So before you commit to the job, raise the steam pressure to about 10-psi pressure and walk through the entire system looking for leaks. If they're there, they should show up under the increased steam pressure. Once you see them, you'll have a better idea of what you're up against, and you'll be able to make the decision as to whether or not you should move on from here.

Next, if the system looks sound, you can take out the tools and go to work. You'll have to remove the innards from all the thermostatic radiator traps and float & thermostatic traps near the ends of the main (if you have them) so the water will be able to flow freely. Again, be especially aware of any old Vapour equipment.

There's a big difference in size between the supply and return lines in a steam system so balancing will always be a challenge when you're running on hot water. Leave the old radiator supply valves in place if they're working. You'll be able to use them for balancing the flow and the heat once you have the system up and running.

Most two-pipe steam systems are similar to two-pipe, direct-return hot water systems. The big difference is that, unlike a hot water system, the return line doesn't increase much in size as it returns to the boiler. With steam, the lines are picking up condensate, not the much greater flow of water you expect to see in a hot water system. Because of this, you may have to redo the return lines to accommodate the greater flow you'll be seeing once you've converted the system. Are you beginning to see why I prefer to fix old steam systems rather than convert them to hot water?

The good news, however, is that each two-pipe radiator will have a return that's at least 1/2" in size. A 1/2" line can safely carry 1-1/2 gpm of hot water and that translates to about 15,000 Btuh, which is usually more than the typical radiator will need to put out to heat the average room.

If you're going to keep the old boiler, you'll have to change the trim, getting rid of the gauge glass, the low-water cutoff, pressuretrol and near-boiler piping. If you're replacing the boiler, you'll still be getting rid of the existing near-boiler piping and setting the boiler up as you normally would with a hot water system. If you're not sure how to do this, get a copy of Pumping Away, which is also in the Books & More section of www.HeatingHelp.com.

Size your circulator based on your heat loss calculation, not on the existing radiation. Take your gross Btuh load and divide by 10,000 to get the GPM for the pump. To figure pump head, measure the longest piping run, from the boiler to the furthest radiator and back. Allow six feet of pump head for each 100 feet of piping in that longest run. That's a rule of thumb, but it works well.

Use a good air separator. Every hot water system needs a good air separator.

Sizing the compression tank is tricky because there's going to be so much more water in those old steam pipes than you'd expect to find in a hot water system of the same Btuh rating. This is important because you base the size of the compression tank on the system's total water volume. Figure out how much water you have in the system, and then talk with the folks who sell the compression tanks. Don't depend on their quick-sizing charts in this case. They base those charts on standard hot water systems, not steam conversions.

You'll have to get a count on how many feet of each size pipe you have in the system to estimate the total water volume. Here's a chart that will help you figure out how much water you can expect to find in each foot of pipe:

If you've gotten this far, the next question will be, How do I control this hybrid I just created? The best way I know is to run the system on continuous circulator with an outdoor-air reset controller. It costs a bit more to set it up this way, but it is by far the best way I know to operate any hot water system, be it old or new.