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waste water heat recovery

Heat exchanger to recover heat from waste water
  (+16, -3)(+16, -3)
(+16, -3)
  [vote for,

With energy prices skyrocketing in next years we should save energy wherever possible. After covering the walls, roof and floor with styrofoam we can cut the cost of heating to a reasonable amount. What becomes next on the expenses list is hot water.

In an average household 4 people shower once per day for about 5 minutes, that gives about 200 litres (52 gallons) of hot water (39C) that runs down the drain almost uncooled (maybe 35C). The minimalistic solution to energy saving would be to close the drain and leave the hot water in the bath tub until it cools down. This saves most of the heat (if heating is necessary at that time) but has many drawbacks - high moisture in the bathroom, possible growth of many dangerous creatures in the water, etc.

Since the water heating system usually consists of a hot water tank with cold water inlet, I suggest that a counter-flow heat exchanger should be installed into the inlet cold water piping and into the waste water piping. This setup would recover the heat from the waste water (which is always warmer than the cold water) and pump this heat to the cold water flowing into the hot water tank. There are only minor changes in current building standards that would have to be changed for this to work the best - the waste water piping should be insulated and it should be laid near the cold water pipe that goes to the tank...

My main suggestion is to make this heat recovery mandatory for all new buildings in colder regions, because flushing expensive energy down the drain is costing lives...

slovakmartin, Aug 27 2005

Shower Heat Exchenger http://gfxtechnology.com/
A simple coil of copper pipe for a heat exchanger. [JephSullivan, Aug 29 2005]

Drain Line Heat Recovery http://www.google.c...at+recovery&spell=1
a good idea [Laughs Last, Aug 29 2005]

My more complicated version Energy_20saving_20shower
which also removes steam [Loris, Aug 30 2005]


       I love anything to do with eco-friendly and eco-consciounce design. therefore a +. So let me get this right, you're proposing to reuse the wasted hot water from our showers to heat the cold water which will be used as hot water once warmed up? Or is it the steam from the hot water that you want regulated to the cold water pipe?
chocolateraindrops, Aug 27 2005

       It is the first of the two. The steam that is in the room is hard to catch :) and it makes no sense to capture that part of energy if it's winter and you need to heat the room anyway.
slovakmartin, Aug 27 2005

       So would you mix the recycled warm water with the fresh cold water? Maybe have an internal filter for this?
chocolateraindrops, Aug 27 2005

       A bit less steam from those pavement vents could cost lives of public sleepers ... certainly not as many lives lost due to tub falls, I daresay.
reensure, Aug 28 2005

       Waste water drains are gravity fed, you would need to add a water pump to put some pressure behind the heat exchanger. Also, for more efficiency, you could make the heat exchange the low enthalpy end of a heat pump. +bun
TheBearMadeMeDoIt, Aug 28 2005

       A major installation for the domestic market. Do heat exchangers exist that can recover enough from such a deltaT range?. How cheap are they?. Nice thought, though.
gnomethang, Aug 28 2005

       So all waste water would be pooled around the cool water inlet to the water heater? Pooled around the pipe I mean so that the cool water is pre-heated?
Antegrity, Aug 29 2005

       This is baked by a co-worker of mine. He plumbed his shower drain through a homemade heat exchanger, and he claims that it does save a significant amount on his gas bill each month. I plan on installing a similar device in my own home, when I get one.   

       However, since I haven't seen anything like this widely available, I'll bun it. [+]
Freefall, Aug 29 2005

       I've thought of doing this (but I don't own my own home right now). A simple way to homebake one of these would be to wrap copper pipe tightly around the drain pipe of the shower. You could put some solder on them to increase thermal contact (unless your drain pipe is PVC). Then maybe put insulation around the whole thing. Then you run the cold water through the copper pipe before it enters the water heater. I've included a link to a similar commercially baked product.   

       There are also a couple of similar ideas in home:shower:recycling.
JephSullivan, Aug 29 2005

       //A major installation for the domestic market.//
I still stand by this!.
gnomethang, Aug 29 2005

       //the waste water piping should be insulated and it should be laid near the cold water pipe that goes to the tank...//   

       You can´t have both. Either use the waste water pipe to heat the cold water, or you try to keep the heat from the water by insulation.   

       For the first you have the problem that you need to coincide the desire for heated water and the disposal of it.   

       For the second you need a device to store the energy released by heat.   

       Also the link´s (not <Loris>´) are about water heaters (boilers) only. I for one prefer central heating.
Susan, Aug 30 2005

       It would have to be waste water from the shower only I presume.
Antegrity, Aug 30 2005

       As Pa've points out, the water from your shower head cools down a lot before it goes down the drain. There may be enough heat left in the water to be worth recovering from the drain, but it isn't going to be all the heat that came out of the shower head. Much of the heat goes into the air in the bathroom.   

       The water spraying from a shower head is in droplets, giving a large surface area for heat exchange. The cooling droplets fall down through air that is rising as it is heated, setting up a counterflow heat exchange. Evaporation takes place, cooling the water further.   

       The heated air in the shower enclosure rises, which draws in cold air at the bottom. If the shower curtain is snug to the walls, it will bulge inward from the air pressure. Some folks mistake this bulging for a demonstration of the Bernoulli principle. It is instead a demonstration of how much heat energy is being lost from the water.
baconbrain, Sep 01 2005

       I like it. I use a similar heat recovery system with my clothes dryer in the winter months. +bun
Monty6, Sep 01 2005

       Had the same idea myself...check that GFX link - Residential models for about $1200. I think it might take a very long time to make back your 1200. If you're handy with a torch, I think you could solder some copper tube to a copper drain pipe for less than $100 and two hours.
wittyhoosier, Mar 08 2007

       I had this idea too... guess the old tinfoil hat needs another couple of layers.   

       Would work best with bath water. Maybe useful on the washing machine as well.   

       specific heat of water = 4.2 kJ/kgK (K is Kelvin. Kelvin = Celsius + 273) Important thing is that the unit size is the same. ie 10 kelvin warmer is also 10 celsius warmer   

       volume (mass) = 4 L/min X 10 min = 40L per shower (40kg). This is using the most modest shower head @ 1 gpm (4 Lpm). And a substantial soaking @ 10min.   

       temp diff is a big variable... depending on how hot your shower is, and how cold your tap water is... I'll use 30C temp difference.   

       Energy = specific heat X mass X temp diff = 4 X 40 X 30 (you can double check the units if you doubt) = 48,000 kJ = 13 kW*hr = $0.80 (approx) ... yikes is this really how much a shower costs me? Yup, I just double checked an "energy calculator" on the net.   

       I'm pretty sure that heat exchanger efficiencies are going to limit this possible cost saving dramatically, but it seems to make sense none the less.   

       To address susan's concerns... the drain water would need to route (insulated) to a point BEFORE the hot water tank, at which point, the water entering the tank would be pre-heated.   

       Heat exchanger! Don't reuse the same water, we're not TOTAL hippies here.   

       Note that this doesn't work so well with a tankless water heater.
knowtion, Mar 09 2009

       Was searching for shower heat recovery on google and found it's on HB! Seems this is baked...   

       Check out www.showersave.com   

       It's expensive, and as wittyhoosier mentions, a good DIY attempt could be made.   

       Basically, I'm pinching the showersave idea of a vertical tube with water running down the inside surfaces (same way cows milk used to be cooled - why I remember this I don't know). There's plenty of room down the side of the heaing pipes in the kitchen below the shower.   

       This though, only gets so far. As baconbrain mentions, lots is lost to the air. In comes heat recovery ventilation.   

       So, combining heat capture from the air and the water, a good gain should be made and the heating bill reduced.   

       The heat recovery ventilation system works very well to capture the energy in a bath too, if it is left to cool.
saedi, Apr 04 2011


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