Engineer

steam to hot water conversion

you may well have enough leaks from the old system, so that the problem will not be heat, but keeping people dry. just pressurize the system with 30 psi soapy water, and you will see what could be in your future. i would plan on new piping, and rads and you should be O.K.--nbc

Why, oh why

are you doing this?  I sincerely hope that there is a very very good reason indeed, for you are in for a myriad of problems, of which how the hot water flows through the radiators will be by far the least of your worries.

This is not to say that conversion of steam to hot water can't be done; it can, and sometimes it even works pretty well.  What it is to say is that it is very rarely a good idea, at least in my humble opinion.

Ideally you would get the steam system working properly; it isn't that hard to do, and there is a lot of help available right here on the Wall, and sometimes there is a good steam pro. right in your area.  It would surprise me greatly if getting the steam system to work right wouldn't be a great deal cheaper than changing over.

However.  If your heart is set on hot water, item 1 is going to be to check all the piping and radiators for leaks, even pinholes.  NBC's approach is as good as any, but I would add a bit to it.  Pressurize the entire system to 30 psi with water (I wouldn't use soap, as it is hard to remove completely) and close it up with a pressure gauge on it.  If the pressure gauge drops, you've got leaks, even if you haven't got gushers.  All -- and I do mean all -- the leaks will have to be fixed before you do anything else.  I would plan on replacing most of the piping and, probably, a good half of the radiators at this point.  This is probably just as well, as it is quite likely that the piping arrangement which works so well for steam will work for water; leaks or no, significant repiping will be in order to control where the hot water goes and which radiators get it.

Having done that, you also need to check to see that you will have enough radiation.  Keep in mind that a radiator fed by steam is at about 210 Fahrenheit.  A radiator fed by hot water is at, at the most, 180 and more likely 160.  That difference may not seem like that much, but it is -- your radiators will put out only about two thirds as much heat as they did on steam.  Will that be enough?  If not, what do you plan to add?

But as I said above, why, oh why, are you planning to do this?  I am not against a good hot water heating system, planned as such -- they're great.  But if you already have a steam system, fix it up!  Don't change it!  We'll help you fix it!

Just Don't Do It!