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.

The first cold weather of fall has prompted a service call to a residence because all the windows in the house were sweating on the inside. The customer explained that all seemed well with the heating system until the windows began to sweat and he wanted to know if the heating system could have anything to do with the problem.

Bob arrives and talks to the homeowner who tells Bob that he just wants to make sure the heating system is working properly. For some reason the windows began to sweat when the weather became cold. It all seems to relate to turning on the heating system.

Figure 1. Furnace draft test. A illustrates furnace updraft. B illustrates furnace downdraft. (From Practical Heating Technology, by William Johnson, published by Delmar Publishers.)
Bob goes to the partial basement where he finds a gas furnace. He starts the furnace and everything looks good. The furnace is an older natural draft furnace and the flame is good and blue and seems to be burning well. He holds his hand in front of the draft diverter to see if the flue gases are going up the flue and they appear to be, but he decides he will do the candle test that he learned in school to be sure (Figure 1). He goes to his truck and brings back a candle and holds it in front of the draft diverter; the candle flame is pulled into the diverter, so the furnace is venting correctly. Bob is about to go to the homeowner and declare that the furnace is performing fine when Btu Buddy makes an appearance and says, "You know the furnace is venting correctly now, but maybe you should try some other tests and ask some questions."

Bob asks, "What kind of tests and questions?"

Btu Buddy then says, "Didn't we have a service call last winter when the furnace was down drafting because there were open fireplaces that pulled air out of the house? (See "Btu Buddy 10: Working With A Gas Furnace That Is Down Drafting.") What would happen if the owner were to be operating several exhaust fans at the same time? What if the vent system has a loose connection between the furnace and the chimney and all of the flue gases were not going up the chimney? Any of these would cause flue gases to move into the basement area and filter upward into the house. Remember that a large part of the flue gas is moisture. You may also want to look at the clothes dryer vent to be sure it is venting outside like it should. It is obvious that the air is loaded with moisture because the windows are sweating on the inside. You must look for any source of extra moisture."

Bob says, "Boy, there sure is a lot to think about. This could be a complicated service call."

Btu Buddy says, "It could also be simple. You just have to do your investigation. Talk to the homeowner about some of these items that we talked about."

Bob goes to the homeowner and asks, "Have you been using your fireplace much this year?"

The homeowner says, "No, we haven't had a fire this year at all in the fireplace."

Bob then asks, "Have you had any reason to run the exhaust fans a great deal?"

The homeowner says, "No, we cook with gas and run the kitchen exhaust when we cook, but we have been eating out for the past two weeks because of a busy schedule."

Bob then asks if he could see the clothes dryer. They go to the basement and the clothes dryer is a gas unit. Bob thinks he has found the cause of the moisture so he asks the homeowner to turn it on for a running test. He goes outside where the vent terminates and it is venting very well. The vent only went through the wall.

Bob thanks the homeowner and goes back to the basement scratching his head with more questions than answers.

He goes to the furnace for a look at the entire flue system. The flue is piped with single wall vent pipe. This would not meet the code today, as this is an unconditioned space. The vent leaves the back of the furnace and is piped through the crawl space but he could not see where it enters the chimney because it goes around a corner. Bob puts his coveralls on and crawls along the path of the vent. When the vent goes around a corner to the other side of the chimney, he finds the problem. The vent pipe had been jarred loose and is venting out into the crawl space. He looks closely and there are no screws in the vent pipe. When it was assembled, the installer did not fasten it correctly.

Bob comes out from the crawl space and Btu Buddy asks, "What did you find?"

Bob explains what he found and tells Btu Buddy that he is going to fasten the single wall pipe with screws, as it should have been in the first place.

Btu Buddy tells Bob, "That will be a repair, but it will not bring the venting system up to the modern code. Why don't you explain the non-code situation to the homeowner and give him an option for a repair or replacement vent pipe."

Bob fastens the pipe together with screws. He also looks at all the connections and makes sure that they are all fastened correctly. This puts the system back in good working order. He then goes to the homeowner and explains the situation.

The homeowner says, "It has worked well all of these years. Why should I pay to change it?"

Bob explains, "The furnace will continue to vent correctly as it is, but the single wall vent will sweat and eventually deteriorate, and the code was changed to make systems safer from this action."

The homeowner says, "Go ahead and get your installation crew to change it to meet the code and thanks for doing a great job of explaining the system to me. When will my windows clear up from the sweat?"

Bob responds, "It will take a few days, but the sweat will gradually go away. We found the source of the moisture and stopped it."

As Bob and Btu Buddy are riding away, Bob asks, "Why didn't the homeowner smell the gas fumes?"

Btu Buddy explains, "The furnace was burning good and clean, and the products of combustion have a very faint smell. I smelled it when we went in the house. Even a good burning gas flame gives off a light smell because part of the products of combustion contains aldehydes, in the same family as formaldehyde. It is not easy to smell, but if you have ever smelled and knew what it was, you would probably notice it the next time."

Btu Buddy tells Bob, "You did a good job with the customer on that call. When you make the customer part of the call, they have more confidence in you and your company."

Bill Johnson has been active in the HVACR industry since the 1950s. He graduated in gas fuel technology and refrigeration from the Southern Technical Institute, a branch of Georgia Tech (now known as Southern Polytechnic Institute). He taught HVAC classes at Coosa Valley Vocational & Technical Institute for four years. He moved on to become service manager for Layne Trane, Charlotte, N.C. He taught for 15 years at Central Piedmont Community College, part of this time as program director. He had his own business for five years doing installation and service work. Now retired, he is the author of Practical Heating Technology and Practical Cooling Technology, and continues as a co-author of Refrigeration & Air Conditioning Technology, 5th Edition, all published by Delmar Publishers. For more information, he can be reached at 704-553-0087, 704-643-3928 (fax), or

Publication date: 12/27/2004