# Solar water preheater design



## mdharris68 (Sep 28, 2006)

I am planning to build a solar water preheater using an old copper coil made of a single layer of 8- 3/4" tubes 8 ft long with about 4 inches between them. I also am using a type of open water heater as my storage tank that has a heat exchanger built in for my potable water. Also has an electric element in case of dark days. My question is this, should I plan to use a circulating pump from tank to coil and back or can I rely on thermal siphoning? If I use a pump, how can I regulate the water flow as to have the best gain in heat to my water? Thanks for any help you can provide.


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## wy_white_wolf (Oct 14, 2004)

Not sure how thermal siphoning will work with the type of tank you're using. Is the collector going to be hooked to the tank or the heat exchanger coil?

For thermal siphoning to work the tank has to be higher than the collector. All piping running from the collector to the tank has to run uphill or you'll end up with a vapor lock that'll prevent the siphon. 

If the collector is hooked to the heat exchanger coil you will need an expansion tank somewhere in the loop.


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## YuccaFlatsRanch (May 3, 2004)

See the latest issue of Mother Earth news. Good article there.


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## michiganfarmer (Oct 15, 2005)

I reccomend using a pump.

I have built 3 outdoor wood furnaces. We have built heat exchangers to heat our domestic water for all three. We have tried to let thermal siphoning work on all three. We cant get enough temperature differential to make the water flow.


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## mdharris68 (Sep 28, 2006)

wy_white_wolf said:


> Not sure how thermal siphoning will work with the type of tank you're using. Is the collector going to be hooked to the tank or the heat exchanger coil?
> 
> For thermal siphoning to work the tank has to be higher than the collector. All piping running from the collector to the tank has to run uphill or you'll end up with a vapor lock that'll prevent the siphon.
> 
> If the collector is hooked to the heat exchanger coil you will need an expansion tank somewhere in the loop.


The way this water heater is designed is this: The water heater has a coiled tube heat exchanger that hooks to potable water. The tank is an open vessel with a plug on top to fill it with water. The heat element heats the water and the potable water picks up it's heat as it passes through the coil. 



If I use this type of water heater with a homemade water/heat collector, and I had the tank higher than the collector, how would I drain down the collector when the sun goes down and it stops collecting heat? Is it possible that it's not neccassary to drain down at night if it is a passive siphoning situation? 

Also, is it critical to have the hot water (from the collector) leave the top of the collector back to the tank and feed cold water from the bottom of the tank to the bottom of the collector?


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## greg273 (Aug 5, 2003)

mdharris68 said:


> The way this water heater is designed is this: The water heater has a coiled tube heat exchanger that hooks to potable water. The tank is an open vessel with a plug on top to fill it with water. The heat element heats the water and the potable water picks up it's heat as it passes through the coil.
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> ...


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## greg273 (Aug 5, 2003)

A small pump with a thermostat would be ideal. Check out 'March pumps', and the 'El sid'. They make small hot water circulating pumps.


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## SolarGary (Sep 8, 2005)

mdharris68 said:


> The way this water heater is designed is this: The water heater has a coiled tube heat exchanger that hooks to potable water. The tank is an open vessel with a plug on top to fill it with water. The heat element heats the water and the potable water picks up it's heat as it passes through the coil.
> 
> 
> 
> ...



The thermosyphon design should be fine as long as the tank is above the collector. These are very common collectors in some areas -- If you search on SolaHart, this is one of the commercial designs of this type.

These collectors don't drain for freeze protection -- you need to use antifreeze. Be to use non-toxic antifreeze (eg propylene glycol not ethylene glycol)

The tank needs to be well insulated to prevent heat loss and freezing.

If you decide to use a pump, the simplest way to set it up is to use a DC pump and power it with a small PV panel. This way, no other control is needed -- when the sun is bright enough to run the pump, it will be bright enough to heat the water in the collector -- March and Ivan Technology make pumps of this type (as well as others). If you use an 120VAC pump, then you need a "differential Controller" to run it -- this has sensors that measure tank temp and collector temp, and turns the pump on when the collector is hotter than the the tank water. Search for GoldLine GL30 to find info on the most commonly used one.

There are a coupld thermosyphon collectors on this page:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#Thermosyphon

Gary


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## mdharris68 (Sep 28, 2006)

Well, I set this heater up and tried the siphon method. Then I researched why mine woudn't work and found out that I should have had positive upward slope from bottom to top for siphoning to occur. Now I have installed a small grundfos pump, (free from a boiler changeout) and I have questions concerning controlling the pump. I know I have to switch the pump on when the collector reaches temp, and switch the pump off when the water gets hot enough. Are there any other control points I am missing?


P.S. I looked up the GL-30 and may have answered my own questions, but feel free to tell me how you may have made a working design to do the same thing as the GL-30 does without actually buying one. I do heating and cooling for a living and am going to try to build my own control before spending more money on this project.


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## SolarGary (Sep 8, 2005)

mdharris68 said:


> Well, I set this heater up and tried the siphon method. Then I researched why mine woudn't work and found out that I should have had positive upward slope from bottom to top for siphoning to occur. Now I have installed a small grundfos pump, (free from a boiler changeout) and I have questions concerning controlling the pump. I know I have to switch the pump on when the collector reaches temp, and switch the pump off when the water gets hot enough. Are there any other control points I am missing?
> 
> 
> P.S. I looked up the GL-30 and may have answered my own questions, but feel free to tell me how you may have made a working design to do the same thing as the GL-30 does without actually buying one. I do heating and cooling for a living and am going to try to build my own control before spending more money on this project.


Hi,
The way the differential controller works is a bit different than you described.

It has a temp sensor in the collector and another in your hot water storage tank. It turns the pump on when the collector temperature goes above the tank temperature by an amount that you set -- usually about 8F. This is a little better than the scheme you describe because it waits until the collector is actually hot enough to add heat to the tank. 
The controllers also have some logic to prevent the pump from short cycling when the collector is hot enough to start the pump, but then immediatly cools down once water starts going through it. And, most of them have an overheat setting that turns the pump off when the tank goes above an upper limit you set -- e.g. 180F.

It does seem (to me) like the differential controllers go for a pretty high price when you consider how simple they are.

If you go ahead and build one, please let us know what kind of circuit you come up with, and how it works out.

Gary


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## mdharris68 (Sep 28, 2006)

Thanks for pointing out the oversight on my part about the water temp cooling down when the circulator starts. Although I don't like to try to re-invent the wheel, it would be nice to find a variable speed pump that would cycle up or down to get the optimum amount of heat transfer based on collector temp. Well I guess I'll just get the controller and make this thing work.


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