How to size PEX?

[Buzz Saw]

I've searched high and low in the forum on how to size PEX for my heating system. I need to go from my manifold to my heat exchangers.

I know I've read it on here in the past but I don't think I'm searching using the right verbiage.

Any one have anything to help me out.
Charts? Graphs? Equations?

Thanks guys!


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[jebatty]

This might help.

 

[taylorfarms]

First, how big are your exchangers BTU's, at what delta-t?

Say a 100k BTU exchanger with a 20 degree design delta t

1 gal of water weighs 8.34lb. so for every GPM of flow, you will have 8.34(weight of water)x20(delta T)=166.8x60(hour) 10,008 BTU per hr of

so roughly you want 10gpm of flow, now go to the charts that jebatty posted, and figure out how much head you will have with different sizes of pex.

smaller pex is cheaper up front, bigger pump is not, more electricity is not either

 

[Buzz Saw]

First, how big are your exchangers BTU's, at what delta-t?

I have two water to air heat exchangers . The big one is over sized at almost double capacity at 180btu with a delta T of 20° The propane furnace it is installed in is rated at 100btu.

The other WTAHX is also oversized at 160k on a 80k furnace. This is also delta T of 20°.

I over sized them to get more heat sl out of the lower temps in my GARN.

I have yet to buy my HX for my domestic hot water and radiant floor. The floor will likely be(30 plate) 120k and the DHW will be ??? Probably a minimum of 30 plate as well. Thinking of using badger pipe plate exchangers.

I will be circulating with Grundfos 15-58 FC pumps.

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[taylorfarms]

Not knowing how long your runs are I would personally go with 1", as the price difference over 3/4 is minimal. and with the 15-58 you can vary the pump speed to what flow you actually need.

the bad part is that increasing fluid flow will not generally help your WTA HX's kick out more btu's at lower temps, slowing down the fan can help with plenum temps though, just a thought.

 

[Buzz Saw]

With a rough guess I think my longest run is right at the 100' round trip. Shortest run will be about 10' round trip.

I was surprised at the minimal price difference between 3/4" and 1". The down side to 1" is that the only place I can find 1" oxygen barrier PEX in 20' sticks is Menard's and those are special order....

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[Bob Rohr]

Plumbing and heating wholesalers generally stock up to 1" pex, both straight and coils. You do want to use barrier pex.

If in fact you need to move 100,000 (10 gpm) and 180,000 BTU 18 gpm 1" would be the best choice. !-1/4 would be better yet for the 180,00 HX. I doubt you need 180,000, the HX may be sized to move that much, I doubt you furnace is sized for to move that size load?

 

[tom in maine]

My 2 cents worth. 3/4" up to 50k btus. 1" for up to 100k and 1 1/4" for over 100k.
With a 20F Delta T, you are moving 5 gpm at 50k, 10gpm at 100k and 15 gpm at 150k.

You can find threads through Hearth where folks undersized piping and were not happy with performance.
I think most PEX fittings are a bit more restrictive than copper fittings and their use should be minimized if possible.

 

[Countryboy1966]

Some of the best advice I read on this forum:

-When in doubt with pex, size up one size.
-Know your heat demand at the point of use do you know how many BTUs need to move through the pipe.
-Water holds only so many BTUs
-You can only push so many BTUs through a given diameter size of PEX.
-Pipe length and all the fittings also have a head pressure affect and need calculated as part of what you need when sizing a pump.
-If you are not sure - Keep researching until you are 200% certain you understand what you need.
-Lines in the ground are not the place to skimp.

Not sure in what medium that 100' loop is in (round or through a house), but regardless of what you are trying to push through it for a heat demand, I'd go with 1-1/4" OXY barrier pex.

This link is pretty useful.
https://www.pexuniverse.com/pex-tubing-technical-specs
https://www.hearth.com/talk/threads/pex-btu-ratings.66226/

Check a lot of the older threads from 2007-2010 Era. Thats where fuels were through the roof and a lot of people like me learned what not to do.