piping size for radiators

bauer

Hello,



Piping size question.... home has radiators in a two pipe system on one zone. the cast iron main supply pipes are 2-3" in diameter, and each radiator is tapped off with 3/4" cast iron.



My question is --- do these main supply lines need to be so large? If not, what exactly do I need to do to calculate their size? I'm assuming I can easily pipe the lines to the radiators with ox-barrier pex, but looking for info to find the size of the main pipes.



Trying to clean up the basement... a little efficiency gained from more modest piping would be nice, but mainly I'd like to get them out of the way because they cause a lot of headaches (quite literally).



thanks!

SWEI

It all starts with a room-by-room heat loss calculation

From that, and a survey of the existing radiation, you can determine the flow required for each radiator and for the system as a whole.  From that will come pipe sizes.

Tinman

Is this a residential application?

If so, does the house heat well during the coldest days of winter?



Does each individual room heat well also?



On the boiler data plate, what is the boiler's Net BTUH?



With that number, pipe size can be determined. The home run system you're describing is a great way to go in many cases. Pex-Al-Pex supply and returns home runned from the boiler room to each radiator, TRV's on the radiators, manifolds in the boiler room...sweet!

bauer

thanks...

tinman,



it is residential. the house heats enough for the radiators that we have... i say that because it is a 150+ yr old home, and only about 70% of the home has radiators. the rest is baseboard electric or wood stove. The rooms with radiators heat adequately.



boiler BTUH is 126,000



On a HR setup, is there a manifold for the pex at the boiler that the HR's come back to? We currently have only one zone so I was planning on TRV's to help.

Tinman

Is that

the "Net BTUH"? That's very important. If so, and you are certain you don't plan on adding radiation to meet the heat loss of the entire structure, then 1 1/4" pipe is what you need with a 20 degree delta T.



If there is ANY chance of adding more later, I would certainly do as SWEI suggested and do a heat loss load calculation.



Yes, you could set up one supply manifold and one return manifold in the boiler room.

bauer

thanks again...

if i were going to add radiators for the rest of the house --- I'm assuming I would need a new boiler to meet that demand, right?



is that 1 1/4" mains splitting off to--- what, 3/4" branches to the radiators?

Tinman

If you're going to do the home runs

with pex-al-pex, the only mains you'll have are in the boiler room...from the boiler to the manifolds. I can't say whether you would need to upgrade the boiler for additional radiators or not without a load calculation.