Reply: Conceivably he could chug away for a few more decades, especially if you continue providing annual maintenance. During that time, however, "Old Reliable" will probably cost you several thousand dollars more than you would have spent on a replacement.

O.R. was born in an era when heating fuel costs were measured in dimes rather than dollars. Therefore, just like auto makers of the 1950s and '60s, the boiler companies ranked fuel efficiency way down on their list of engineering priorities. Skyrocketing energy costs and government mandates have since turned that list upside down.

The buzzword you hear from them now is "Aw phooey" meaning AFUE (Annual Fuel Utilization Efficiency). This is expressed as a percentage that measures a combination of combustion efficiency and standby losses. Most modern boilers have AFUE ratings above 80% with some models topping 90%. As of 1992, all boilers and furnaces made in the U.S. are required to have an AFUE of at least 78%.

Your beloved O.R. probably has an AFUE in the 50-55% range. That is abou is about a third less efficient than a modern unit bearing an AFUE of 85%. Figure out how much you spent last year in fuel costs and divide that by three. This roughly reflects how much money you will save each year in fuel expense by putting in a new boiler.

Now figure how many years it would take for those savings to surpass the cost of getting a new boiler installed. Many times it pays for itself within three or four years. (A good heating contractor ought to be able to help you calculate with more precision what your cost savings and payback might be.)

Something else that might surprise you is how much smaller new boilers are compared to the old models. The "footprint," i.e. the amount of floor space they take up in your basement, may be only half as much as O.R.'s.

You don't specify whether your boiler is steam or hot water. Steam is generally a little less efficient than hot water, so if you decide to replace O.R. you may wish to find out how much more it would cost to convert from steam to a hot water system. Likewise, if you have an oil boiler, you might realize savings converting to gas.

Of course, if you have a sentimental attachment to "Old Reliable," you can always say "Aw phooey" to AFUE.