[Greenbuilding] thermosiphon questions for the solar DHW experts

[Reuben Deumling]

Yes to those points about connecting the upper ports to each other and the
lower ports. That's how I did it. Around here I've found that the way we
get thicker pipe insulation is to buy two sets, one that fits around the
pipe, and a second that fits around the already insulated pipe with a
correspondingly large ID. As to high temp pipe insulation, I'll look into
that.

As for the rise/run ratio that is un-promising. The tops of the tanks are
5' apart; the bottoms 8' for a run of about 35'. I think we'll try the
thermosiphon route and if that is disappointing we'll add a pump. Any
suggestions for what to use? We have a Grundfos UP rated at .74A kicking
around. But if something better has come around since then I'd be
interested.

Thanks very much.

On Fri, Oct 31, 2014 at 4:24 PM, Frank Tettemer <frank at livingsol.com> wrote:

> 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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