What size Pex-B Oxygen Barrier Tubing for patching in replacement radiators?

HandyFS · November 2022

I can use some guidance on pex sizing for supply/return lines and patching in some pex to get replacement cast iron rads connected.

Existing Piping:To give some more insight, the system is a hot water 2 zone setup with a Taco 007 circulator pump on each zone. It has 1"+ black pipe that has Monoflo tee's to each radiator. Coming out of the tee it mostly transitions to 1/2" copper. The 1/2" copper is then transitioned to 1/2" steel/galvanized/black pipe nipples to mate to the radiator valve and return union.

My plan is to sweat on a copper to pex adapter and then use pex to go from this connection, to a brass drop ear that then stubs up with black pipe.

What Pex to use? Oxygen Barrier:
For general plumbing work, I have the tools for the stainless pex crimp rings, so I am planning to use something that is compatible. I know boilers and radiator type system require Oxygen Barrier pex, so I'm planning to use that. With pex crimp rings, I'm guessing Pex-B is most widely available. I've seen some use Pex-AL-Pex, but I don't think I need that, I think Pex-B will get by ok since these are fairly straight short runs that I can easily support if a bend is needed at all.

Is this regular Oxyben Barrier Pex-B tubing ideal for what I am doing?
1/2" Oxygen Barrier PEX-b Tubing
https://www.supplyhouse.com/Bluefin-T050-300-OXY-1-2-Oxygen-Barrier-PEX-b-Tubing-300-ft-Coil

From reading around, I know a lot of tradesmen and professionals prefer Pex-A for a lot of reasons. I have the crimp rings so I think Pex-B may be easier since I'm used to working with it and have the tools already if its ideal.

Pex Sizing and Transitioning to Pex:
Is 1/2" pex sufficient in this scenario, or should I go with 3/4"? The system currently has the 1"+ black supply pipe, which then monoflo tee's off to 1/2 copper. I was hoping to cut the existing copper and then use a copper-to-pex sweat adapter and use pex to go from that copper stub coming out of the monoflo tee and then mate it to the bottom of the nipple with a 1/2" PEX x 1/2" NPT Brass Female Adapter (or 3/4"). None of my lengths are extreme on the 1st floor, none run more than 2-3 feet max to the inlet/output of a radiator. On the 2nd floor I am replacing 1 radiators supply, return lines, those probably run about 10-12 feet from the monoflo tee as they work their way up and sideways to the radiator position on the outer wall. I've read hot water systems are typically sufficient on 1/2" but want to confirm since cost is negligible. Is 3/4" recommended for the longer 2nd floor run, or will 1/2" be sufficient? Currently that 2nd floor line is a 1/2" steel pipe of some sort. If this questions requires BTU sizing of radiators on that 1st floor zone are around 40,000 BTU's. 1/2" is currently carrying the load but I can go 1/2" or 3/4" with neglible difference in cost if that's more ideal since I know pex inner diameter is typically not as big as 1/2" copper overall. Is 1/2" ok or should I go 3/4"?

hot_rod · November 2022

1/2" pex can move 10- 15,000 btu/hr easily. 20' or so is not an issue. What does the room or radiators need to put into the space? If you need 40,000 at a radiator, 3/4 pex.

Pex A can be assembled with crimp rings, and for short runs, look a buying straight 20 footers instead of coils. Especially if you go up to 3/4" pex.

mattmia2 · November 2022

Since it is monoflow you want to be careful not to make the runnouts any smaller than they were originally. I might even be a little concerned about fittings that don't preserve the full diameter. I might do it in 3/4 to be safe(assuming it is 1/2" copper).

Ironman · November 2022

Beware about downsizing the runout’s to the rads. On a Monoflo system, the cone in the tee, along with the common piping between the tees, produces enough resistance to force some of the flow through the radiator. If you reduce the size of the runouts, you’re adding more resistance to that part and the flow (or a large portion) may find it easier to go straight through the tees than divert to the radiator.

A 1” line carries at least four times more than a 1/2” line. And pex carries about 60-70% of the corresponding black iron size

psb75 · December 2022

Uponor pex A and its fittings are much better "all around." The i.d. of the fittings are much closer to 1/2" than the other brands. But you need the expander tools. And Uponor is generally at a higher price point.

HandyFS · December 2022

mattmia2 said:
Since it is monoflow you want to be careful not to make the runnouts any smaller than they were originally. I might even be a little concerned about fittings that don't preserve the full diameter. I might do it in 3/4 to be safe(assuming it is 1/2" copper).

So 3/4" is sounding like a better plan then. I could easily use 3/4" pex to 1/2" nipple stub ups. I have to double check the nipples but I'm almost certain they are 1/2", same as the spuds going into the radiators.

Ironman said:
Beware about downsizing the runout’s to the rads. On a Monoflo system, the cone in the tee, along with the common piping between the tees, produces enough resistance to force some of the flow through the radiator. If you reduce the size of the runouts, you’re adding more resistance to that part and the flow (or a large portion) may find it easier to go straight through the tees than divert to the radiator.

A 1” line carries at least four times more than a 1/2” line. And pex carries about 60-70% of the corresponding black iron size

So is 3/4" pex ideal, or would that possibly throw the flow off too much and send more water than expected to a rad throwing off the flow to ones further down the supply line?

psb75 said:
Uponor pex A and its fittings are much better "all around." The i.d. of the fittings are much closer to 1/2" than the other brands. But you need the expander tools. And Uponor is generally at a higher price point.

This makes sense, I see a low level expander tool that would probably get the job done that I could buy, I was just trying to not invest in more since I already have the expander tool. Think 3/4" pex would be ok with regular crimp ring style pex, or will that throw the monoflo balance off?

Ironman · December 2022

A 1” pipe has twice the capacity of a 3/4”.

If the original lines were 1”, that’s what I’d go back with.

Something smaller MAY work, but it may not. There’s no simple formula to apply here. Water will take the path of least resistance and if you create more resistance in the runouts, then you may get little or no flow.

The cost of using the correct size tubing is not that much greater than trying to cheap out with something smaller. And if the smaller doesn’t work, then you’ll have to rip it out and do it over with the proper size.

hot_rod · December 2022

Rehau and Mr Pex are also A Pex. Several or the B Pex brands approve of the expansion fitting, Zurn for one.

It is a long calculation, but you could calculate all the piping and get an exact answer🤔

Modern Hydronics shows the steps and has examples. I suspect with some digging you could find the formulas at B&Gs website.

It’s basically a pressure drop formula that shows where flow goes.

Certainly no harm in upsizing to get closer to the original design.

mattmia2 · December 2022

If you have too much flow you can throttle it with the radiator valve(although this is tricky with monoflow). If you have too little the fix is bigger pipe.

HandyFS · December 2022

Ironman said:
A 1” pipe has twice the capacity of a 3/4”.

If the original lines were 1”, that’s what I’d go back with.

Something smaller MAY work, but it may not. There’s no simple formula to apply here. Water will take the path of least resistance and if you create more resistance in the runouts, then you may get little or no flow.

The cost of using the correct size tubing is not that much greater than trying to cheap out with something smaller. And if the smaller doesn’t work, then you’ll have to rip it out and do it over with the proper size.

The original lines are 1/2" copper off the monoflo tee which then stem up to 1/2" black pipe. So I'm going to use the same 1/2" black pipe, its just the tubing between the tee and the black pipe I need to decide on. So with pex being less restrictive, I can either go 1/2", or 3/4". 1/2" possibly passing slightly less, and 3/4" passing slightly more. Either way it transitions back to 1/2" black pipe the way it was. In this scenario, would you go 1/2" or 3/4"?

hot_rod said:
Rehau and Mr Pex are also A Pex. Several or the B Pex brands approve of the expansion fitting, Zurn for one.

It is a long calculation, but you could calculate all the piping and get an exact answer🤔

Modern Hydronics shows the steps and has examples. I suspect with some digging you could find the formulas at B&Gs website.

It’s basically a pressure drop formula that shows where flow goes.

Certainly no harm in upsizing to get closer to the original design.

The lengths between the monoflo tee and the black pipe (The 1/2" copper) which stems up to the radiators ranges from 2 feet -3 feet length give or take, so its only that small portion that I'm looking to calculate or decide on. I could just re-use 1/2" copper and it would stay exact, I was just trying to save myself some sweating work.

In the case of a monoflo tee and its current 1/2", assuming slightly more would be more (3/4") ideal than slightly less (1/2")? As Mattmia2 mentioned, I could always put slight resistance on the line by dialing back the radiator valve slightly.

mattmia2 said:
If you have too much flow you can throttle it with the radiator valve(although this is tricky with monoflow). If you have too little the fix is bigger pipe.

That seems to make good sense, I think at least based on the radiator valve restricting and giving slightly less flow so the monoflo can work as intended. In this case, would you go for 3/4"?

Ironman · December 2022

Pex is more restrictive, not less. You should be okay with 1/2” if that’s what was there before.

HandyFS · December 2022

Ironman said:
Pex is more restrictive, not less. You should be okay with 1/2” if that’s what was there before.

My bad, I meant to say more restrictive (Less flow since its inner diameter is smaller than copper).

Doing some calculations of inside diameter.

Inside Diameter:
1/2" Copper = 0.545
1/2" Pex = 0.475
3/4" Pex = 0.681

1/2" Pex is 15% smaller than 1/2" copper
3/4" Pex passess is about 20%+bigger than 1/2" copper

With the monoflo, is 15% smaller or 20% bigger more ideal?

mattmia2 · December 2022

Bigger is better. Monoflow relies on the pressure differences of the diverter tees to induce flow in the emitters. If you add enough resistance over what it was designed for the emitter won't heat or won't heat fully.

HandyFS · December 2022

mattmia2 said:
Bigger is better. Monoflow relies on the pressure differences of the diverter tees to induce flow in the emitters. If you add enough resistance over what it was designed for the emitter won't heat or won't heat fully.

3/4" it is then. Thanks for the extra feedback. I will be doing some rough calculations over the weekend and then working to get the system all back together soon after.

mattmia2 · December 2022

To sum it up, 1/2 probably would be fine but 3/4 wouldn't hurt and is a trivial cost difference. Just don't keep doing this every time and ending up with 2".

What size Pex-B Oxygen Barrier Tubing for patching in replacement radiators?

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