Replace iron rad supply/return with PEX?

Ken Farmer

Am renovating my basement and need to replace a number of iron supply/returns hanging 18" below the ceiling so I can re-route the pipes to gain some headroom but I'm not sure what diameter pipe to use to ensure adequate flow to the rads.


My current boiler is an old low-mass boiler with a Taco 007 circulator. Boiler runs at about 18psi / 160F.
The supply/return headers at the boiler has 1.25" O.D. nipples but connect into larger 2.25" O.D. main supply trunks and 1.75" O.D. return trunks, which seems weird to me.


All 16 cast iron rads in my house (3 storey) are supplied via 4 main trunks - with four branch runs to rads from each of the mains.

My rads are of different lengths and height - ( from 18"-72" long and from 24"-48" tall, but most are between those extremes ). Most of them have 1.25" O.D. supply/return nipples but four of them (the larger rads) have 1.75" O.D. nipples.


I will be replacing the boiler with a condensing boiler after all the basement renovations are done and the new boiler room is complete - in about 8 weeks or so.


There is no zoning of the existing supply/returns but am planning to install zone manifolds with the new piping so I can control the heat better on different floors.


My wife is arthritic and is very sensitive to cold so I really want to ensure that the new rad supply/returns are sized correctly.




a) Can I use pex for this or should I use thick-wall copper? I was thinking of using Pex-AL-Pex pipe.

b) Can I use the same diameter pex for all rad sizes? One contractor who came here said 1/2" pipe was plenty, which seems to not be right but maybe the math works out(??).

c) What diameter pipe to the rads is correct to use?

d) Can you recommend a book(s) that I can read about the factors which determine which size pipe to use?


Thanks.

Ken