pipes for hot water heat

I have hot water heat with cast iron radiators. Right now the piping is 3/4 in. copper in the immediate vicinity of the boiler (which is about 15 years old) but it then connects to old steel pipes. The steel pipes are sizable (the mains are maybe 2.5 in. and the loops to each radiator are 1.5 in.). I think the system used to be gravity-driven judging by the size of the pipes and the way the mains slope slightly up away from the boiler. But now there is a pump. I have been thinking I might want to replace the old steel piping, mainly to increase headroom in the basement, but also to reroute some of the lines which go through outside walls. Could I replace the steel with copper, and if so, how to size the lines? Is there any kind of plastic piping that is suitable for this application? Are there particular advantages or disadvantages of doing this I should know about? Thanks,
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here in buffalo ny the first floor has baseboard radiators, the second and third floors have old fashioned radiators. the boiler is from 1990.
sounds just like our house next door. here's some of what i learned since 1978: 1. first insulate the house [oops we didn't include the basement]. all radiators must be inside insulated walls and heated in winter or they will freeze and break like one of mine in a back stairwell when we turned off its supply line in the basement to save money. you will now have to shovel snow around the house since you are not warming up the walkways with your warm basement like i am. but there are less icicles forming on the gutters from wasted heat rising into the attic. 2. your basement may have comfortable but waste heat from all those properly sized pipes for your radiators without modern insulation on them. if you do what you suggest it will get cold down there and you won't need any headroom because you will be living upstairs in the comfortable insulated and heated habitable area of the house. 3. if you are going to reduce the amount of water in your system it will affect the way your system operates. your local hvac guy will be trying to figure this out to adjust the boiler for this and considering your tankless boiler. 4. if you are thinking of replacing cast iron radiatiors stop now. it takes wall to wall baseboards full of modern radiator piping to replace a big hot old radiator. 5. add as many zone valves and zones and thermostats as you can afford if you update your system. we changed from 2 to 4. 6. hang a copy of your boiler manual and the manual for every component of your system that you searched for at www.google.com and printed out by the boiler but above flood level. also list your annual maintenance steps that you perform regarding the expansion tank and so forth at the end of each summer.
i found basement insulation info in great detail at: http://www.buildingscience.com/resources/basements.htm
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You dont want pipes in outside walls they will freeze.
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Mixing tube and fin baseboard and ci radiators is a problem with balance too. If you do replace, use ci baseboard of the same capacity.
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Couple things in response/clarification:
-- Yes I want to keep the radiators, like them a lot. -- I am not too concerned about basement waste heat, currently have a teenager living down there. (If he's good, someday we might let him come upstairs.) -- currently some of the (steel) pipes to the upstairs radiators go through outside walls. While that has not caused problems in the 10 years we have occupied the house, it certainly is not real energy-efficient; one purpose of redoing the pipes would be to get away from that. -- mainly I am curious if anyone has done this sort of thing and has any tips. Also, curious if it would be practical to use CPVC or other plastic pipe rather than copper.
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Heh, heh, he'll be there for life.

I'm not sure that he gains would offset much of the cost of a re-do of them. If the wall cavity is insulated, there shold be little loss around the pipe.

If anything, PEX would be my first choice. While it is capable of taking 180 degrees, it is designed for 140.
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You dont use plastic for a boiler, think copper or galvanised.
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