Sizing PEX supply pipe for cast iron radiators.

Richie921

I added a second story and finished attic to my home.  I am using old cast iron radiators for heat (hot water syatem) My question is how do I size the pex supply and return lines to each radiator (I'm zoning each room with zone valves off a manifold at the boiler). Thanks

Richie

Bob Bona_4

Richie

1/2" pex will do a 15,000 BTU rad based on a 20 degree Delta, and that's a BIG rad! Don't think you'll be hefting THAT to the attic! Some guys will use 3/8" pex, but I run 1/2"..fittings are more common.

This is a job for pex with an aluminum layer. If you run conventional pex you will have expansion and noise issues as it heats. Run PAP such as Viega's Fostapex or Uponor's MultiCor.

Sounds like a nice job.

Richie921

Thanks Bob

Hey Bob, thanks alot. I was looking at the Pex-Al-Pex and now you guaranteed I will use it.  I assume when you say 1/2 inch will handle a 15,000btu radiator, that is based on 180 degree water temp right? Also please educate an electrician. What is a "20 degree Delta"? Thnx again.

Richie

Bob Bona_4

right.

1/2" can carry 15K BTU based on a Delta T (temperature difference) of 20 degrees. Which is the typical target for systems that are to run up to 180 degrees on a cold day. The return water should be 160 based on that pipe size (and other things outside this discussion).

It would be worth a look at ways to reset that water supply temp for the warmer days. The radiators can be proportionally warm to offset the room's heat loss, and the heating appliance will use less fuel.

Taco, tekmar, etc, make some nice bolt on outdoor reset controls to accomplish this. 

You can also sub-zone room by room by using thermostatic radiator valves in place of the usual hand valves that go on one side of the rads.



 

Richie921

Sorry it took sooo long...

Thanks again Bob.  1/2 inch PAP it is with thermostatic valves for each room on the manifold at the boiler.  I am looking at the weather respoinsive controls too. Thanks again and sorry it took sooo long.

Richie

World Plumber

HePex

Make sure you use HePex or you will rust the radiators shut in a short time.

NRT_Rob

PAP

has an oxygen barrier, the aluminum. Uponor's variant of this pipe is called "Multicore".

Steve Ebels_3

I concur

1/2" tube will supply all you need unless the rad is a monster.  The greater the temp drop you can get, the better a condensing boiler will work. (if that's what you are using)

I would also second the use of a Pex-AL-Pex product like the afore mentioned Viega Fosta-Pex.

Richie921

Fittings for PEX?

So I was still researching this and came across a post saying that PEX fittings are actually a smaller diameter than the pipe so you have to go bigger just to maintain the flow through the fitting. Does this mean that if 1/2 inch would supply enough water to the rads I actually have to use 3/4 to account for the smaller diameter at the fittings?  Also,  does anyone know the formula to calculate the required flow in gpm for each radiator assuming 180 degree water? Thanks again.

Richie

Mark Eatherton

Sounds like you've been reading a CDA web site....

they HATE plastic tubing :-) Yes, the fittings do have a higher pressure drop than copper fittings, but it does not represent the whole length of the tubing. It represents more feet of tubing than the tubing alone. I wouldn't worry about up sizing, unless 5/8" is cheaper than 1/2", in which case I'd say gopher it.



Tubing size is dictated by heat loss calculations. Radiator output is dictated by EDR. Operating temperature is dictated by the calculated load and existing EDR. Flow rate is a function of designed temperature differential, which in your case could be 15 degrees F.



1. Do a heat loss calc.

2. Do an EDR survey

3. Do a flow/pressure drop study.

4. Size pipes and pumps.



In the Book Store on tis site is a book called EDR. you will need that. There are numerous free on line heat loss calculators.



Also, in regards to getting rid of big pipes for the smaller pipes, you may run into trouble where the drop pipes come out of the ceiling. Some of these can't be easily removed or shortened, and you may end up with some funky looking piping connections at that point (think periscope). Some times you can get the radiator on the floor directly above the drop to disconnect and shorten it, but mains running to the second floor pose a problem.



Good luck with your project.



ME

Richie921

Thanks a lot.

I appreciate the response.  i'm glad I don't have to deal with any existing drops like you were saying.  It's ALL coming out. The old system was single pipe steam.  I'm gonna look for that book you recommended too.  Is there any downside to a pipe that's too big? Let's say 1/2 would be fine, is there a downside to running 5/8 or 3/4 (pressure drop, etc.).  I might be able to get 3/4 pex-al-pex cheap (read FREE) from a buddy who bought 2 rolls and didn't use it and just wants it out of his basement:). Thanx.

Richie



P.S. Just out of curiousity, how come it seems like the pipes in old buildings with hot water rads had such BIG pipe feeding them? Maybe this was before pumps so they needed more room. Just my uneducated guess!