[Greenbuilding] thermosiphon questions for the solar DHW experts

Reuben Deumling 9watts at gmail.com
Fri Oct 31 17:05:47 CDT 2014


On Fri, Oct 31, 2014 at 2:28 PM, Topher <topher at greenfret.com> wrote:

>
> 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.
>

I think I'm going to need a little help with that.
supply line: 1 elbow at terminal end
return line: 1 elbow at each end where the line leaves/enters the tank.
There are a few couplers along the way to join 20' sections that intrude
slightly into the ID of the pipe.
I'm attempting to get a steady slope, which PEX doesn't facilitate as
easily as some other materials I might have chosen.

>
>  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.
>

You said it better, yes.
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