Splitting and rejoining water supply lines

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aerie

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I am making a heat exchanger for my shower trap arm. I have a 2" section of copper drain pipe that I want to coil the incoming water supply line around, before the supply feeds into the 3/4" inlet of my on-demand water heater. I already tried and failed at coiling 3/4" soft copper tubing around it. I see in the commercially available ones (pictured), the supply line is split into multiple smaller lines which has the advantage of more surface area for contact in addition to the smaller diameter tubing being more workable. I'm considering this approach but am wondering if there are any implications for downstream pressure or flow when splitting a line into multiple smaller lines and then rejoining them before feeding into the water heater. According to the table linked below, splitting into two 1/2" lines would maintain the same cross-sectional area as the single 3/4" line. The commercially available ones split into four lines but according to the table linked below, there is no diameter of pipe that will quadruple to match 3/4" cross sectional area. Is it better to have the cumulative cross-sections of the coiled lines exceed that of the 3/4" pipe so the coiled volume acts as a sort of reservoir?

I'm leaning towards two 1/2" lines because the tubing and fittings are easy to find at it would only require one tee on each end. Lastly, can anyone comment on whether it is reasonable to expect to coil 1/2" around a 2" diameter (wish I had asked about 3/4" before trying it).

Also, not sure if this is relevant but the heat exchanger is the only portion of my plumbing system that will be copper. The supply line will be pex before and after the coil and the rest of the DWV system is ABS.

*cross-sectional areas were taken from this chart: Copper Tubing Size Chart ASTM B-88 | Engineers Edge | www.engineersedge.com

thanks, aerie
 

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aerie

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Sounds like a complete waste of time, money, and effort, but to each his own.
It's for a tiny home with an electric on demand water heater which, due to limitations of our electrical service size, may be slightly under sized for the coldest days of winter in our climate. I'm doing it to increase my chance of a hot shower when it's -30 out. Plumbers in my area have said the commercially available versions they've installed actually make a significant difference to the load on the water heater. It's free heat, might as well use it.

If anyone reading this is curious about the question of splitting supply into parallel lines, I got a very well-explained lesson in fluid dynamics and a walk-through of the calculation on this other forum. Splitting and merging supply lines
 

Jeff Handy

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I just see potential for leaks, freezing of tiny pipes, unless maybe a commercial version is more robust.

I see it bursting somehow when -30.

I see a gizmo that will haunt you later.

It might be great, but my spidey sense says no way.

Is there a crawl space or basement under this shower?

If so, and if it is heated space, could you install a storage tank to preheat the supply to the heater with just ambient heat?

The water coming right from your utility or well would likely be pretty cold

A twenty or thirty gallon tank should hold enough for a decent shower.
 

aerie

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The exchanger is fine if it works. The factory units work, the ones I’ve seen are like 4-6’ long though.

A tempering tank would work.....sized based off demand.
We're using the on demand water heater because unfortunately we don't have room for a tank in the tiny home. The trap arm length is 5' to give room for the exchanger, while still within code for venting. cheers
 

aerie

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I just see potential for leaks, freezing of tiny pipes, unless maybe a commercial version is more robust.

I see it bursting somehow when -30.

I see a gizmo that will haunt you later.

It might be great, but my spidey sense says no way.

Is there a crawl space or basement under this shower?

If so, and if it is heated space, could you install a storage tank to preheat the supply to the heater with just ambient heat?

The water coming right from your utility or well would likely be pretty cold

A twenty or thirty gallon tank should hold enough for a decent shower.
The pipes including the heat exchanger will be in an insulated and heated space so there are no concerns about them freezing. I was just saying outside air temps can dip to -30 in my area, implying (1) that a warm shower is a nice way to warm up and (2) that incoming water temperature would be low, probably a couple of degrees (C). I don't see much more potential for leaks than there would be connecting to a commercial unit.
 

Twowaxhack

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We're using the on demand water heater because unfortunately we don't have room for a tank in the tiny home. The trap arm length is 5' to give room for the exchanger, while still within code for venting. cheers
Sounds like a plan, build it and show it to us.
 

JG plumbing

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Can I ask why you don't just buy the one you want instead of trying to make one. No doubt it can be done, but why not spend $80?
 

JG plumbing

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That may be right. Anyway yeah you can bend 1/2"copper around a 2"pipe. I wouldn't want to do it. I can't really even think of a good method of doing it.

Maybe run a good long braze to start along the outside of the tubes to stick them together. Then you need a decent length of waste to grip in a chain vice. It should be fairly easy after that. Good luck, lol.
 

Jeff Handy

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I would think you would want something like 1/4 inch copper ice maker line.

I’m wondering how it would hold up to long term internal friction or turbulence.

Whatever you build or buy, it should have a bypass so it does not wear out from operating when not needed.

What about building something that preheats the water by taking heat from your heating system or furnace?

If you have forced air, you could run some skinny pipes off a manifold, and run them along the ductwork, or inside the ductwork.

If you have a boiler, wrap around the hot pipes of that instead.

Just kick up the thermostat a few degrees before your shower, so the heating system keeps the water preheated during your shower.

You could add some fins to the pipes to absorb more heat, like a skinny version of a baseboard radiator.

Then wrap everything up to improve heat transfer.
 

Mitchell-DIY-Guy

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Wow.
Hardly “free heat” if you have to spend a lot of money trying to engineer a system to work for you. Seriously? Tiny house? Undersized water heater? Undersized electric service? -30 F temperatures? AND you want nice hot showers as if you lived in a normal sized house in a slightly different climate without all your limitations?
I’m sure there is some code about gray-water heat exchangers and implementing them safely. Even if you live off the grid in some unincorporated mountain area, following some code standards makes sense even if not required or inspected.

If a commercially manufactured unit can’t work for you regardless of the reasons, give up the science project.
 
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