I plan to replace the 60-year-old furnace in my home. It is a steam-radiator system which was adapted to gas in the 1940s. How may I determine the proper furnace size? Also, can an automatic flue damper be added, or are these dampers primarily for reduction of the loss of warm air from a forced-air system rather than loss of warmth from a boiler? finally, are there any obstacles to my replacing the boiler and radiators with a force-air system other than the need to cut holes in the floors, disguise the ducts to the second floor, etc.? Peter S. White
First of all, dampers are used with boilers as well as furnaces. You should speak to a contractor about the size of chimney in your home. It's probably very much oversize. What does the code call for?
The contractor should be able to figure the proper furnace size for you by using something like the Manual J form, put out by the National Association of Home Builders, or its equivalent, for calculating heating loads on a building.
The furnace should be sized so as to work effectively on the coldest anticipated day in an average winter.
If you buy a gas-fired system, get an intermittent ignition device and a stack damper. They will cost more but should pay for themselves by the savings. The vent damper almost eliminates the problem of having an oversize stack.
The ignition device would be needless, however, if you're a disciplined person who goes downstairs in the spring and shuts off the gas pilot light and then turns it back on in the fall.
There is no need to go to a duct heating system unless you are interested in including a central air-conditioning system in the home at the same time. Clearly, there is no particular advantage, from an energy point of view, of the hydronic (hot water) system over a hot-air system in the heating mode.
Of course, you might want to tear out the old steam radiators and replace them with small fin-tube units instead.