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.

In this edition of the Btu Buddy series, the dispatcher calls Bob first thing in the morning before he leaves his home and tells him that a customer with a gas furnace has a strange smell in the house. Bob tells the dispatcher to call the customer back and have them turn the furnace off until he can get there, within the hour. It is very cold outside, about 10 degrees F, but the customer has two fireplaces that can keep the house warm enough until Bob can get there.

Bob arrives and the customer explains that, when the furnace is operating, he smells something peculiar and he's concerned. It smells like his gas grill when it first starts up.

Figure 1. This is a typical natural draft furnace with a draft diverter operating correctly. (All figures are from Refrigeration & Air Conditioning Technology, by William Whitman, William Johnson, and John Tomczyk, published by Delmar Publishers.)
Bob turns the furnace on and goes to the furnace room in the basement of the house. The furnace is operating correctly; it is an atmospheric draft system with a draft hood (Figure 1). Bob removes the burner compartment door and observes the flame; it appears to be burning properly, all blue and no visible yellow tips. Bob is thinking there is nothing wrong with the furnace; there must be something else wrong. Maybe the homeowner has some other odor problem. Btu Buddy then enters the scene and asks, "Is the furnace drafting like it should?"

Bob says, "I don't know. All looks well."

Btu Buddy tells Bob, "Why don't you check the draft with a match, just to be sure."

Bob says, "We did this in school by lighting a match and holding it up close to the draft diverter to see which way the air was moving."

Bob holds the lighted match up close to the draft diverter and the flame moves away from the draft diverter. Flue gases are not going up the flue (Figure 2).

Bob steps out back to see what kind of chimney cap the flue has. It all looks normal. He suspected that a limb might have fallen on it and collapsed it, but it looks good.

Bob goes back inside and is standing there scratching his head when Btu Buddy asks, "Can you think of anything else that would cause a flue not to draft?"

Bob says, "A restriction in the flue pipe would, but everything seems to be intact."

Btu Buddy tells Bob, "You need to go talk to the customer and see if there is a clue as to when it happened."

Figure 2. This draft diverter is not venting. This could be called back drafting or down drafting. The products of combustion are spilling out in the furnace area.

A Clue About The Flue

Bob talks to the customer and the customer says that it seemed to start late last night. They had a lot of company with a fire in both fireplaces. He smelled the odor when he went to bed.

Btu Buddy tells Bob to ask the owner what kind of fireplaces he has.

He explained that the fireplaces were the open type with screens only and no glass. There has been a fire in each fireplace all night.

Btu Buddy tells Bob, "Go open the downstairs back door about 4 inches and recheck the draft."

Bob opens the door and lights a match at the draft hood again, and says, "This draft is working now."

Btu Buddy explains, "With those two open fireplaces running, they were requiring so much air that they were actually drawing air down the furnace flue system."

Bob says, "I thought the vent system around the furnace was supposed to provide enough air for the furnace."

Btu Buddy notes, "Look around the furnace for the vents that are required by the codes today. The rule of thumb is that there should be 1 square inch of free surface area below and above the furnace for each 1,000 Btu of capacity for furnace ventilation air and proper venting practices. This is a 100,000 Btuh furnace so it should have 100 square inches at the bottom of the furnace and 100 square inches at the top. Remember, this is free area.

"With most grilles, the louvers take up about 30 percent of the area. A grille with 100 square inches of free area would have an overall area of about 140 square inches. That is, a 10 x 14 equals 140 total square inches; 140 x 0.70 equals 98 square inches of free area. This means that a 10 x 14 grille should be located at the bottom of the furnace and at the top for proper ventilation."

Bob looks around and says, "There is not enough air vent surface area for this furnace. I can tell without measuring. How did they get this past the code?"

Btu Buddy responds, "This house was built in the early 1970s, and homes were not as tight at that time and were able to get ventilation air from the cracks around windows and doors. This house was built during that time. Look at the tag for the insulation that is pinned up on the wall for the date of insulation."

Bob looked and the home was built in 1970.

Btu Buddy remarks, "The oil embargo in the early 1970s caused much stricter building codes in our part of the country. Builders began to tighten up all construction. This meant that mechanical ventilation codes had to be stricter. That is probably the original furnace, or the inspector would have required that proper ventilation be installed when the furnace was changed."

Bob asks, "What can we do now? Can we make the customer install the proper ventilation?"

Btu Buddy says, "No, you can't, but you can explain the circumstances and the owner will probably want to remain safe and get the job done. For now, the owner can let those fires die down and leave the downstairs door cracked so the furnace can get the ventilation that it must have until the fireplace fires go out.

"Then the customer would be well advised to get glass doors for the fireplaces to prevent them from drawing so much heated air out of the house. An open fireplace draws air out, which pulls air in even when it is not burning. At the same time, you can give them a price on installing furnace ventilation. It will not be too complicated, because the furnace room is on an outside wall. Notice the furnace room door is a louvered door. That door should also be changed to a solid door to keep the ventilation air system within the furnace room. That louvered door was installed originally to pull air from the whole house for ventilation air. This system should have been corrected years ago.

"If you look, someone has installed storm doors and windows all around the house. The windows have also been caulked to tighten the house against outside air infiltration. All of this contributed to the problem today. This house is really tight compared to when it was built."

Bob asks, "How much danger was the family in with those fumes?"

Btu Buddy explains, "The fumes that they smelled were the aldehydes that are present in all gas burning products of combustion. These are present in flue gases from good combustion and poor combustion. Poor combustion occurs when there is not enough oxygen to properly burn the fuel; yellow flame tips are the indication of poor combustion. With good combustion, you can still smell the aldehydes, but the gas is not poisonous. It will suffocate you because of lack of oxygen, but is not poison, like carbon monoxide. It is not likely that suffocation would occur, as the oxygen level in the whole house would have to be reduced to a very low level. Most houses would leak in enough air to prevent suffocation. Proper combustion gives off carbon dioxide. Both smell about the same and are dangerous, with carbon monoxide being poisonous. The people were in no immediate danger, but it was not a good situation.

"This house should also have a carbon monoxide detector, as all homes should. I would suggest that you draw up a proposal to install the ventilation system and encourage the customer to have fireplace fronts installed along with two carbon monoxide detectors, one for upstairs and one for downstairs. It is a good idea for all technicians to carry a carbon monoxide detection instrument for situations just like this. We were able to look at this job and find a solution very simply because of past experience."

Bob says, "The experience was all yours. I am still learning. I am glad that all of my service calls are not this complicated. Most of them are fairly easy."

Btu Buddy says, "Most calls are routine. The more experience you have the more routine calls that you have. However, this is a profession where you never stop learning. That is what makes it so interesting. There are always challenges out there to keep it interesting. It could be like an assembly line job without the challenges. This service call was one of those calls that allows you to become a safety inspector for the customer. I think that all customers would welcome a chance to upgrade their system to make it safer. You are not just selling them more service; you are selling them peace of mind.

"No one knew when houses were built in the late 60s and early 70s that the energy crunch would come and that houses would be modified to become tighter. Infiltration was not even considered back then. All construction was very loose. The construction of today creates houses that must have planned ventilation air."

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: 01/26/2004