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sdsolar

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Posts: 6
Reply with quote  #1 
Hi,
I have had a long term interest in solar, but so far no construction. Living in San Diego now, instead of Newfoundland Canada, I am interested in using solar how water to reduce DHW heating costs.construction proposal.gif   I have read and learned a great deal from the posts and very much appreciate the insights I have gained. I have attached my proposed plan to heat the water and I hope for comments on my proposal.  On the right is the traditional approach to heating the incoming city water by running it through a heat exchanger (HE) located in the solar storage (SS) tank and then passing it to the cold water inlet on the DHW heater. The water in the SS tank will be heated by a modified trickle down collector.
The only variation is shown on the left. Instead of the collector return simply emptying into the SS tank, I have been trying to think of way to directly heat the water in the DHW tank. My goal was to reduce or eliminate the gas used now to bring the water up to temp in the DHW tank. The plan is to pass the returning water from the collector though a secondary heat exchanger at the DHW tank. This insulated HE would surround a pipe running from the drain port on the DHW tank to the pressure valve (PV) port near the top of the tank. The PV would remain connected to the DHW tank by a tee. My thought was that by heating the water in this external pipe (copper) I would heat the water in the DHW tank by the thermal syphon action. Thus, instead of the gas heating the water to maintain the tank temperature the heat from the solar collector return would contribute to this. After flowing through this secondary HE, the water would return to the SS tank to raise or maintain the temperature there as well. Clearly the temperature in the DHW tank could rise above the scalding point so a mixer would be required on the hot water output from the tank to the house. I would be happy to answer questions and again, would very much appreciate your insights and help. 
SDSolar

stmbtwle

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Posts: 2,748
Reply with quote  #2 
It should work well. I built something like that years ago for my houseboat using a pool heater and it worked after a fashion. In mine the HEX was not at the dhw tank (no room) so I had to have a second circulating pump. The water was then returned directly to the collector.

My problems were
1) I could only heat what was in the dhw tank and it would cool by morning.
2) The snap-type switch I used would not shut off in the evening, actually COOLING the dhw tank until the temperature dropped to the set point.
3) I had no hot water storage tank (weight, space). It isn't really necessary if your dhw tank is big enough. My houshold dhw system doesn't need one but the tank is 80 gal. You WILL need the mixing valve.

You will probably need a motorized valve to isolate the solar collector at night. Otherwise the thermosiphon will work against you.

Eventually I replaced the boat's system with a propane RV heater which worked well for the application.

Good luck with your endeavor and please keep us posted.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
sdsolar

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Posts: 6
Reply with quote  #3 
The pump to the collector will be shut off at night and the HE at the DHW tank will be empty. Are you suggesting I need a motorized valve to prevent flow through the pipe connecting the old drain and the PV port on the tank at night or was your concern the cool air from the collector at night cooling the DHW in the HE? In other words, prevent water circulation at night through the DHW HE or prevent cool air from entering the HE from the collector? I thought the DHW HE might be a problem at night so I planned to use substantial insulation. 
The DHW tank is 40 gal and the SS storage tank will be in the range of 60 to 80 gal. Haven't done the calculations yet on thermal gains and losses. That is the next step.

Very much appreciate your comments.
SDsolar
stmbtwle

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Reply with quote  #4 
If the collector drains down you don't need a valve. Sorry, I misread your description. I have a pressurized system which does.

You will need an air gap where the hot water from the collector enters the storage tank, to allow drainback and prevent a thermosiphon.

Unless your HE is above the storage tank it may not empty, but I'm not sure it will matter, the air gap should still do its job.

Does it ever freeze there?

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
sdsolar

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Posts: 6
Reply with quote  #5 
The DHW HE will be above the SS tank and the return to SS will have an air break to allow the system to drain completely.

It does freeze here occasionally, but, at most since I have lived here, it only lasts few hours.  
Thanks for looking this over!


stmbtwle

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Reply with quote  #6 
That wasn't shown in your diagram. Emen a few hours of freezing weather is enough to wreck a collector (been there).

However once built you should be able to turn the heating element off for the summer, I do.

__________________
Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
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