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Thermosiphon question
By shaniac, Section Water Posted on Thu Oct 22, 2009 at 02:25:12 PM MST
Thermosiphon placement question.

My neighbor just set up a SHW thermosiphon system that seems to be working well. (AKA his showers are hot.)
It is a home made setup (under $500) and he is using the dip tube in the solar tank as the return back to the panels.
My question is: Would connecting the return (cooler water) back to the panels at the drain at the bottom of the tank be more efficient than connecting at the TOP of the tank using the deepest dip tube? His tank is above his panels.
Or.. would the effect be so minimal on the Thermosiphon that it would not be worth the effort.
Kinda scary what to white guys debate when you live in the hills. ; )
Obviously his system is working, but could we make it more efficient (sans a pump) and would it be worth it to change it over?
As always, thanks for all your great advice.
PS: All but two of my neighbors are now using SHW in some form, you gotta LOVE that!

Thermosiphon question | 4 comments (4 topical)

Re: Thermosiphon question (3.00 / 0) (#1)
by 97fishmt on Thu Oct 22, 2009 at 03:16:41 PM MST

Yes, absolutely. I have a set up with the panels connected hot to top and cold to the drain and it works awesome. I can't imagine it works connected cold to the top. That would make a 90 degree turn at the top, not good for a thermo-siphon to take place, ok with a pump. It would be worth a try if you are looking for it to be more efficient.

Re: Thermosiphon question (3.00 / 0) (#2)
by zap on Thu Oct 22, 2009 at 10:14:54 PM MST

You really haven't given enough information. How deep is the tank and how deep does the dip tube go?

If the deepest dip tube is within a few inches from the bottom then it will make no difference. If the deepest dip tube only goes halfway down then it could improve things hooking up to the drain.

One thing to keep in mind...
Using the dip tube give you a built in sump... a place for all, if any, crud to settle out of the system.

zap

Re: Thermosiphon question (3.00 / 0) (#3)
by shaniac on Thu Oct 22, 2009 at 10:27:49 PM MST

Thanks Zap, looking online at a few tank diagrams, it looks like most cold water dip tubes extend down to just a few inches above the drain spigot. So if your theory is correct, and my loose research is correct, then changing the connection from the top to the bottom should not make too much of a difference. Anyone else have any thoughts?

And what about what 97Fish said about the 90 degree and drop. Depending on tank placement, the 90 might not be avoidable at the bottom but the drop would be eliminated.

Does anyone think we are just splitting hairs here, or would it really be that much more efficient?

Thanks so much!!!!!

[ Parent ]

Re: Thermosiphon question (3.00 / 0) (#4)
by hydrosun on Fri Oct 23, 2009 at 09:41:23 AM MST

Thermosyphon works by the water heating up and becoming lighter and being pushed up by the water that is heavier in the return pipe. As long as the water is drawn from the bottom of the tank the water in the return pipe will continue to be heavier than the water in the solar water heater. That will push the water through any small obstructions like a 90 elbow. Just keep the downward flow of heated water at the tank to a minimum.
The problem with the connections you are using is when water is used from the hot water tank. Cold water being pushed into the cold water supply pipe can just as easily go through the pipes to the solar water heater as through the hot water tank. So half of the flow will bypass the tank and put cold water into the hot water outlet. This may make the water temperature too low and need to be heated by the backup heater. To avoid this problem I put a tee before the pressure relief valve for the hot side of the solar water heater and use the drain for the cold side. That takes the solar water heater out of the loop for using hot water.
Chris

Thermosiphon question | 4 comments (4 topical)

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