Q We had to put a new furnace in our home and they put in a flexible tube from the outside to the furnace. It was to supply fresh air for the furnace, but it seems to me like having an open window in the house. It brings in lots of frigid air, especially this winter. I can't believe it's efficient. Is this tube really necessary?

A Yes, in your situation, it is necessary. Do not remove the tube or plug it, or you could have dangerous air-quality problems in your home.

What you have is old technology. The best furnaces available today are sealed combustion design. That means they have a rigid pipe that supplies air directly to and from the furnace. Indoor air, the air you breathe, is completely out of the loop. The combustion process is "sealed" from your home. These furnaces are safer as well as more comfortable because they don't pull cold air into the living space.

Older furnace designs need the "tube" you describe, an insulated flexible duct called the combustion air supply. It is critically important because it delivers outside air needed for the proper operation of your furnace. If it's removed or is plugged, the furnace can malfunction and release poisonous gases, including carbon monoxide, into your home.

The combustion air duct should never be blocked. To reduce the amount of cold air that flows in, place the discharge end of the duct into a large wastebasket or box. Don't attach it directly to the cold air return unless that's approved by the manufacturer. Doing so can lead to premature failure of the heat exchanger and the production of carbon monoxide. (Also, if you have a central air conditioning system, connecting the combustion air to the return air will make the system less effective.)

For more information on combustion air supply ducts and purchasing a furnace or air conditioner, contact the Minnesota Energy Office at 651-296-5175 or 1-800-657-3710. Or go to www.commerce .state.mn.us. Click on "Energy Information" in the upper left corner.

Ice from hot water?

Q Is it true that hot water freezes faster than cold water when making ice?

A No. Hot water must become cold water before it can freeze. As a result, it takes longer for hot water to freeze than cold water.

However, scientists say the freezing temperature for water that is not pure is less than 32 degrees. Thus, if the hot water were pure but the cold water contained impurities, hot water could cool and freeze quicker than cold water.

Send your questions to Fixit in care of the Star Tribune, 425 Portland Av. S., Minneapolis, MN 55488, or call 612-673-9033, or e-mail fixit@startribune.com. Past columns are available at www.startribune.com/fixit. Sorry, Fixit cannot supply individual replies.