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sizing replacement steam radiators

Taylor_4 Member Posts: 55
I am replacing recessed steam radiators with new unrecessed radiators. Doing heat loss calculations I come in with e.g. 4K BTUs for a radiator whereas the existing radiator is 7K BTUs. Should I go ahead and get a similar-sized radiator (with a 10% reduction for non-recessed, as suggested by one manufacturer), or go with one based on my heat loss calcs? Basically I would take a radiator sized between what my heat loss calcs suggest and what I have now.

Having read Dan's books, I am not surprised that my system is oversized. My heat loss calcs assume the house is insulated, which it was not when it was built. I am assuming that I should not cut back too much on radiator size if I am not replacing the boiler (it's 7 years old, though copper return pipes may have shortened its lifespan). I have already removed one radiator and the house stays warm enough, the system typically runs for about 15-20 minutes every hour which I guess means it's about 3 times larger than necessary. OTOH it's one of the manufacturer's smallest boilers....

On a related question: if I remove a radiator serving an alcove, leaving heating to the radiator in the main room (suitably sized), will I cause any problems? I hear this term "temperature gradient" but don't understand its significance....


  • Brad White
    Brad White Member Posts: 2,398
    Another view...

    We all know that sizing steam boilers is a function of the connected radiation which we all hope also conforms to the heat loss of the rooms they are in.

    When old homes (with assumed radiation proportional to the heat losses) are insulated, that ratio of radiation to heat loss goes up. Good! To a point anyway.

    But... at least the proportionality of radiation to heat loss is maintained. A radiator which may have been 100% or even 120% of heat loss becomes 150-200% of heat loss.

    Here is my concern: The remainder of your radiators are sized now for a much lower heat loss than they can handle and may be double the size they need to be. Your new room has half as much. Same material, great.

    But now you have half the mass/area. Will the space heat at the same rate as the other rooms? Open question, never thought about it. But I am willing to bet or at least ask if this is a concern with steam. It is, to me, with Hot Water.

    But if I am sizing HW radiation in a similar vein, I keep the apportionment the same throughout if I can. If the radiators are 1.5 times the heat loss for older rooms, the new radiator is also 1.5 times the size. Thus they can all use the same water temperature.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • valid point

    It may not matter as much during mild weather, or during regular run cycles, because most radiators won't heat all of the way across before the thermostat satisfies.

    On long run cycles, like coming out of setback or very cold weather, proportion will play a very big role in how fast individual rooms keep up or recover.

    I wouldn't downsize much, unless you downsize all radiators equally, proportionally.

This discussion has been closed.