Pex - what is the correct size?

soundsailer

New member
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0
Hi all -

Question: 33 foot sailboat with water tank in the bow, running to a hot water tank at the stern and has a 3.5 gph fresh water pump providing (currently low) pressure. Currently it is plumbed with PVC but needs to be replaced.

I am going to replace with pex. My question is 1/2 inch or 3/4 inch? I'd really prefer 1/2 inch but worried about flow (I will be needing to use multiple 90 degree fittings). Any insight anyone could provide would be gratefully received, only want to do this once!

Thanks!!
 

IStream

Super Anarchist
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I'd run a trunk line of 3/4" down the length of the boat and then tee off with 1/2" lines to the individual points of demand.
 

El Borracho

Verified User
6,686
2,656
Pacific Rim
A cocktail straw will flow 3.5 gph.

Quite common for yachts to be entirely done in 1/2” or 5/8” hose together with the rather restrictive barb fittings. Depends entirely on the desired and tolerable pressure drop. Our luxury yacht gets by on 1/2” tube with a basic 60 psi pump. Shower, basins, etc.
 

2airishuman

The Loyal Opposition
Hi all -

Question: 33 foot sailboat with water tank in the bow, running to a hot water tank at the stern and has a 3.5 gph fresh water pump providing (currently low) pressure. Currently it is plumbed with PVC but needs to be replaced.

I am going to replace with pex. My question is 1/2 inch or 3/4 inch? I'd really prefer 1/2 inch but worried about flow (I will be needing to use multiple 90 degree fittings). Any insight anyone could provide would be gratefully received, only want to do this once!

Thanks!!
Look at the pressure drop tables. At 3.5 gpm you can run from the bow to the stern and back to the bow again with 1/2" pex and lose 10 psi. There is no reason to use 3/4" on a boat that is smaller than a sooperyackt. The question you should be asking is where do you really need 1/2" and where can you get away with 3/8". With 3/8" you pour half as much water down the drain waiting for the water to get hot, and it's lighter and easier to run.
 

thinwater

Super Anarchist
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Deale, MD
You really do NOT want 3.5 GPM at every tap on a 33-foot boat. It will empty the tank FAST. In fact, the marine faucets are almost certainly reduced way down from home faucets, in part, to prevent that (IME). 1/2-inch is enough.

It's also not like you have a washing machine that will start while you are in the shower. There will seldom be 2 flows at the same time.
 

DDW

Super Anarchist
6,561
1,125
My brother plumbed his house with 3/8 runs to the hot side of all the faucets. He did this to save water. The flow is perfectly adequate, and would not be noticed to be different. A 25' run from hot water heater to shower for example is 0.14 gallons in 3/8, but 0.25 gallons in 1/2".
 

IStream

Super Anarchist
10,863
3,042
So here's why I suggested what I suggested (3/4" trunk, 1/2" branches).

On my boat, I've got a freshwater anchor washdown at the bow, a freshwater transom shower, freshwater (Vacuflush) heads, and the usual galley/sink/shower arrangements. I've also got fiberglass tanks that make the water taste like shit, so I put a whole-house filter system in.

My boat is plumbed entirely with 1/2" tubing and most of the time it's not a problem but if I have multiple points of demand active at the same time or if I want to use a high flow point of demand far from the pump (e.g. anchor washdown), the pressure drops more than I'd like. It's not crippling but if I were to plumb the boat from scratch, I'd do it the way I suggested just to make sure I never ran into the issue. The only downside to running the fatter trunk is more dead volume of cold water in the lines when you want hot water at the point of use, but that's something I'd gladly trade off.

EDIT: Woops, I've been thinking OD when I should've been thinking ID. Pex is spec'd by ID so the actual sizes I should've said are 5/8" and 3/8". Management regrets the error.
 

gptyk

Anarchist
735
404
California
If you have the right tools, PEX-A (Uponor) is far superior than other systems. I personally prefer the Milwaukee m12 tool as it's lighter.

Pex-A has better flow through the fittings. Much easier and faster installation. No crimp rings to corrode.

I'd echo the 3/8 advice. My boat has the 15mm whale pipe from the factory and it takes quite a while to get hot water at the shower/stern shower.
 

toddster

Super Anarchist
4,265
999
The Gorge
I did the old 30ish foot boat all in 3/8. No problems with the flow or pressure. New 40ish foot boat is done in 3/4 which is ridiculous since all the fixtures are low-flow anyway. Really wastes a tremendous amount of water to get hot water to the forward head or the cockpit.
 

CapDave

Member
403
325
Sint Maarten
We have 15mm OD pex on a 57' cat, lots of long plumbing runs. There is some pressure drop with multiple consumers open, but in practice it's really not an issue. I would do the same again if I had to choose.
 

MaxDog

Member
So here's why I suggested what I suggested (3/4" trunk, 1/2" branches).

On my boat, I've got a freshwater anchor washdown at the bow, a freshwater transom shower, freshwater (Vacuflush) heads, and the usual galley/sink/shower arrangements. I've also got fiberglass tanks that make the water taste like shit, so I put a whole-house filter system in.

My boat is plumbed entirely with 1/2" tubing and most of the time it's not a problem but if I have multiple points of demand active at the same time or if I want to use a high flow point of demand far from the pump (e.g. anchor washdown), the pressure drops more than I'd like. It's not crippling but if I were to plumb the boat from scratch, I'd do it the way I suggested just to make sure I never ran into the issue. The only downside to running the fatter trunk is more dead volume of cold water in the lines when you want hot water at the point of use, but that's something I'd gladly trade off.

EDIT: Woops, I've been thinking OD when I should've been thinking ID. Pex is spec'd by ID so the actual sizes I should've said are 5/8" and 3/8". Management regrets the error.
For anchor wash down and other high demands the limiting factor will be the pump. Bigger tubing is unlikely to help there.
 

soundsailer

New member
5
0
Thanks all, really interesting discussion. This thread has convinced my to go the 1/2 inch route (pex B). Frankly, the fact that I can have a very limited set of spares and that everything is easily available from box stores is extremely attractive. I would like the weight savings of the 3/8th however i worry about water impurities over time causing clogs.

In the perfect world I would use Pex A, however it comes back to easy availability of spares and cost with limited downside.
 

climenuts

Anarchist
726
292
PNW
You're not going to get clogs in 3/8" piping but the pressure drop is significantly more. at 1.5 GPM 3/8" PEX-B is 15.6psi/100ft and 1/2" is 3.6psi/100ft. 1.5GPM is typical flowrate for a modern low-flow showerhead or low-flow kitchen faucet and you would notice less than ~10psi.

Be aware that the pressure rating of the pipe is at room temperature and is halved at 180F. Shouldn't be an issue with boat pump pressures but FYI. Cheap PEX-B doesn't have great quality control.

Pipe erosion at high temperatures is a significant issue with PEX (all types). While with room temperature water velocities up to 10ft/s is acceptable at 180F it's 2ft/s which would limit a 1/2" line to 1.1 GPM (not counting fitting constrictions). Very common to see 180F+ when connected to the engine HEX. Low use probably masks this issue on boats but if you're using the boat frequently beware. Using RO (watermaker) water makes erosion much worse than most city water due to the reactivity of the water.
 

climenuts

Anarchist
726
292
PNW
Alright I'll bite. ASPE (American Society of Plumbing Engineers) Plumbing Engineering Design Handbook Volume 3: Special Plumbing Systems.

Specifically:
1663783407768.png

Note: High-Purity Water describes the purity based on the water conductivity instead of mineral content or other means. Lower conductivity -> higher purity.

ASPE doesn't discuss PEX specifically in this context but the common theme between PVC, CPVC, PE (PEX), PP, and PVDF is it's the additives, dyes, antioxidants, etc. leech out the piping causes embrittlement, erosion, weakening, etc. Piping used for low-conductivity RO is specifically engineered to avoid additives and is not equal to the PEX you're buying at the store.

I'm not saying a boat's watermaker is producing ultra-low conductivity RO water but can be significantly less conductive than City water with newer membranes. Obviously as membranes age conductivity goes up. Some cities like Vancouver with mountain snow-melt water supplies are very low conductivity to begin with (leading to wide spread erosion problems) but other supply types are significantly more conductive than a well maintained RO system.
 

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