Q - My doors let a lot of cold air leak in all winter. Now that the weather is warmer, I plan to weatherstrip them. What types of weatherstripping should I use for the best airtight seal? S. F.

A - Even though the worst of the cold winter weather is past, this is an excellent and more pleasant time to weatherstrip your doors. By spending about $25 on materials, you can save up to $100 each year on your utility bills and be more comfortable.In the summer if you air-condition, airtight doors have a more significant impact on your utility bills. Since the indoor to outdoor temperature differences are less in the summer, a greater percentage of the energy losses are due to air leakage.

The selection of the proper type of weatherstripping depends on your door and how often it is used. For a seldom-used door, adhesive-backed foam weatherstripping is inexpensive and very easy to install. With heavy usage, however, it will not hold up and provide a good airtight seal.

If the mating surfaces between your door and door jamb are not too uneven, a metal V-strip is very effective and durable. Since it is mounted in the door jamb, it is fairly well protected from damage.

The V-strip is available in rolls. You nail the longer leg of the "V" to the door frame with the "V" pointing in the direction the door opens. Then bend out the free leg a little to form the "V." When the door closes, the free leg springs against the door edge to block air leaks.

Another easy-to-install type of weatherstripping uses a rolled vinyl bulb. You nail the backing strip on the bulb to the door jamb. The bulb should extend slightly past the edge of the door stop so the door compresses it a little when it is closed. One drawback to this is that the bulb is exposed and susceptible to damage by children.

The best but most difficult to install weatherstripping is interlocking metal strips. One strip is mounted on the edge of the door and the mating strip is mounted on the door jamb. This is fairly well protected from possible damage.

When the door is closed, the two legs of the mating strips slide together. This results in a very airtight seal. You must carefully align the mating strips during installation or the door won't close smoothly or seal well.

You can write to me for UTILITY BILLS UPDATE No. 017 showing a selector guide for door weatherstrip-ping and diagrams and do-it-yourself instructions for installing the various types. Write to James Dulley, The Deseret News, 6906 Royal Green Dr., Cincinnati, OH 45244. Please include $1.00 and a self-addressed envelope.

Q - I always hear about Btu of heat from my heating contractor and the utility company. Exactly what is a Btu of heat and what does it mean in everyday terms? C. W.

A - Technically, a Btu of heat is the amount of heat required to increase the temperature of one pound of water one degree Fahrenheit. All of your furnaces, air conditioners, and space heaters are rated in the number of Btu of heat that they can produce in one hour of operation, called BTUH.

In everyday terms, a Btu of heat is approximately the amount of heat given off by burning a wood stick match. When a furnace is rated at 60,000 BTUH, this refers to the heat content of the fuel that it burns. The amount of heat that actually warms your house is less and depends its efficiency.