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Clanking radiators after hot water boiler replacement

Farian Member Posts: 4
I have a hot water system with cast iron radiators that was converted this year from a gravity system to circulating in conjunction with replacing the boiler. The new boiler is a Buderus GC144/5 with outdoor reset. Previously, the system was pretty much silent but now there is occasional, loud clanking of some of the radiators. The radiators have been bled (and rechecked) and there was no air in them, so I'm guessing that the clanking results from expansion/contraction of the cast iron. The clanking is somewhat random but, generally, there's a smaller clank when the heat first comes on and then occasional louder clanks while the radiators are cooling down. I'm guessing that with the gravity system, the temperature changes were more gradual and that might be a factor. Also, because the old boiler had a quite massive pilot light, the low temperature of the radiators (between heat calls) was probably warmer than it is currently. Is there any way to fix this? Could changing the water temperature setting affect it? I've been reading about continual/constant circulation with hot water systems and wonder if that could help. Thanks for any thoughts on this.


  • the_donut
    the_donut Member Posts: 374
    Cut some plastic milk gallon containers and place under feet if expansion is forcing radiators to move, this will let them slide. Look for tight or no clearance around pipes, cut floor around to allow movement, won’t take much, 1/8” or less is all that is needed. I doubt it’s thermal expansion of radiators, otherwise you would experience it on startup on old system from dead cold. You could also throttle back supplyin every radiator or insert orifice on supply of each radiator.
  • Jamie Hall
    Jamie Hall Member Posts: 23,536
    Use that outdoor reset to get the circulating water at the right temperature to maintain the space -- there are several control strategies and devices for this -- and then run the circulating water constantly (secondary pump) with the boiler running just enough to provide the heat.

    Pretty standard operating procedure these days... if your installer can't figure it out, find one who can.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Farian
    Farian Member Posts: 4
    Thanks much for the suggestions. Do you happen to know if there's a minimum water temperature for this boiler to operate properly? And if so, what it would be?