Buildings

Smelly House

Q: Recently, I visited a friend's house. Upon entering I noticed an annoying odor which seemed to come from the fireplace. The fan switch on the thermostat for the central air conditioner was set at the " on" position (running all the time). The odor bothered me enough that I mentioned it to my friend. She said she didn't smell it, but I suspect she's become accustomed to the odor which reminded me of a smell I noticed on a visit to a blacksmith booth at a farm show, creosote and metallic-like. Could the smell be coming from the fireplace? What would cause this to happen?

A: What you describe is fairly common. It's likely that when the air handler fan is running it's causing negative pressure in the room with the fireplace, causing air to come down the chimney. In the summer, it's a problem with the odor and with humid outside air coming into the house. In the winter, it can be much more serious if it's causing backdrafting of the fireplace. Combustion gasses, including oxides of nitrogen, water vapor, carbon dioxide, and carbon monoxide, could be coming down into the living space.

Your friend needs to have the house checked to determine the cause of the negative pressure. Since the air handler fan is running all the time, the likely causes are leaks in the supply ducts, pressure imbalances caused by closing interior doors, or both.

It's also possible that the air handler fan is not what's causing the negative pressure. Other exhaust fans, such as the dryer or a kitchen exhaust fan, particularly one of the downdraft super-sucker types, could cause enough negative pressure to pull air down the chimney.

A whole house exhaust fan could also do it, as could a powered attic ventilator. If it's the whole house fan, a solution may be to make sure you cover the fireplace opening tightly and open enough windows when that fan is operating, so that it's easier for the air to come in the windows than to come all the way down the chimney. If it's a powered attic ventilator that's causing the problem, simply disconnect the electricity, remove the unit, take it out into the back yard, and shoot it. It's unlikely that it's doing anything good for your house.

The next question to consider is this: Why does your friend want to have the fan running all the time in the first place? In general, we don't recommend letting the air handler fan run all the time. During both the cooling and heating seasons, if operating the air handler fan causes pressure imbalances in the house, it can be substantially increasing infiltration (300% or even more), leading to higher bills, comfort problems, and moisture problems. In addition, in the cooling season if the air handler fan continues to run after the compressor has shut off, the air flowing over the wet coil will simply re-humidify the house by picking up the water and putting it back into the air stream.

There needs to be a pause of at least 20 minutes or so each time the compressor shuts off during which the fan isn't running. This will enable the water to run off the coil, into the drain pan and then into the house drain or into the yard, next to the dead attic fan. It's possible to install a control that will do this, but the question remains — why run the air handler all the time?

Some people do it so that the air in the house is being filtered more. If you have allergies or similar problems, that may be worthwhile, IF you shut the fan off while the water drips off. For most allergic people, the worst thing you can do is raise the moisture level in the house. This strategy also assumes that you've installed a high efficiency filter and replace it or clean it as necessary. Running more air through the typical fiberglass furnace filter will do little or nothing for cleaning the air in your home.