Solar hot water heater question and grey water irrigation

ncamyJanuary 4, 2011

Can I use a drainwater heat recovery unit on a solar hot water heater? I want to be able to extend the heating capacity of my hot water. DH really wants a solar hot water heater. We have a south facing roof that backs up to an alley. I need the ability to occasionally take longer than normal showers. Normal is 4-5 minutes each for two people. Long is 20-30 minutes for one ( with multi-shower heads) plus 5-10 minutes for 2-3 others, but not simultaneously. A drainwater heat recovery unit was suggested to me on another forum, but I'm not sure they realized that we really want a solar hot water heater. I'd also like to recycle my grey water for irrigation. Any advice?

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Here are a couple more links I found out there: [not heat recovery, but reuse!]

As I mentioned in the other forum, I'm not sure what the best way to incorporate the drainwater heat recovery into your solar system. You could have it run through a seperate coil in your solar hot water tank, but I'm guessing you wouldn't get much heat exchange there. You could pre-heat the water before it goes into your solar tank, which might be the best plan, but this may slightly reduce your solar heat collection. The last option would be to use it to heat the cold water coming into your shower. If you typically mix the hot and cold water in the shower, this would heat up the cold water, so you could reduce the amount of hot water you mixed with the cold water.

In the end, it will depend heavily on how you design your solar system. I'm assuming you'll be working with a solar designer, s/he would likely have the best ideas for how to integrate the two together.

    Bookmark   January 4, 2011 at 3:45PM
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Drainwater heat recovery you should look up photos of it online. Bob villa video at Habitat for hummanity build in NY could help some. Coils of copper around another pipe... Of course you can do it. Water has to come in to your solar system some how and water has to leave your bathtub... the question is how doable. Where does the city water come into your home? Where does the bathtub water drain go? If you have a basement and these two pipes are close to each other than wrapping up that cold incoming water with the hotwater that is going down the drain should be done. 15% Savings?

I have a strange do it your self idea. Bread Box water heater. Basically, you take an electric water heater. strip off the insulation and then put it in a box. Paint it black. Set up the cold city water to come into it and pre heat that cold water in your bread box so that the and the water that comes into your inside water heater is warmer.


    Bookmark   January 10, 2011 at 1:39AM
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DIY heat exchangers are incredibly ineffective. Most drains are PVC and transmit very little heat.

Re gray water - check with local authorities before you spend any money. In many places, it is illegal to use untreated gray water for irrigation.

    Bookmark   January 10, 2011 at 12:48PM
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"The last option would be to use it to heat the cold water coming into your shower."

I know about drain heat recovery options, but I had never though of putting it on the cold water side for a shower. Thanks, moandtg, for contributing. I will look at it in a different way.

This sounds like the best option. When you start stacking water heating options on top of each other, you don't get additive economy. For example, if you save 50% with solar pre-heat and 20% with your shower hot water in series you will not get 80% savings. It is a little hard to compute the exact figures because it will depend on the weather.

In addition, the higher the temperature change, the more efficient the heat transfer is. By pre-heating the water going to the solar, you actually decrease the efficiency of the solar heating process. That is why you are better off installing a cost-effective way to heat the cold water. Note that I am not saying that drain heat recovery devices are cost-effective. I have not evaluated them.

If you pre-heat the cold water to the shower, you should get additive heat efficiency. In addition, the plumbing will be greatly simplified and less costly. The heat losses in the between the heat recovery unit and the other water heater will be gone as well!

    Bookmark   January 18, 2011 at 7:59PM
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