# pre-heating water



## SquashNut (Sep 25, 2005)

Does any one have a pre-heat tank for their hot water?
I am talking about a solar type not wood.
Or a system made of black EMT on the roof to preheat water.
Any ideas to save money heating water.


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## fishhead (Jul 19, 2006)

I was thinking a person could preheat water and use it to dehumidify at the same time.

My plan was to pump replacement water into a tank and have a radiator connected to the tank. A temperature sensor would sense when the water in the tank was cold and circulate it through the radiator or it could be done passively. A fan would blow basement air through the radiator or maybe there's a passive way of doing that too. That would cause the water in the basement air to condense on the radiator where it could be collected and it would preheat the water before it went into the water heater.

If a household used 100 gallons/day that's 833 lbs of water to be heated with the preheater.


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## SolarGary (Sep 8, 2005)

Hi,
There are lots of DIY batch solar heater designs here:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#Batch

The batch heater is basically just a black painted tank inside a glazed box in the sun. They work well, but if you get freezing temps in the winter, they have to be disconnected and drained.

I have heard of people just running the cold incoming water through a tank located in the basement -- this might preheat the water up toward room temp if it says there long enough. I've thought that just running the water through a 300 ft coil of 1 inch diameter PEX sitting in the basement might do the same thing more efficiently because it has more surface area. I think these schemes are pretty limited in that if your water comes in at 50 to 60F the basement preheat might get it a few degrees warmer, but it needs to go all the way up to 110F or so to be useful as hot water.

12V Man heats all of his hot water with just the pilot light on a regular gas heater.

Gary


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## fishhead (Jul 19, 2006)

I just did a quick calculation and 100 gallons of water heated from 50 degrees F to 60 degrees F would provide 15,067 btu of energy for dehumidifying.

833 lbs (100 gallons of H20) times 454 (grams/lb) times 10 degree rise = 3,781,820 calories

3,781,820 calories/251 (calories/btu) = 15,067 btu

That's going from 50 degree well water temp to 60 degree basement temp.

If anyone checks the math and it's wrong please let me know.


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## SolarGary (Sep 8, 2005)

fishhead said:


> I just did a quick calculation and 100 gallons of water heated from 50 degrees F to 60 degrees F would provide 15,067 btu of energy for dehumidifying.
> 
> 833 lbs (100 gallons of H20) times 454 (grams/lb) times 10 degree rise = 3,781,820 calories
> 
> ...


Hi,
I get:
(100 gal)(8.33 lb/gal)(10F)(1 BTU/lb-F) = 8330 BTU

Where the 1 BTU/lb-F is the specific heat of water.

If all of that energy went into condensing water, the amount would be:

(8330 BTU)/(970 BTU/lb) = 8.6 lbs or just over a gallon.

Where the 970 BTU/lb is the heat of evaporation for water.

I guess that's a maximum in that the coil has to get to a temperature below the dewpoint of the air, and some of the coolth might go into cooling the air and other things? But, an interesting idea -- sort of a double benefit.

Gary


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## fishhead (Jul 19, 2006)

Thanks Gary!

My other idea was to make a dry well where the basement air could be circulated down into the 50 degree earth where it would cool and drop some moisture in the process. That would make a good storage building for things that need to be cool and dry.


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