Welcome! Here are the website rules, as well as some tips for using this forum.

# Changing pipe size

Member Posts: 2
I have a hot water boiler with cast iron radiators. The outlet pipe out of the radiator is 1 inch cast iron which sizes up to 2 inch pipe for 20 feet then sizes back down to 1 inch before going into the wall and attaching to the radiators. Can I change the 2 inch pipe to 1 inch without ruining the system? Can I use copper or pex instead of black iron?? I need to change the run to allow for a new room being put in. I will be adding about 4 extra feet to the pipe.

• Member Posts: 314
pipe replacement

Yes you can swap out the 2" for 1" might have to put an union in but I would definetly keep the pipe steel in this case.
• Member Posts: 2,398

this system is a gravity hot water system. If it is and you reduce the pipe size, the flow dynamics will likely change. On large pipe systems the pressure drops are relatively low so a modest increase to you is magnified as a large one to such a system.

I would keep whatever pipe you change out the same material and size as exists.
"If you do not know the answer, say, "I do not know the answer", and you will be correct!"

• Member Posts: 1,405

Presuming that the 2" portion of the pipe is horizontal there shouldn't be any consequence if you reduce it to 1". If the 2" portion is vertical, there might be a problem (assuming this was a gravity system that's now converted to forced flow).

Why the difference? If they dead men were forced to use a long horizontal branch run (and 20' is a very long horizontal branch run for a gravity system), they would use fantastically oversized pipes in the horizontal as an attempt to compensate for the lack of gravity flow ability.

Under forced flow the head loss in the branch piping is still going to be near zero--just like the original--even though the replaced horizontal portion is much smaller.

If however the pipe is vertical, be VERY wary. Adding even a very small amount of head loss to a single branch in a gravity system is a near certain way to greatly reduce or even stop the flow in that branch.
• Member Posts: 2
pipe change Copper?

thanks for all the advice. This definately looks like an old gravity system and the replacement piece is the horizontal run. As long as I use dieletric unions on all the conections why not copper?
This discussion has been closed.