[Greenbuilding] thermosiphon questions for the solar DHW experts

Fri Oct 31 21:03:12 CDT 2014

smart alec answer: the sun

slightly longer answer: when the tempering valve malfunctions.

At least that is my experience.

On Fri, Oct 31, 2014 at 6:53 PM, Clarke Olsen <colsen at fairpoint.net> wrote:

> How could water get hot enough to melt foam insulation?
> Clarke Olsen
> clarkeolsendesign.com
> 373 route 203
> Spencertown, NY 12165
> USA
> 518-392-4640
> colsen at taconic.net
>
>
>
>
> On Oct 31, 2014, at 8:18 PM, Reuben Deumling <9watts at gmail.com> wrote:
>
> 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/ <http://www.greenfret.com/>
>> topher at greenfret.com
>>
>> --
>>
>>
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