Shower wastewater heat recovery

[Wayne Whitney]

Hello,

I'm interested in the idea of recovering some of the heat from the wastewater of my shower, and I'm wondering about building my own heat exchanger. For simplicity, I'm just thinking of preheating the shower cold water feed with the shower drain water.

My first option would is to use a piece of 1.5" or 2" copper DWV pipe for the drain portion and to attempt to coil 1/2" soft copper type L pipe around the copper DWV pipe. How tight a bend can one make with

1/2" soft copper pipe? Are there any measures I could take to improve the contact and heat exchange between the DWV copper and the soft copper coil?

The other option I know of is to use a 1.5" copper type M pipe inside a 2" copper type L pipe, where the inner pipe is the drain, and space between the pipes is pressurized with cold water. For a given length of heat exchanger, how would this design's performace compare with the coiled design? I'm not sure if this design would fly with my inspector, though.

Cheers, Wayne

[Brad Bruce]

Wayne Whitney wrote in news: [e-mail]:

Check this out

I think I remember such a system on This Old House or Bob Vila's Home Again

Brad

[yellowbirddog]

I get the feeling that you wouldn't even come close to recovering the cost of a heat exchanger system from the microscopic water heating savings.

Secondly, if really feel you must do this, leave the design of heat exchangers to heat exchanger manufacturers. Although, if you tell them what you're doing, they might tell you that you're wasting your money (unless they really want to sell you something).

[Percival P. Cassidy]

How long do you or other members of your household spend in the shower anyway?

I reckon I have the hot water on for about 20 secs., then turn it off and soap myself, then rinse myself off with cold water; the water is flowing for a total of about 1 min. When I wash my hair, the water may be flowing for as long as 3 min.

Perce

On 10/25/05 06:55 pm Wayne Whitney tossed the following ingredients into the ever-growing pot of cybersoup:

[yellowbirddog]

P.S. If you want to save money, just reduce the temperature on your water heater. Waste comes in when you combine extremely hot water with cold water to get a tolerable temperature.

Also, you can use low-flow shower heads (shudder), put a shutoff valve near the shower head to maintain the Hot/Cold mix while you soap up, or take shorter showers.

You could also insulate your water heater if you live in a hot climate or if the water heater is in the garage.

[Mark]

most of the heat is already recovered into the air in the bath...

check the water going down the drain,,, it is already pretty cool...

if it isn't then i suggest you switch to a shower head with finer drops...

the water evaporates and cools, there is probably little heat to recover in the drain water...

Mark

[nicksanspam]

No. Altho a better shower enclosure can raise the drain temp.

Nick

[wkearney99]

Trouble is the water's probably not 'warm enough' and you'd have to leave it in contact with the cold water 'too long' to make it practical. Not to mention the hassles of dealing with gray water crud building up inside whatever exchanger might be involved. This alone would make it rather impractical as the amount of build-up would rapidly degrade any expected recovery. By the time you have enough surface area to recover whatever heat is present you've also created a prime location for the various soap, skin, hair and other waste products to build up on it. Then you've got a real mess on your hands which will undoubtedly take either MORE hot water or harsh chemicals to clean.

Better to turn back the hot water heater temp and avoid wasting the hot water by having to crank up the cold to find a comfortable bathing level.

[Lawrence Wasserman]

Wouldn't it be simpler to just take a bath instead?

[scott21230]

I would also recommend a bath instead AND get one of those one demand water heaters.

[nicksanspam]

Yes, if you let the water cool before draining it.

Nick

[Goedjn]

For most places, you're better off pre-heating your water-supply with a solar collector, and ignoring the waste-water heat entirely.

[Anonymous]

If you really want to recover lost heat;

insulate....Insulate....INSULATE !

then

Figure out a way to recover the heat ( but not the humidity ) from your DRYER ! Now that's a heat loss.

[Beachcomber]

The Graywater from just a shower alone is probably not enough to economically justify a heat exchanger.

However, if you run a dishwasher, the wastewater from one complete cycle will usually be hot enough and of sufficient quantity to consider the possiblity of some sort of heat exchanger. This combined with a shower graywater system and you might save some money.

The conventional solution is a large, insulated graywater tank that contains a heat exchanger (copper coils) ahead of your regular hot water heater. The insulation preservers the temperature gradient to keep the hottest water at the top where the largest surface area of the exchange coils are located. The copper supply line must be isolated from the tank water so that there is no leakage (or biological hazard from a backflow). Also, since the tank will contain standing wastewater, some provision of cleaning or flushing the tank when necessary should be provided.

These are the basic physical requirements. I don't think that most US plumbing codes allow graywater systems though.

Beachcomber

[Larry Bud]

3 minutes? Takes me at least twice as long to wake up in the morning!

[Edwin Pawlowski]

Explain please. If you want say, 100 degree water to shower in, you can get it by heating to 100 degrees, or mixing different portions of hotter and colder water to get to 100 degrees. The Btu needed to heat the water is the same.

[yellowbirddog]

No. Energy is wasted. As an anology, imagine that you need 25 psi air, but instead of running the compressor until it gets to 25 psi, you run it until it gets to 100 psi, and then vent the tank until it gets back down to 25 psi. A lot of energy is wasted. In fact, you could have run a pneumatic motor during the venting process.

[Chris Lewis]

Bad analogy.

Dumping air from a tank until you hit 25PSI is an open system - you're dumping energy into the external environment.

Mixing hot/cold is a closed environment, you're not dumping energy anywhere except into the shower. Ignoring 2nd order effects, mixing 190F water (produced by heating 40F cold) with 40F cold to accomplish 100F, uses precisely the same amount of energy as directly heating the same (total) amount of water from 40F to 100F.

[The 2nd order effect is the fact that hotter water radiates more BTUs per hour from the pipes than colder water - it's essentially linear based on the temperature above ambient. The better you insulate the pipes, the less this matters.]

[Chris Lewis]

According to Beachcomber :

What some codes prohibit is dumping greywater into the ground separate from the black water system.

There's nothing that prevents you from having a grey water system that ultimately goes the same place as your black water. Ie: running grey water into a heat exchanger, and thence into a sanitary sewer (or septic system).

However, there _may_ be some areas that prohibit grey water in sanitary sewers.

[yellowbirddog]

If you heat up a hot water tank to 190F and only use a tiny bit of hot water to mix with cold water to get a warm shower, the energy from the rest of the 190F water tank will exit the tank, and moreso the longer it's left sitting, and at a much faster rate because of the higher temperature difference. In addition, copper water pipes transmit heat great and aren't typically insulated.

You've wasted energy running the tank up to 190F. Those "2nd order" effects dump energy into an external environment.