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Hot Water Radiator tube diamater

We have 6 9 section hat water radiators all sandblasted and ready to go. we will be using a two pipe system. one pie hot to all radiators, and one pipe return. since I am not a plumber, what diameter of pipe is standard in an installation such as this? in the home we removed these units the pipes were over 2" ???
we plan on running lower temp water longer...

Also, would Pex work for this type of retrofit?

thanks ya'll


  • that's 6, nine section radiators ....NOT 69! : )
  • Steamhead
    Steamhead Member Posts: 16,723
    Those radiators came off an old gravity system

    which is why the pipes were so big. Since you'll be using forced circulation the new pipes will be much smaller.

    A Burnham radiator book I have states that any rad up to 100 square feet can be supplied with 1/2-inch pipe. You didn't tell us what type of rads they are (column, tubular etc) or their height- but in Dan's book "EDR" the biggest 9-section rad I could find was 90 square feet, and most were much smaller. I think it's safe to say that the branch lines to your rads can be 1/2-inch.

    Sizing the mains takes a bit more work. You need to know how much heat you need to move thru the main. Start with a heat-loss calculation of your building. Then determine what your highest water temperature will be on the coldest day of the year (usually 180 degrees with big old rads like yours- at this temp each square foot of radiation emits 150 BTU/hour). Then you'll know how much water you need to move and can size your mains from that.

    Dan has two books that will really help you along. One is "EDR" which has radiator size info so you can look yours up, and the other is "Pumping Away" which is all about piping hydronic systems the right way, and contains pipe-sizing info. You can order these on the Books and More page of this site.

    As far as piping materials- I don't see why PEX wouldn't work but don't have much experience with it myself. If that were my system I'd use copper or steel, like the Dead Men did.

    The above probably sounds like a fair amount of work- and it is. I know you'd be better off finding a Wethead to handle the job. Use the Find a Contractor page of this site to locate one near you.

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  • Joel_3
    Joel_3 Member Posts: 166
    We use

    We use 1/2 pex to the rads, actually i prefer fosta pex for this (pex alpex) as it holds it's shape and doesn't exspand as much. We would use copper or steel for the mains and put a ball valve at each branch so you can isolate the rads .then run some type of weather responsive controll either via a mixing valve or whatever way you want. trvs on the rads if you can swing the cost. if not just the ball valves let you ballance it out pretty well with a little expierementation.

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