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# Using 3/8 PEX and 1/2 PEX on same manifold

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Member Posts: 22
Is it possible to mix 3/8 and 1/2 PEX within the same manifold zone. Here is out setup...
We have a zone manifold with four loops. This is for the upstairs with three bedrooms and one bathroom. The bedrooms will be hardwood flooring, using 1/2 PEX staple up from underneath the subfloor with aluminum heat transfer plates. The bathroom is title. We would like to use 3/8 PEX in the bathroom between the subfloor and title. Is it feasible to use 3/8" and 1/2" PEX within the same zone? And if so, do we keep the 3/8" loop the same length as the three other 1/2" loops, or is there a calculation to keep the one 3/8" loop and three 1/2" loops balanced?

• Member Posts: 23,318
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For any given length of pipe, if the 3/8 and 1/2 are hooked to the same manifold, the 1/2 will have almost twice the flow rate as the 3/8. Actually, this may be what you want, since the tile is a much better conductor and emitter than your hardwood floor will be.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 22
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Jamie, agreed on the "tile is a better conductor" and this might work out.

Here was my limited understanding and thinking based on the layout we came up with. We are trying to keep all the loops close in length, but then did not know about the 3/8"...
1. Loop 1 - 302 feet - 1/2" PEX - Hardwood under subfloor staple up with Alum Transfer Plate.
2. Loop 2 - 305 feet - 1/2" PEX - Hardwood under subfloor staple up with Alum Transfer Plate.
3. Loop 3 - 304 feet - 1/2" PEX - Hardwood under subfloor staple up with Alum Transfer Plate.
4. Loop 4 - 195 feet - 3/8" PEX - Tile above subfloor, 3/8 installed as part of underlayment.
I think there are too many variables here for me, with limited understanding, to fully determine if this will balance out. Any online calculators that would run the numbers on something like this? I can change the loop lengths (some) if it helps to balance things out. Or do we try and balance after the fact with the manifold flow adjustments?

Thank you
• Member Posts: 23,318
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the three half inch loops will be very close to equal; the 3/8 loop, being shorter, will have perhaps 3/4 the flow of each 1/2 inch loop. Depends on how many fittings and all.

I'd definitely use manifold flow adjustments at this point -- and take your time. Radiant is slow to respond!
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 556
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I would change the 1/2" to 3/8"

it is a world of difference installing 3/8" in the joists than 1/2".

So much more flexible. What is missing to determine what you can get away with are the desired/calculated flow rates for each zone/loop? That will also determine what size pipe you can use.

Dave H.
Dave H
• Member Posts: 22,157
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How do the temperature requirements match up. Loop 4 may need a much lower SWT if it is a tile set? It's tough to have multiple temperatures off 1 manifold.

The staple up under subfloor and wood flooring may require 20F or warmer SWT, compared to loop 4?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 22
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Thank you for the comments / suggestions.

My hope was to supply the same SWT (supply water temp of about ~120-125) to the entire manifold (and loops). My thought process to "balance" the heat in each room and to make each room comfortable would be using PEX length adjustments (during the construction process) and manifold flow adjustments (after construction). The goal was to get the lengths close to "balanced" based on the different size PEX and materials being heated.

I felt leaving the 1/2 PEX in place under the hardwood would transfer a bit more heat to the area/zone which should need a little more because of the staple up. Using 3/8 PEX in the area with the tile has the benefit of fitting between the subfloor and tile and tile has a better heat transfer rate. Our hope is this would get us close to balanced. The big question for me is what length of the 3/8 PEX would "balance" knowing the 3/8 is under tile and the 1/2 zones are under the subfloor.

I am guessing there is no easy answer to this, as there are too many variables.

Thank you
• Member Posts: 22,157
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3/8 is usually limited to 200 maybe 250' loop lengths. 1/2 is commonly 300' loops. Some installers will push that to 333' to use 1000' coils efficiently.

Bathrooms may not have enough floor space to put in 200- 250', so the shorter loop could be balanced at the manifold.

As you mentioned it just guesstimates at this point.
A radiant software simulation program allows you to play with loop length, flow rate, spacing, etc to best match the tube to the load.

Personally I prefer 6" spacing in bathrooms, for nice even floor temperature.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 556
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You want to "balance" the system? The easiest way I have found is with a thermostat.
Break that manifold into zones with actuators. Playing with loop lengths so they match and "tweaking" the manifold can drive you to the looney bin, sometimes we way over think the tweaking.
Whats your ultimate goal? Write that answer down and design the system around what you want in your home!
You only have once to do it right!

Dave H.
Dave H
• Member Posts: 22,157
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You don't mention what type and size boiler. Use caution if you start micro-zoning, a thermostat in every bedroom and bath for example. You may get the system in to some serious, inefficient, possibly destructive short cycling issues.

if you have a cast boiler, both mixing and return protection need to be piped in.

Modulating boilers and or buffer tanks may be the best approach to numerous and micro zoned systems, if you want room by room thermostatic control.

Better yet a TRV type control, a modulation, proportional, non electric solution.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 7,569
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Here is what you are proposing would look like with a typical circulator. I made few assumptions along the way, but this should be close. I don't see any issues with what you are proposing.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 22
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Thank you everyone for the comments. The heat source is a Richmond (RMTGH95) 199k BTU LP Tankless Water Heater. 199,900 Max output 11,000 min output (btu/hr). The complete system has 3 zones (ground floor, 1st floor, 2nd floor). Each zone has it's own Grundfos Alpha2 pump and manifold. Zone 1, and 2 is "balanced" meaning all the PEX is the same length and size on each zone. Zone 3 is the zone we are talking about and I was just not sure I wanted to break this zone into two. I felt that it best to attempt to have one loop of 3/8 PEX with the three other loops of 1/2 PEX on Zone 3.

Zman, thanks for your calculations. I don't know anything about the Hydronic Circuit Simulator, but find it interesting that the 3/8 will give off more heat, than a single 1/2. I assume the calculator takes into account the floor material.

Thank you
• Member Posts: 7,569
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The tankless water heater has higher internal flow resistance than a typical boiler. You definitely want to pipe it primary secondary. Do your homework and use the right size circulator.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein