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Old 09-27-2010, 09:58 PM   #1
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Using the heat generated by the hot water chimney?


I had an idea, why not have the cold water line wrap around the hot water heater chimney to basically cover it all in a spiral (some kind of flexible copper pipe maybe?), with good contact, then the whole thing be insulated? The cold water would get preheated before it gets to the tank. I touched that pipe and nearly burned myself, that is a lot of heat basically going straight outside, and then started thinking, why waste this heat?

Of course, when hot water is needed it would take a while before the heater fires up and gets that pipe hot enough but it would probably be enough to compensate for when you start to run out of hot water.

This could also have other uses such as a heated floor somewhere else in the house. Perhaps in a closet, so by the time you take your shower and all, when you go to grab your coat to leave, it would be nice and warm.

I suppose doing something like this would be against code as you're probably not suppose to rig stuff on a chimney like that, but is this a concept that has ever been thought of or used?

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Old 09-27-2010, 10:08 PM   #2
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Using the heat generated by the hot water chimney?


Illegal dangerous and verbotten. It will cool down the moisture in the fumes, condense them in the chimney/freeze it up and give U CO poisoning. They do make high efficiency condensing tankless water heaters (google Navien) but they start at around $3000 for a Good one.

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Old 09-27-2010, 10:59 PM   #3
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Using the heat generated by the hot water chimney?


Hmm probably why it's not something that is seen then. I suppose the tankless probably has a blower inducer motor to push the fumes away vs letting gravity do the work.

I did hear of this new thing where the hot water that goes down the shower drain goes through a heat exchanger, and that heat is used for stuff. Guess that's a safer alternative to messing with the flue.
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Old 09-28-2010, 07:36 PM   #4
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Using the heat generated by the hot water chimney?


Thanks for the good idea RS.

My upstairs bathroom is directly above the downstairs bathroom. The electric water heater is in a closet in the down bath. Tub drain for both baths is right next to the water heater cold and hot water lines. Might be easy to use the wasted warm tub drain water to pre-heat the cold going to the water heater.

Do you have any links to that device you are referring too?
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Old 09-28-2010, 07:44 PM   #5
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Using the heat generated by the hot water chimney?


I saw a episode of This Old House Hour with it. IMO it will cost more to buy /install than it is worth. Soapy water will sludge it up and reduce the heat transfer rate so it won't work well. Some kind of "grey water" heat recovery device/heat exchanger.
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Old 09-28-2010, 07:50 PM   #6
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Using the heat generated by the hot water chimney?


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Originally Posted by Earnie View Post
Thanks for the good idea RS.

My upstairs bathroom is directly above the downstairs bathroom. The electric water heater is in a closet in the down bath. Tub drain for both baths is right next to the water heater cold and hot water lines. Might be easy to use the wasted warm tub drain water to pre-heat the cold going to the water heater.

Do you have any links to that device you are referring too?
I can't recall where I saw it, or if it was just something someone told me, but then I got the same idea with the hot water exhaust pipe, as that gets bloody hot. But if it would cause a problem then best not to touch it. I suppose if I had an inducer motor it would be less of an issue? But the electricity needed to run the motor kinda offsets whatever you'd gain from the heat.
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Old 09-28-2010, 07:56 PM   #7
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Using the heat generated by the hot water chimney?


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I saw a episode of This Old House Hour with it. IMO it will cost more to buy /install than it is worth. Soapy water will sludge it up and reduce the heat transfer rate so it won't work well. Some kind of "grey water" heat recovery device/heat exchanger.
I was thinking that as well, also the water that goes down the drain has lost probably more then half it's heat by the time it hits the person and falls to the ground. Most of the heat goes up as steam or is absorbed by the body.

Now another thing that would be cool is a HRV that also hooks into the home's various forms of exhaust such as kitchen and bathroom fans. Could give full control over air exchange instead of relying on random cracks that air can come in to replace air being exhausted out. Maybe something we will see in the future.
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Old 09-28-2010, 08:00 PM   #8
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Using the heat generated by the hot water chimney?


I have it in my house. Most newer higher end homes in my area use one centralized exhaust fan for everything except cooking exhaust which is greasy and needs it's own fan. I unhooked my central fan/removed it and installed my own HRV in place which I got a sweet deal on as I am a contractor.
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Old 09-28-2010, 08:44 PM   #9
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Using the heat generated by the hot water chimney?


I did some hunting and found these.

http://www.ecodrain.ca/

http://gfxtechnology.com/contents.html

http://www.retherm.com/index.htm

http://www.trimlinedesigncentre.com/page.php?197


Interesting soap comment. I don't believe any mentioned the soap problem but some did say the device was self cleaning.
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Old 09-28-2010, 08:48 PM   #10
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Using the heat generated by the hot water chimney?


A residential water hater is roughly 78 to 80% efficient.
And around 36,000 BTUs an hour input. So about 7,920 BTUs an hour goes out the chimney. So if you could transfer all that heat to the water. You would be able to raise 24 gallons of the cold incoming water by 40 degrees. If you took a shower that lasted about 60 minutes and the water heater's burner ran the entire time.

Doesn't matter if the water heater has an inducer or not. It would still cause condensation in the chimney, and rot out the chimney.
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Old 09-28-2010, 09:00 PM   #11
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Using the heat generated by the hot water chimney?


I think the initial cost would be HIGH as copper is very expensive and if you had to pay a plumber to install it it would be at least $500 or more. Would take a LONG time to break even. You can buy a LOT of gas and electricity where I am for that money. Having done quite a bit of plumbing in commercial blgs etc I have seen what soap etc does to drains. When new the exchanger may be efficient, add some sludge and it decreases quickly. Of course a salesman/brochure will tell you what you want to hear. Real life tends to be a bit different.
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Old 09-28-2010, 09:08 PM   #12
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Using the heat generated by the hot water chimney?


They make a heat exchanger for the drain line. that the coil is wrapped around the drain line, and none of the waste water ever touches the actual heat exchanger coil.

I see them once in a great while. The people claim it helps.
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Old 09-28-2010, 09:16 PM   #13
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Using the heat generated by the hot water chimney?


I am a keep it simple guy. I have seen fouled up heat exchangers for chillers/steam heat exchangers/ condensors etc etc and did water treatment for them etc. Would have a hard time proving to me with some long tern studies that they would pay for themselves. Would probably get a better bang for the buck with better windows/insulation stopping air leaks.
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Old 09-28-2010, 09:21 PM   #14
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Using the heat generated by the hot water chimney?


They seem to work best with families that have only one full bathroom, and 3 or 4 people that take a shower one after the other.

The coil around the drain line. never has any direct contact with the waste water. And since it doesn't get real hot. It doesn't get a build up of minerals or scale, like a boiler's tankless coil does.

The fresh water line of a house has less air in it then a tower does.
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Old 09-28-2010, 10:27 PM   #15
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Using the heat generated by the hot water chimney?


It all gets real interesting when you study steam boilers and chillers and large equipment. Wall thickness and design of heat exchangers yada yada yada. Worthwhile on large units with a large Delta T and continuos flow etc. Hardly worthwhile for a measly 10 -15 minute shower. Will take that long to heat the darn thing up. Even worse if you use a low flow shower head.

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