Downsizing radiator supply pipe

gadolphus577

I have an old hydronic boiler system with cast iron radiators. The system has a circulator near the boiler.

The supply and return pipes in the basement are iron and about 3 inches in diameter. I need to replace one.

Can I replace it with copper or pex that is one-inch in diameter — meaning I'd have a one-inch diameter pipe with 3-inch pipes on either side? Or would that constrict the flow too much?

Thanks in advance for your replies. And please be nice; I'm an amateur — although I did read Pumping Away because these old systems are really interesting.

Steamhead

This sounds like it was originally a gravity system. DO NOT decrease the pipe size. If you do, the system will heat unevenly.

EdTheHeaterMan

That depends on the location of that ppe you want to replace. @Steamhead is right if the pipe restricts the flow to only a portion of the system. If that smaller diameter pipe is going to restrict the flow to every radiator in the system, Then you may be OK with a smaller diameter. How small you can go will depend on the heating needs of your home. Using a rule of thumb for residential heating systems where you have a 20° temperature difference between the supply side of the boiler outlet and the return side of the boiler inlet, a 1" pipe will only move about 80,000 BTU of heating energy per hour.

A 1-1/4" copper pipe can move about 140,000 BTU/h

If this is a DIY repair, I would stick with the same size pipe. If you are asking because a professional is suggesting it. I would ask them to put it in writing that this repair will have no effect or how balanced the system is next winter. Can he place a performance bond to guarantee his work will not inadvertently compromise your heating system performance?

gadolphus577

Thanks to you both. It is a pipe that supplies the entire system so I gather I'll be OK as long as it's large enough to meet the heating energy requirements. I'll probably do 1 1/2 inch copper, which seems like the largest that's readily available (and that I can cut).

pecmsg

your paying to heat a lot of extra water with those oversized pipes.
consider repiping all the main pipes in the basement. You’ll also gain some head room.

Pictures would help.

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1802612#Comment_1802612

@pecmsg believes everyone has unlimited funds that can be spent on saving energy cost. I don't believe that re-piping your basement will amount to that much operating cost savings. It may even hurt if you reduce the total water content of the system to the point where you end up with short cycling. Then Bob Rohr might suggest a buffer tank to add more water volume to the system to solve the short cycling problem that @pecmsg created by reducing the water volume.

Just messin' with you! I'll bet he is using your favorite thermostat!