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New Copper Pipe In Gravity Hot Water System

Tim_48 Member Posts: 1
Hello, We own a 1920s duplex where each floor/unit has its own gravity hot water system (coal boiler converted to oil). Each radiator has two pipes. We are remodeling the second floor and moving a radiator a couple feet. The plumber is installing 3/4" copper pipe to the radiator from the original 1” inner diameter steel(?). Should I be concerned about the reduced diameter? Should I be concerned about the heat loss of the copper? Should I be concerned about corrosion between the different metals?
Thanks, Tim


  • Brad White
    Brad White Member Posts: 2,398
    Yes and a maybe

    Modifying a gravity hot water system is sometimes more art than science. The good news is that the radiator in question is higher in the building than say a first floor radiator. The higher the radiators the greater the flow potential.

    The size difference will affect flow because the pressure differences are relatively small compared to forced (pumped) hot water systems. The paradox is, with small pressure drops (otherwise fairly large piping), small differences become magnified relative to the whole.

    Really though, the question is, by how much will flow be affected? A little less flow, say 25% less, would be hardly detectable as the output would still be about 95 percent of what it was before.

    If the radiator is over-sized compared to the room's heat loss, you may never notice. If it is marginal and the other rooms are over-radiated, it could be a cooler room. Lots of variables but there is really only one way to find out. :)

    I would want to see the same pipe size used regardless. It is enough that you are changing other variables (lateral distance however small). Besides, you are only talking a couple of feet, why not go for at least the same size as before?

    Provided the metal to metal connections are made with a brass nipple or brass ball valve, I would not worry about the two metals. A direct connection of two dis-similar metals in the presence of an electrolyte (water) will develop a potential (micro-voltages) which can start corrosion as you suspect. Despite this, some such joints have lasted for 50 years with no sign of deterioration.

    My $0.02

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Steamhead (in transit)
    That plumber

    is applying the usual conversion from galvanized to copper water-supply pipe. In that situation, going down one size shouldn't cause a problem.

    But in an old gravity hot-water system you definitely do not want to reduce the pipe sizes. The resistance in these big old pipes is so low that if you reduce the pipe size, thereby increasing the resistance, you may get little or no heat in that radiator. The water will just go elsewhere.

    Tell the plumber to use 1-inch pipe. The change from iron to copper, as Brad says, shouldn't be a problem.

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  • zeke
    zeke Member Posts: 223

    In going from 1 inch to 3/4 inch you decrease the flow area by 50% which means you could get as much as a 50 % drop in flow and a significant loss of heat transfer.
    I would strongly agree with steamhead on this. Don't let the plumber do 3/4.
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