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Steam to hot water conversion

I'm estimating a steam to hot water conversion. I haven't done one since the 80's and that was with my father who understood steam and hot water. Customer wants to use the existing two pipe radiators. Steam came into the top taping and returned on opposite side bottom tap. They would like to reuse the piping going to the second floor radiators. I plan on installing all new supply pipng in basement and to first floor ratiators. I have had a heat loss done and i'm told some of the radiator are undersized for hot water.

Here are my questions : 1. Can I use the pipe entering the top of the radiator as the hot water supply ?

2. This existig steam system was installed in 1948. The house heats nicely, will hot water running through the radiators make them loose much btu's ?

3.Any other advise would be appriciated.

Thanks, Jamesy


  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I'm estimating a steam to hot water conversion.

    Why? Especially when you say "This existig steam system was installed in 1948."  So it is relatively new, Whatever is wrong with it, can it possibly be improved by converting to hot water more economically than figuring out what the existing troubles are and fixing them?
  • Jamie Hall
    Jamie Hall Member Posts: 23,289
    Why on earth

    Would the homeowner be wanting to convert a steam system installed in 1948 to hot water?  That is practically new!  Unlless it has been really hammered by some class A knucklehead, it's going to cost a lot less to get it running like new -- or better -- even allowing for a brand new boiler.

    In answer to your specific questions...

    Can you use the existing top entry?  Yes, but it won't heat as well as a bottom entry would.  You will have to change the valve on the entry, and remove any traps or other applicances on the outlet.

    Will you lose BTUs from the radiators?  Indeed you will.  Lots of them.  Just how many "lots" is varies, of course, with the temperature you run your hot water system at.  You can come up with a rough approximation, though, by dividing the difference between the water temperature and 60 by the difference between 212 and 60.  For example, if your water temperature is 160, you would divide 100 (160 - 60) by 150 (212 - 60, roughly) and decide that the radiator, with hot water, will produce about two thirds of the heat that it would have on steam.  That's approximate; there are more exact formulae.

    In order to meet the heat rquirements with the old radiators, (see above) you will probably have to run a pretty high temperature in your hot water.  This will remove any possible efficiency gains (which would be marginal anyway) from the conversion.  Even then, unless a lot of insulation has been added you may have to add radiation to maintain design temperatures.

    The old radiators may not be able to take the pressure of a hot water system -- since a hot water system runs at about 40 times the pressure a steam system should.

    So... you ask for advice.  My advice would be... don't do it.  And good luck.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Dave in QCA
    Dave in QCA Member Posts: 1,785
    Rarely a good Idea

    First to answer your questions:

    The piping entering the top of the radiator can be used as the supply connection.  Hot water system were often piped this way.

    Changing from steam to hot water will reduce the heating capacity of all radiators by 1/3.  That is, they will be capable of putting out about 62-66% of what they would have been able to on steam.

    Any advice.... DON"T do it!   I have personally seen several of these conversion projects.  They were designed by engineers and installed by very reputable contractors.  The anticipated savings were never realized and furthermore, portions of the system never worked acceptably.  Once done, there is usually never a chance to go back.

    For many years, I misunderstood the rating plates on boilers that gave both hot water and steam ratings.  My interpretation of the NET ratings was that the boiler would put out much less heat when set up for steam.  I was wrong!   The NET ratings, as I'm sure you probably know refer to the amount of radiation that can be connected, and steam usually is sized for a 1.34% oversize factor for piping loss and pickup.  Especially that pickup.  On hot water, the piping loss and pickup factor is usually 1.15%.  A decent steam boiler is going to perform in the 80%+ area.   If the homeowner is dreaming about the saving that will be had with a 90%+ system, it needs to be remembered that they only work at that high efficiency when the return water is very cool.  This is possible in low temp radiant systems and sometimes in cast iron systems, as long as the radiation is over sized for the current conditions in the building.  If you have to run the boiler on the hot side to compensate for the radiation being too small, the savings are out the window.

    Steam to Water conversions are indeed all the rage at the moment.  I first came to this website looking for help in doing exactly the same thing to the steam / vapor system that is in a building that I just purchased.  I thought I knew that steam was a bad system and terribly inefficient.  I learned how misinformed I was.  

    I really hope you have time to change the course on this project and that "the Train has not Already left the Station!"
    Dave in Quad Cities, America
    Weil-McLain 680 with Riello 2-stage burner, December 2012. Firing rate=375MBH Low, 690MBH Hi.
    System = Early Dunham 2-pipe Vacuo-Vapor (inlet and outlet both at bottom of radiators) Traps are Dunham #2 rebuilt w. Barnes-Jones Cage Units, Dunham-Bush 1E, Mepco 1E, and Armstrong TS-2. All valves haveTunstall orifices sized at 8 oz.
    Current connected load EDR= 1,259 sq ft, Original system EDR = 2,100 sq ft Vaporstat, 13 oz cutout, 4 oz cutin - Temp. control Tekmar 279.
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