[Greenbuilding] thermosiphon questions for the solar DHW experts

[Frank Tettemer]

I agree with Topher, around calculating pressures.
However, I have a fast and dirty way to calculate:
For every 2 feet of horizontal pipe, there needs to be one foot of vertical, for the flow to have any decent speed.

The other point to clarify is around how, and what ports are used, for the upper tank's connection to the lower tank.
  
Assuming that the lower tank is the one being heated, then, for gravity to drive this circulation well, the flow from the upper tank will be from the lowest outlet of the upper tank, (i.e., usually what is manufactured as the "drain faucet"), flowing down to the "drain faucet" of the lower tank.

This is gravity talking here; pulling the coldest water downward, and having it enter the lower tank also at it's coldest port of entry. The return flow of warm water, from the lower tank, works best exiting the tank from the upper outlet of the lower tank, to a top inlet in the upper tank.

If you think well about using gravity to pull the coolest water from the upper tank, down to the lowest level of the lower tank, then, the return flow of the warm water will be driven by the cold water displacing the warm. The water being pulled down will be the elevater device for the hottest water in the lower tank, up to the level of the hottest water in the upper tank.

Take care not to use the dip tubes improperly! What ever circulation pathway that you design, do it so that stratification is Encouraged, not Diisturbed, in both tanks. That way, each tank will have it's coolest water near the bottom, so that gravity can do it's job of driving the circulation. Mixing, or stirring, the water of the different temperatures, in either tank, will disturb stratification, and that will disturb circulation.

I agree with using 3/4" pipe for gravity circulation. Yes, that is a lot of water to be heated, But the 3/4"diameter offers less resistance and friction than 1/2" pipe.
So, it's imperitive that you seal and insulate the pipes very well. You can buy higher than normal temperature insulation for 3/4" pipe. I would use higher temp foam , so that the one day that it overheats, it doesn't also melt the foam all over the pipe, sticking and hardening there for ever. (Speaking from a personal, lousy experience).

Frank Tettemer
Living Sol ~ Building and Design
www.livingsol.com
613 756 3884


On 10/31/2014 2:38 PM, Reuben Deumling wrote:

> (1) Does it seem reasonable to assume that this system will adequately
> thermosiphon itself without the need of a pump? We have a spare pump but
> would prefer not to use it.

I would calculate the effective length of pipe (i.e. adjust for elbows
etc.) and then determine the pressure due to temperature difference, and
flow rate for that pressue.  That should get you there.

> (2) My question is whether I should install shutoff valves at both ends of
> the two 35' runs connecting the two tanks, or just at one end, and if just
> at one, which one?

I am a huge fan of shut-offs and unions.  Be able to replace anything
which breaks without draining the entire system.


  My thinking is that with 70' of  3/4" that is a lot of

> potential hot water we might as well not be heating.

The problem isn't the capacity of the pipe, but rather the surface area
of the pipe.  70 feet of 3/4" PEX holds 1.3 Gallons, at a temperature
difference of say 60°F is 641 BTUs.  The heat loss through the insulated
pipe on the other hand is 183 BTU/hour.

Thank You Kindly,

Topher

-- Topher Belknap Green Fret Consulting Kermit didn't know the half of 
it... http://www.GreenFret.com/ topher at greenfret.com

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