A recent column explaining the cold-air supply into homes -- that plastic-and-wire tube or duct that runs from the foundation and ends on the basement floor -- generated additional questions on the topic. Called the combustion air supply, this duct supplies fresh, outdoor air to the furnace and water heater. It helps protect occupants from noxious fumes, including carbon monoxide, generated by these appliances.

Don't plug the line

Q I was told by a heating contractor that houses built before 1994 don't need that duct to supply air to the furnace, that the house is leaky enough. Since my house was built before 1994, he told me I could plug the hose or disconnect it.

A That contractor is wrong. Do not plug or disconnect a fresh-air duct or combustion air supply.

It's dangerous to assume that a house is leaky and that appliances won't spill poisonous exhaust into the basement. At least three studies reveal that Minnesota homes -- even ones built in the 1930s -- are surprisingly tight. Some are as tight as homes built today. Why? Because the craftsmanship was good, and the home's owners have updated them with newer windows, doors and insulation. In one study, about 70 percent of the houses tested were tight enough to cause a water heater or furnace to malfunction and spill noxious byproducts of combustion (such as carbon monoxide) into the house. The combustion air supply helps mitigate the danger and is required by the state mechanical code.

Door can affect air flow

Q It gets so cold in my furnace room that other items could freeze. Should I just keep the door ajar to allow heat from the rest of the basement into this room?

A It's unlikely that the room gets that cold; a thermometer could verify that.

Meanwhile, do not close the door. That can interfere with the balance of air that comes in and out of a home through natural ventilation, which could cause other problems, said state energy specialist Phil Smith. If you want the door closed, install two vents into the room, one high and one low, so air can move between the room and the rest of the living space.

Pressure is the key

Q Cold air is heavier than warm air. The fix is to lift the end of the hose from the floor to the same height as it enters. Then air will come in only when it's needed, right?

A That's no solution. Air density plays a role, but air pressure plays a greater role, Smith said. The building code specifies that the duct has to end within 12 inches of the floor. Do not crimp, bend or shape the tube in any way that would impede the flow of air.

Avoiding cold air

Q I got a new furnace, but now I have one of those tubes in my basement. I hate the cold air that pours out of it. Could it have been avoided?

A Yes. The furnace you bought is old, 1980s technology. It's perfectly good at heating your house, but it has a design weakness. It needs a combustion air supply to make it safer for occupants.

The newer design, a sealed-combustion furnace, is different. It has a small, hard plastic (PVC) pipe that runs air directly to the furnace for combustion purposes and another one to carry byproducts of that combustion out of the house, usually out the side of the house. This design completely seals the furnace burner from the indoor air, making it far safer than previous furnaces.

However, before you get angry at yourself or your heating contractor, realize that a sealed-combustion furnace alone won't eliminate the need for a combustion air supply. You would need a different water heater, too. If you also had a sealed-combustion (or power-vented) water heater, you wouldn't need the combustion air supply.

If you are considering replacing a furnace, and your water heater is older, replacing them at the same time is a good idea. Then you can get sealed-combustion designs and eliminate the need for a combustion air supply duct into your home.

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