Reducing diameter of radiator piping, will it effect the system's heat output?

[pclimber]

I posted previously asking about the feasibility of moving some radiator piping that runs along the ceiling in my basement. I finally had a contractor come out and take a look, and what he was suggesting was replacing the existing cast iron with copper at a reduced diameter.

I believe the primary supply / return lines now are about 3" cast iron and maybe 1.5" or so for the branch supply and return lines. What he wanted to replace this with was 11/4" copper for the primary supply and return and 3/4" for the branch lines. His rationale for the reduced diameter was that when the system was installed originally (the house is from 1925), the system would have been gravity based and that required a larger diameter pipe. Now that the system mechanically circulates the hot water, smaller diameter copper would be OK.

Is this legitimate? I want to make sure I'm not crippling the heating in the front half of my house by making these changes. To lend credence to his explanation, I do see that when the boiler was replaced the primary supply and return cast iron pipes that service it feed into narrower (probably 11/4) copper to actually enter and exit the boiler.

 

[johnjh2o]

Yes he is correct. If the system was at one time a gravity feed system and is now using a circulator the pipe size can be reduced. Is he replacing all the piping? If that is the case a one pipe system may be a better choice.


John

 

[pclimber]

Thanks John, that makes sense.

I wasn't planning on having the entire system replaced, the piping in the room I'm redoing services 5 of about 14 radiators, so it'd be significantly more work to have the whole system done.

I'm not too familiar with the different configuration options for hot water radiator heat, what is a one pipe system and what's the advantage?

 

[johnjh2o]

Where you now have two pipes that run around your basement you would have one. Reducing the amount of water in the system that needs to be heated. Thus reducing heating costs.

John