Thermosyphon DHW using thermal storage tank?

afblue said:

Can I thermosyphon from an elevated hot water tank into a DHW coil in my thermal storage tank?

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There are many of us here that are convinced that incorporating gravity into the design the definitely the way to go.

But I couldn't pull it off in my Munchkin basement and I don't know of any reports from anyone who has tried it.



--ewd

If you are going for a thermosyphon, the system is going to rely solely on the difference of buoyancy between the hot and cold legs.
Since this is not a great amount of driving force, you probably should consider 1" tubing for the loop. There should be minimal 90 degree elbows.
I don't think that PEX is going to be any different than insulated copper in making the thermosyphon work. Regardless, both types of tubing should be insulated.

You do not need a big hx in the tank. It can be rather small since it will thermosyphon 24/7 as long as there is a temperature difference.
It would be better if the bottom of the hot water tank is at the top of the storage tank. I suspect the coil you are showing will not work well as a thermosyphon.

I would use a vertically oriented grid of tubing in the storage tank, so hot water is rising while cooler water is dropping to the bottom of this hx.
I would place it near the top of the storage tank, where the hottest water is.

Thermosyphons work on temperature difference. The vertical separation of the two tanks will enhance the flow.
Coils are probably going to have too much restriction. If you have a lot of 3/4"tubing, manifold the tubing vertically (it needs to easily flow up!) and make the manifolds out of larger diameter tubing.

When the time comes for solar, and you are considering an antifreeze system, then put that coil heat exchanger in the bottom of the tank.

If you are not going to thermosyphon (for any number of reasons that make it impractical), then use a coil and a small bronze pump.
Of course, that coil should be in the upper reaches of the tank since that is always the hottest zone.
In that case, you can stack them, although this is not critical for good performance.

afblue said:

My next question of all this, is how do you plumb a continuous hot water loop with a thermal mixing valve so you always have hot water at the taps, do you connect it to the cold side?

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I think you just tie in in the vicinity of the cold port of the DHW tank and the cold inlet to the mixing valve. The recirc will draw the stay-hot return through the mixing valve C port along with a little bit of hot water through the H port to make up for lost heat. Any hot water flow from the tank will be replaced by warm water returning from the loop, so there isn't a problem of cold water constantly being mixed with the hot if that's what you're getting at.

With a good sized DHW tank you only need enough flow through the coil to restore the tank before the next demand. A hundred feet just seems like an awful lot, at some point the extra length will be too much resistance. Sidearm heaters get away with a whole lot less pipe, and what you're making is a sidearm without the outer shell.

As Tom points out, the more vertical displacement the better. If the electric tank could be placed in a closet upstairs somewhere that would really help in guaranteeing that it would perform well.

It's pretty strait-forward to estimate the rate of flow you could expect, just calculate the weight of a square inch column of the hot water and likewise for the cold water and get your pressure differential. It doesn't take much pressure at all to move a half a gallon a minute through a couple dozen feet of 0.75" pipe.

Hopefully you can keep us posted with pics and diagrams when you get it all set up, as well as performance updates.

Will

I am off grid and have thermosiphoning working quite well for DHW and not so well for shoulder season heating.

My solar array delivers sufficient power most of the time and my batteries cannot absorb all of the sun during peak daytime hours.
So, I have an electricity dump to 48V water heater elements.

The DHW is provided by the SuperStor stacked above a 40 gallon electric water heater with two 48V water heater elements as shown in the foreground in the picture below.

The radiant floor is heated by the Boilermate stacked above a 20 gallon electric water heater with one 48V water heater element in the background in the picture below.

The DHW all summer long is provided by the solar array dumping to the SuperStor and no fires are needed.

I do not have the shoulder season worked out.

The setup is definitely for anyone on the grid but works well if off grid with excess electricity.

Back to the thread - the thermosiphoning works well in this setup; however, everything is pressurized and the heat is added from about 10AM to 3PM throughout the summer. So, I am not sure about your unpressurized tank heating through thermosiphoning.

You can definitely move heat from the storage tank to a higher water heater the way you drew it, probably not fast enough to meet the expectations for hot water though.

All of Tom's suggestions are right on for improving the heat transfer, raise the water heater to increase the "draft" just like a chimney, shorten the coils or make bigger diameter or manifold several short coils.

I use the drain port on an electric water heater tank for the supply AND return using a pex "reverse dip tube" coaxially to minimize mixing. With 1/2" pex and a tiny taco pump I probably still get more mixing than ideal. Generally though, the drain port is NOT recommended because it can plug up. If you could figure out how to use it the lower element hole would sure be handy.

Also you mentioned a solar coil. In my line of thinking the main advantage of unpressurised storage is the simplicity of an open drainback solar system, no coil needed. If you go to the trouble of having a glycol solar system then you would want a direct transfer between the solar glycol and the DHW storage tank, this would bypass your storage, but you could still use the unpressurised storage to preheat the incoming cold water, and the cool glycol from the DHW to heat the storage, basically making a two stage heat exchanger. You will significantly reduce the output of the solar by using the glycol loop to heat the storage and then the storage to heat the DHW.

I'm not sure what you meant by thermosiphon working better when the water is heated rapidly. Obviously a high temp differencewill make a thermosiphon work better, but the beauty of a thermosiphon is that it is automatic and will keep working as long as there is heat to transfer, even if the rate is much less than with hot storage and a DHW tank full of cold water.

afblue said:

I am not quite sure if I am understanding, you correctly? I want to heat a DHW tank with a thermosyphon loop in my thermal storage tank. The thermal storage will be charged by means independent of this setup. I am just wondering if the thermosyphon is going to be able to pull heat from the thermal storage into a normal electric hot water tank.

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Sorry, the picture got cropped...

I am actually doing the reverse - I am heating an indirect water heater with an electric water heater through thermo-siphoning.

The thermo-siphon loop is through the connections hot to hot and cold to cold with the siphoning occurring from the electric heater below.

My storage tank is in the garage a floor above both water heaters in the picture. Therefore, I am not as lucky as you to be able to thermo-siphon from storage.

Some discussion about thermo-siphoning at https://www.hearth.com/econtent/index.php/forums/viewthread/23613/P0/