Idea for energy saving - wastewater for preheating

[Dr Batsen D Belfry]

All,

A friend of mine had a thought on possible small yet not insignificant energy savings. He thought of the possibility of putting bathtub wastewater into a heat exchanger and passing the cold feed through it prior to filling the hot water tank.

We figured the hot water coming from the shower/tub, dishwasher, washing machine is already spent. To use it to preheat the incoming water could reduce the cost of heating water over the long haul, resulting in lower electric and gas usage.

Not sure if this could work or not, but I think it is worth looking into.

Thoughts?

DBDB

[Deja Q]

1. And how do you prevent people from peeing in the shower?

:rofl:

In seriousness, if the water goes to heating into steam for a generator or used to fertilize whatever, there is always a logical use...

Only the inherent complexity of reality that hinders the idea.

[One_Life_To_Give]

2. Already been done - sort of

Can't remember where off the top of my head. But the idea of putting a heat exchanger on the waste water line has been done. Although the application I remember was to help heat the air in winter.

I don't see why it won't work for pre-heating water though.

[DavidMS]

3. It already exists

http://www.toolbase.org/Technology-Inventory/Plumbing/drainwater-heat-recovery

http://www.eere.energy.gov/consumer/your_home/water_heating/index.cfm/mytopic=13040

So its certainly a known and a good way to increase efficiency.

[chaplainM]

4. Nature got there first: in duck's feet!

It's called a "rete mirabile."

These birds have a network of arteries and veins called a rete mirabile. When hot blood comes down to their feet through the arteries, the heat from the blood is transferred to the cold blood going up through its veins in a countercurrent mechanism.

The heat in the blood is exchanged over from the hot artery into the cool vein, so the heat is not lost from the bird. It actually comes back into the bird.

This system enables the birds from losing all their body heat through their feet, and keeps them just warm enough so they don't freeze, ability we unfortunately don't share with our feathered friends.



http://www.treknature.com/gallery/North_America/Canada/photo9035.htm

[Porcupine]

5. These heat exchangers don't work well in the home

as they are rather expensive and the amount of recaptured heat is small. It is much more efficient to just add a solar heater and hot storage tank to your existing water heater. Google "Solar hot water" and more information than you ever want to know will emerge.

For restaurants, hotels and commercial laundries these units can same thousands of dollars. Of course there are also water heaters available that are MUCH more efficient than your average home unit also. I worked at a small clothing dying operation and we had a heat exchanger on our waste stream.