The vice president of my company had a heating contractor remove his furnace and replace it with two units to provide two heating zones. Soon after the units were put into operation, an objectionable odor occurred whenever the units were used.
The contractor who installed the new units had visited the home several times, but had not been able to determine the cause of the odor. I was asked to visit the home to see if I could help.
Some time later, I looked over the installation. The only thing I thought might cause trouble was the use of a common return-air system by both units. I removed the front of one unit's casing to check the control setting and found it was set properly. While the casing was off, the burner started. The burner and blower had been running for some time on the second unit. I watched the first unit to see how long it would be before the blower started. It did not start, but the burner continued to operate.
Because of the improper connection to the return-air duct, air was drawn backward through the second unit when the blower on the first unit began to operate. This prevented the warmed air in the second unit from flowing upward to actuate the control. As the burner continued to operate without affecting the limit switch, I believed the insulation on the inside of the casing was being heated past its critical point, creating an odor.
I contacted the insulation manufacturer and was told that an odor would occur if the insulation was heated above its critical temperature. The manfacturer said that if this occurred, the only solution was to replace the insulation and control the burner operation so it would not happen again. I discussed my findings and the information from the insulation manufacturer with the heating contractor. He said there was no way the insulation could have overheated. He admitted that the return-air connection was wrong, but did not believe it was the cause of any trouble.
I knew the president of the company that had made the problem units. I called him and presented my problem and conclusions. He agreed and said something would be done to solve the problem. I expected him to send new casings to the heating contractor. Instead, he had two new furnaces delivered to replace the problem units. The contractor installed the new units and reworked the common return-air connection. There were no more complaints of odors. The moral of the story: Always operate equipment within its design limits.