Today's generation of gas furnaces can boost indoor comfort levels at the same time they cut heating bills.

Considering this combination of benefits, is it any wonder that so many homeowners are replacing their old, inefficient units with these modern, high-tech models?

If you're thinking about replacing your old gas furnace, the Trane Home Comfort Institute reminds you that selecting the right size furnace is important. A unit that's too small will lack the ability to give you the comfort you want. A unit that is too large will cost more to buy and operate.

The heating experts also remind you that the size of a gas furnace refers to its heating capacity not its physical dimensions. The capacity of a heating system is measured in BTUs (British thermal units) per hour.

The correct furnace size depends on the size of your home, its construction and insulation, the way you use the house and the normal winter temperatures in your area.

A qualified heating and cooling dealer can estimate the heat loss of your home by measuring the amount of wall, roof and floor space exposed to the outside, and the amount of insulation you have.

Using a mathematical formula that includes the desired indoor temperature and the local outdoor temperature ranges, he can then recommend an appropriate size for your new furnace.

In many cases, older homes have furnaces that are larger than necessary, and homeowners are pleased to discover they can replace it with a smaller capacity unit.

This situation often exists because of the addition of significant amounts of insulation or because of other energy conservation measures that have been taken to substantially reduce the heat loss of the house.

The increased efficiency of today's gas furnace is another reason. For example, a new, smaller furnace that's 90% efficient can produce as much heat as a larger capacity furnace that's only 60% efficient.

The heating contractor can also tell you if any changes are needed in the venting of the furnace. In most cases, the new furnace is placed in the same location as the old and uses the same venting system. However, many of today's high-efficiency gas furnaces can be vented out a side wall similar to a gas dryer.

If you're thinking about installing a new furnace, the Trane Home Comfort Institute notes it's also a good time to think about adding accessories that can increase comfort levels even more. These include an electronic air cleaner, a humidifier and a programmable thermostat.

And, if you don't already have it, it's also a good time to think about adding central air conditioning. Be sure to tell your heating contractor of your desire so that he can match the needs of the cooling system with that of the gas furnace.