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Old 03-10-14, 03:43 PM   #1
Tom Brown
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Default Shower pre-heat idea

Hi. First post.

I'm renovating a bathroom and have had a thought of using 3/8" pex for the cold line to the shower. Why can't I run a loop of that same 3/8" pex through the floor of the shower, on it's way to the cold water valve, to recover some of the wastewater heat? It will be thinset and tile, anyway.

I know the efficiency will be small but the cost will be almost zero and I don't want the floor of the shower to be uncomfortable.

The hot will be 1/2" pex and I run an anti-scold valve to limit water temps to about 115*F. We don't generally mix in much cold water, anyway.


What do you think? Please be gentle. lol!


Last edited by Tom Brown; 03-10-14 at 03:50 PM..
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Old 03-10-14, 03:58 PM   #2
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Welcome to the site!

It sounds like a good idea to me. We want to see pics when you do it.

edit: a side effect will be that your shower floor will stay cooler...
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Last edited by Daox; 03-10-14 at 04:06 PM..
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Old 03-11-14, 01:02 AM   #3
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now you got me thinking

Of a simple heat exchanger from the warm drain water to the cold intake water.


Could wrap the cold water(copper) pipe around a (copper) shower drain pipe then into your shower floor to maximize the heat transfer to the cold water pipe.

I am no expert with the heat conductivity of pex compared to copper so I included a link to a pdf file from an expert

http://ascpro0.ascweb.org/archives/c...T192002010.pdf

Thermal Conductivity

Cooper ~ 401W/mk
Pex ~ 0.41W/mk yes 0.41 !

The SI unit W/mK indicates the amount of energy in Watts or one
joule per second taking into consideration the thickness of
the material in meters and the temperature in Kelvin’s

Just using copper with your idea would be good too
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Old 03-11-14, 04:59 AM   #4
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I knew pex was less thermally conductive than copper, but 1000x less conductive? WOW.
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Old 03-11-14, 08:29 AM   #5
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Tom,

In your cold weather area, preheating the cold water will have quick returns. But I would use copper in the thinset shower floor rather than PEX.

I know we have discussed cold water preheat for showers - can someone come up with that set of posts?

My experience was a copper drain pipe with spiral copper water tubing around it. It captured a lot of the BTUs going down the drain and recycled those into the cold water line going to the shower.


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Old 03-11-14, 10:18 AM   #6
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I like that idea of preheating the cold line to the shower with the drain line. I had thought of trying to preheat the cold line feeding the water heater that way, but that seemed to be problematic. Preheating the cold line to the shower instead allows much simpler routing and only has effect when useful heat is literally going down the drain. I like it! Thanks.
Seems to me a thin, small diameter, flexible, copper cold line wrapped around a copper drain pipe is desirable. And where the warming cold line starts at the bottom and flow up the coil would have best heat transfer.

The one caution I imagine is if the hot line is too hot. So hot that there is not enough cold in the preheated cold line to temper it. But for that to happen there would be a bunch of other problems first.

I'm not so excited about using the heat in the shower floor to preheat the cold line as my feet are still enjoying that heat. It has not yet become waste heat.
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Old 03-11-14, 02:05 PM   #7
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Hamsaterpowers mention of cold feet warrants more thought , The shower base will become a chiller of sorts good , cold feet in the summer is not much better then cold feet in the winter.

For fast conduction the copper pipe is superior (good for the drain undertaking)
but in the Solar heat collectors where the heat is constant the test performed by the author in the linked pdf file showed little too no difference in the heat exchanges properties or gains by the various plastic pipes as compared to the copper.

Pex should not be underestimated for use in other hot water designs as it in most cases is just as good as the copper in dissipating heat (the linked test denote that) and may be required by your insurance company or local bylaws. I believe that for New constructions or on Newer built houses not the older houses that have the copper pipe already..
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Old 03-11-14, 02:09 PM   #8
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Also wrapping the drain pipe and coiled cold water pipe with a good amount of insulation would be best.
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Old 03-11-14, 03:08 PM   #9
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Here are a few threads on drain water heat exchangers.

http://ecorenovator.org/forum/conser...-recovery.html

http://ecorenovator.org/forum/conser...-recovery.html

http://ecorenovator.org/forum/conser...-recovery.html
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Old 03-11-14, 06:16 PM   #10
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I'm doing something very similar, built and installed a heat recovery pipe on our shower drain to warm the cold supply to the thermostatic shower mixer valve.

I did this as an experiment to see if it is worth doing as we are extending the house later this year and all the pipework will need altering again anyway.

My recovery system involves an aluminium tube 100mm x 1800mm (4" x 71") with 4 coils of 10mm (3/8") aluminium pipe wrapped around it.
To improve the heat transfer i coated it in glassfibre resin mixed with copper powder, i,ve no idea how much this helps but it seemed like a good idea at the time.

I have some sensors attached as part of my heating datalogging system.
As far as i can tell, it recovers heat quite well.
However the flow of cold water is so low that hardly any recovered heat makes it to the shower.

I'm considering altering the pipework to pre-heat the cold feed to our hot water tank so the flow through the recovery lines would be far greater and hopefully recover a usefull amount of heat.

The only way i can imagine this working properly in the cold feed to the shower is if the hot feed is at a much higher temperature and requires large quantities of cold to dilute it.
I need to check the logging software but i think our cold flow during a shower is only around 0.2LPM out of the 7LPM total shower usage.

Hope this helps.
Steve









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