipe (Figure 3). This dead air space has an insulating effect. Connecting it using the special connections that lock together is part of keeping the integrity of the vent system. You will notice that you do not need sheet metal screws to fasten the pipe connections together as they have their own locking system. If fact, you should never use screws to connect the pipe, only where the pipe fastens to the furnace collar (Figure 3).”

Bob asked, “So what should I do?”

Btu Buddy responded, “Tell the customer what is required to get the job up to code level by informing him about the need for double-wall pipe.”

Bob went back to the customer and explained what had to be done. Bob said, “The fumes you smelled were called aldehydes. These are present in the flue gas. They are not poison, like carbon monoxide, but do alert you to flue gas being present. It is much like the burners on your gas stove; you smell the aldehydes when you light the stove, but you have a nice blue flame and it is safe. The aldehyde odor is very slight, but noticeable to someone with a keen nose. A yellow flame will have carbon monoxide in it and smell the same, like the aldehydes.”

Bob explained what needed to be done and the customer said, “By all means, get the system up to code.”

After Bob installed the pipe, started the furnace, and checked the flame for good characteristics, he oiled the motor and replaced the air filter.

Btu Buddy said, “There is one other aspect to this job that you didn’t notice. We will talk about it over lunch tomorrow. This job is now up to code and all is well.”

As they were riding away, Btu Buddy commented, “The customers all seem to want to do the safe thing and you did the right thing by bringing it up.”

Bob then asked, “Would most service technicians go to double-wall pipe in a situation like this?”

Btu Buddy said, “Some would just replace the vent with single-wall pipe and it would work well, for some period of time. We just can’t be sure how long that will be, but it pays to be safe.”

Publication date: 11/19/2007