I’m remodeling my basement and the over head cast iron heat pipes that supply heat to the radiators are positioned low and a bit from the walls. A plumber suggested replacing them with copper pipe and lifting them closer to the ceiling and walls where they can be enclosed in a soffit. My question is will a smaller copper pipe reduce the efficiency of heat supplied by the radiators throughout the house? The current cast iron pipes are about 3.5 inches wide.
Many of these older heating systems used very large size cast iron pipe because they did not have any hot water pumps to circulate the hot water and they operated at very low or no pressure. The large piping provided a kind of thermo-siphon effect which allowed hot water to rise and cold water to return without a pump. In this case, the large pipe size was not due to a high water flow rate. If your new system now has a circulating pump, you will not need large piping since the higher flow rates and system pressure to remove air can transfer much more heat per hour.
A 1-1/2″ copper pipe has a flow rate of over 20 gallons of hot water per minute with a nominal pressure drop, which is probably more than your pump can pump. A 1″ copper pipe can flow 9 gallons per minute. Except for the main headers, I would bet most of your piping run-outs to radiators are only 3/4″, but this size pipe can still flow 5 GPM in each leg which will add up to the main header flow rate. Remember, a smaller pipe costs less to insulate.
Hope this helps,