sizing O2 barrier PEX in a home run distribution system

roundrightfarm

I'm setting up a dist. system for a home that will have a Buderus panel radiator in each room. Each radiator will be connected to a central supply and return manifold in the boiler room. I have a BTUH calculation for each room and have sized the radiators accordingly for my design water temp. (140F - 120F). How do I decide the appropriate diameter of O2 barrier PEX to use? It seems like 1/2" is commonly used, but how can I know if 3/8" or 5/8" might be more appropriate?

SWEI

Short version for SDR9 PEX: Keep your flows under 1.5 GPM on 3/8", 2.5 GPM on 1/2", and 4 GPM on 5/8."

Really, really short version: 3/8" PEX should handle almost any residential radiator made.

Zman

SWEI said:

Short version for SDR9 PEX: Keep your flows under 1.5 GPM on 3/8", 2.5 GPM on 1/2", and 4 GPM on 5/8."

Really, really short version: 3/8" PEX should handle almost any residential radiator made.

To add a bit more, if you know your BTU requirements per radiator.
Use the universal hydronic formula BTU/HR = GPM x Delta T X 500.
With 1/2" pex, 2.5 GPM will give you 25,000 at a 20 degree delta T.
Another question you should be asking is how long are your runs and how large a circulator do you plan to use?
If your typical radiator is less than 15,000 BTU and you don't have piping runs longer than 150' out and back, 1/2" pex with a smaller ECM circ is probably where you will want to end up.

roundrightfarm

If your typical radiator is less than 15,000 BTU and you don't have piping runs longer than 150' out and back, 1/2" pex with a smaller ECM circ is probably where you will want to end up.

Only my longest runs will approach 150', out and back. Many will be much shorter. I do have some rooms that require more than 15,000 BTUH, but these will be divided among 2 radiators, so all radiators are less than 15, 000. By the universal hydronic formula at 20F delta T, this puts all my flows under 1.5 GPM , suggesting 3/8" all around.

I do see that there is significant pressure drop in 3/8" vs 1/2", so I understand why I might choose 1/2" here for the ECM circulator. But I've also read that less than 2ft/sec flow can cause problems with the water moving trapped air along, leaving air pockets at high spots. 3/8" will move the water faster, but I don't see where the install would have any high spots. Does it make sense to oversize the PEX to help with pressure drop, or will too low of a flow be a problem in 1/2"?

I mean, with TRVs and ODR water temp control, it seems like ultra-low flows would be commonplace, especially if some unoccupied rooms are on TRV frost protection mode and others at full heat. The ones in frost protection would have very low flows. 3/8" costs about as much as 1/2", so price is not an issue. Is low flow really something to worry about in this type of system?

Zman

With Pex tubing, especially a homerun system, low flows are more a theoretical problem than a real world problem.
I ran a quick calculation using various loops with a grundfos alpha. With 6 random loops flowing, the 150' 1/2" loop was flowing at 1.56 gpm. A 150' loop of 3/8" was flowing at .7 gpm.
It sounds like 1/2" would be appropriate for your application.
If you post the details of your design, you will get more accurate feedback.

bmwpowere36m3

I ran 3/8" PEX (Roth AluLaser PAP) for my home-run setup with Buderus panel rads... on my longest run (~110' total, 12k BTU rad) I can only get 0.6 GPM with the Alpha on speed III. However that is sufficient to run a 20-25* delta for the most part. The loops are capable of running 0.6 to 1 GPM (plenty).

Don't forget to factor in the radiator and boiler-side piping (boiler, piping, manifolds, etc...).

roundrightfarm

If you post the details of your design, you will get more accurate feedback.

This system will have 23 radiators for a total design BTUH of 113,500. They will run through two 12 circuit manifolds supplied through a buffer tank.

I have not calculated my total run length for each rad, but it is a 2 story house, plus basement and finished/heated attic. (4 levels) 4 radiators in the attic (9k BTUH each) will have the longest runs, approximately 130' each, there and back. The basement is heated too, so some runs will be quite short. I'm guessing around 1700' total. Please let me know if I can offer any more detail.

If low flow is not a real problem, and I can't see in this context why it would be, I'm leaning more towards the 1/2" just to give the pump an easier job.

SWEI

If you have tools, fittings, and adapters for 3/8" you can certainly use that on the shorter runs. It might even help in balancing a bit. If you don't, you might as well pipe the whole thing using 1/2" runouts.

Zman

I think 1/2" is the way to go for your setup. I sounds like you have loops with significantly different lengths. Manifold with balancing capability would also be a good idea.

roundrightfarm

I think 1/2" is the way to go for your setup. I sounds like you have loops with significantly different lengths. Manifold with balancing capability would also be a good idea.

Definitely planning on that. Who's got the best flow meter/balancing valves out there? The ones by Caleffi look pretty sweet.