Affordable large diameter potable water pipe -OR- Tempering cold water supply to shower

I am trying to creatively reduce the hot water demand for our shower.  I have a wood stove in the basement and the water supply for the bathroom passes right over the stove.  The basement ceiling isn't finished yet.  I've noticed that the "cold" water is actually nicely warm when I turn on the faucet for the first few seconds.

Long term I'd like to heat my hot water using the wood stove but I'm not ready to engineer that just yet.

So my desire is to modify the cold water supply to hold more water in the ceiling above the wood stove so it warms up.  Then when I'm showering the "cold" water that is actually warm will allow me to use less "hot" water.  My initial thought was to run cpvc or PEX pipe back and forth in the joist cavity so that there are several gallons of water contained in that loop of piping.  The problem with this is that I'd need to stuff a ton of pipe in there to hold much water.  I did some math with 1/2" PEX.  With 1000 feet of 1/2" pipe I'd get 10.2 gallons of warm water storage (1/2" would allow me to bend the pipe instead of using fittings).  My next thought was to see if there is some 3-6" diameter pipe that I can run back and forth one time to hold enough water for a shower.  PVC pipe is cheap but I'm not sure it's safe or legal.  I found 2" cpvc pipe but it's $80 for 20'.  If there's a way to put a tank in the ceiling that could work as long as the new cold water rushing in didn't bypass the warm water.  

Maybe I'm just trying to be too creative.  Does anyone have any ideas for a way to do what I'm dreaming of?  Or should I just spend my energy working on a way to heat the "hot" water with the wood stove?

Thanks!

I think you should skip the tempering project in favor of a preheater for your conventional water heater.
You will probably spend the same amount either way.
Even non-potable pipe of large diameter is expensive.

You could  tap into the cold water supply at that point above your wood stove, running copper from there down into a coil,suspended in a large pot of water atop the stove.
From there,more copper all the way  to the water heater. The water heater maintains the water temp,and adds to it if needed.
A float valve like those in a toilet(but metal) can keep the water in the stove top pot full.That water is a buffer ensuring the water in coil never gets hot enough to turn to steam.
If you don't want the steam from the pot loose in house,a lid with copper line can divert it to the outside,or better yet,loop it back into the pot after it condenses. That lid shouldn't be tight,lest we cause a steam explosion.

What I have described would be ugly to most eyes,but I think it would work,and be safe.

Thanks William!  I checked at my hardware store and they said that solid core PVC pipe can work for potable water supply if the temp stays under 140 degrees.  I did some math and a 10' length of 4" pipe with a 180 bend at the end and 10' coming back would hold about 14 gallons of water.  I'd be into the project about $50.

I like the preheater design with the open water bath to avoid steam issues.  But I agree that it would likely be ugly.  My issue with preheaters is that I believe if they don't preheat enough water to get you through a shower, they just send cold water on into the water heater anyway.  Or in other words, if the coil in the pot of hot water holds one quart of water, that only helps the water heater out for the first quart of water it draws.  After that the cold water that feeds through that coil is only heated a bit as it rushes through the pot of hot water.  Unless I'm underestimating the amount of heat transfer you'd get?

One possible plan I had for water heat was to put a coil or radiator of water next to the stove (more hidden from view on the right side) and tie that into the water heater so that it directly loops water into the water heater tank.  The water heater is 2' away from the wood stove behind the wall to the left of it.  But I would not have good control of the temperature of the water and the water heater's t&p valve would be the safety for the system.  

Maybe I could incorporate your open vessel with a loop back to the water heater...  Have the pot of hot water on or next to the stove.  Have a float that keeps it full.  Let's assume it maintains 160 degrees.  Have a coil of copper pipe in the pot that is connected to the top and bottom of the water heater.  The hot water in the pot rises to the water heater and is replaced by less hot water from the heater.  Or it's on a circulation pump that temperature controls the system.  If the pump stops or we don't need more heat in the water heater tank or the circulation fails, the water in the coil just heats up to 160 and sits there (no explosion).  If the fire is out and we need hot water, then the water heater wakes up and heats the water itself.

For my hot water needs, I think the vast majority of my energy usage is in keeping the water hot.  Our usage is pretty low so I think heating the incoming water is less of an opportunity for us.  So that may be skewing me away from the preheating option for the hot water.  

But I don't have any actual experience with preheaters or appropriate technology in the hot water arena so I'm guessing a bit.  Thanks!!!

You make some excellent points.
I think the heat would transfer well from boiling pot water to the cold water coil,but there is a way to guarantees better transfer.

Running  a loop from a heated vessel of water we could pump water in a jacket around the line that supplies cold water to the water heater.
A 3/4" water jacket around a 1/2" supply line can be made with standard copper fittings. If the two streams of water are made to run in opposition directions this is called counter-flow and it is very effective for heat transfer. Many home brewers use a conter-flow set up to rapidly cool their wort .

If you do anything with the water heater loop, it will be important to keep the end near the stove from actually radiating heat when the fire is out.The thermostat  controlled pump could help with this.

If maintaining water at temp is where most of the energy is being expended, you might want a smaller tank,with lots of extra insulation.
My clothes washer is only hooked up toward water, so showers are our biggest hot water use here as well.
I have considered slot if ways to address this,including instahot electric heaters at each shower and faucet,and running the cold water line through a barrel of the hot water that goes down the drain.

I'm liking the idea of the thermostatically controlled pump more and more.  Then I don't have to rely on a thermosiphon to work perfectly, I'd have control over the temperature and the coil entry and exit can be at the same elevation.  I agree that a counter-flow heat exchanger would be a good way to increase heat transfer to incoming cold water.  If the thermostatically controlled pump is moving the water slowly enough, it should pick up enough heat from the tank of hot water which would keep the system a bit simpler.  

I've considered running the incoming shower cold water through the drain pipe but it seems like it would just increase the likelihood of a plugged up drain.  I've seen companies that sell heat exchangers that you put on your drain but the one I saw needed a 4-6' vertical drop.  Mine is horizontal to the main stack and the main stack is hard to get to.  Maybe a counter-flow heat exchanger with 3" pvc pipe around the 2" drain pipe would allow the drain to be unobstructed.  But then you'd have a gallon of cold water sitting there when you start the shower.  By the time the drain water heats up and starts to transfer heat to the incoming cold, your shower would be halfway over already.

I love the idea of the open pot of water to keep the circulating water from getting over 212 degrees.  Do you know of a float system that is commercially available other than a toilet tank float?  That should work but I'm hoping there's something smaller and sexier.  If it fails to stop feeding in water, it would be a mess.  If it fails to start feeding in water it would probably be safe.  I'm dreaming up a copper tank that is 18" high, 24" long and 3" wide that sits next to my stove.  The coil of copper in it absorbs heat and I just need to keep that tank full as it evaporates.  The circulating pump would have to come on when the water heater tank goes below a certain temp and the water in the tank is above that temp.  The pipes to and from the tank would be insulated.  I'd have a lid on the tank that keeps most condensation in but won't prevent expansion.  I'd mount it in a way that if it gets too hot I can move it farther away from the stove until I find the perfect distance.

I'm pleasantly surprised at how well this water heater holds in its heat.  It is a year old and electric.  The surface of the tank is always cold to the touch.  I also have hopes of heating the water in the summer with a solar setup of some sort.  Then I'd only need to turn on the water heater breaker when we have company.