Base the size of the replacement boiler on the connected load, not the building’s heat loss. The boiler’s ability to produce steam must match the system’s ability to condense steam. It really doesn’t matter what the building’s heat loss is. The radiators and pipes are going to condense steam at a certain rate, and if the boiler is too small (which usually happens if you size it based on the heat loss), the steam will go just so far. You’ll wind up with a building that has either hot rooms or cold rooms, and there’s not much you can do to cure the problem at that point.

Go from room to room and survey the radiators. Then use a suitable pick-up factor for the piping. If you look in a boiler manufacturer’s catalog you’ll see they’ve already built in a pick-up factor. It’s the difference between the Net load and the D.O.E Heating Capacity load. It’s standard to increase the Net load by a factor of 1.33 nowadays to get to the Heating Capacity. If you notice that radiation is missing from the building, though, or if you see there’s no insulation on the pipes, the piping pick-up may be more than you expect. In cases such as these, protect yourself by using a larger pick-up factor. Measure the radiation and multiply it by a factor of 1.5. Then select the boiler from the D.O.E. Heating Capacity column, not the Net column. You’ll be glad you did this when that boiler starts for the first time.

If you find the building is over-radiated, consider removing radiators, or better yet, use thermostatic radiator valves at the existing radiators. Don’t downsize the boiler figuring each radiator will get "half-hot." What will actually happen is that a portion of the radiators in the building will get fully hot while the others stay stone cold.

If the boiler goes back to the Coal Era, know that it may be 75 percent larger than it has to be. It was standard to increase the boiler by that much in the old days because engineers weren’t certain the owners could make proper coal fires. By oversizing, they pretty much guaranteed there would be enough heat – even with lousy firemen. A contractor friend regularly measures the radiation and comes up with steam boilers of, say, 60 horsepower where 100-horsepower boilers are already on the jobs. The smaller boilers work beautifully. Never base the size of a replacement steam boiler on the old boiler’s rating. Survey those radiators. It’s the only way to be certain.

Be aware of water levels. This is more important in gravity-return systems than it is in pumped-return systems. If the new boiler’s water level is too high, there may not be enough vertical space between the boiler water line and the end of the steam main (in one-pipe systems), or the lowest steam trap (in two-pipe, gravity-return systems). The result? Returning condensate backs up into the mains causing water hammer. This usually happens in the middle of the steaming cycle.

If the water level of the new boiler is too low, there’s a chance you may turn what was a wet return (a pipe below the boiler water line on a gravity-return system) into a dry return (a pipe that’s above the boiler water line). Here again, your reward for making this mistake is water hammer. If you find an old boiler that’s sitting up on a pedestal, know that it’s there to cover a return line on the building’s first floor. This was once a very common piping practice in apartment buildings. If you install your new boiler off the pedestal you’ll have to create a false water line to avoid water hammer. My book The Lost Art of Steam Heating goes into detail on how to make one of these wonderful devices. You can get a copy in the Books & More section.