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re: thermosyphoning freeze protection
14 jul 2003
daestrom  wrote:

>...perhaps you haven't considered how long you can keep the riser at 60
>degf?  if the hot water tank is 50 gal. and the return water perfectly mixes
>with the tank, it will cool from 60 to 40 in one day.

an inch of foamboard outside the 16 ft^2 surface of a 6"x6"x8' box makes
its conductance to outdoor air 4 btu/h-f. a 50 gal tank has 400 btu/f, so
rc = 400/4 = 100 h. at -10 f, 40 = -10+(60-(-10))e^-t/100 makes t 34 hours. 
the water might cool from 120 to 40 f in 100ln(50/130) = 96 hours. 

>given that it is designed *not* to mix, it may only have an effective
>volume (as far as the riser temperature is concerned) of 16 gal or less
>due to the deliberate stratification.

thinking about the aug 2003 hp article, it might be a lot more.
and the tank may slowly destratify by diffusion over time,
especially with some thermosyphoning flow. 

>the riser temperature will probably drop down to ~40 within 8 hours.

the header pipe has about 1.6 ft^2 of surface. with r4 insulation inside
vs outside the header box, rc = 1000 hours, so it might cool from 60 to 
40 in 340 hours, when it's -10 f outdoors.

>a cold night followed by a cloudy day or two could ruin the system unless
>the electric pump is available or something else is done to protect it (more
>insulation, drain-down, auxilary heat to the riser, etc...).  given that it
>is pressurized from the water supply, one might end up with a lot of water
>damage from some burst pipe/fitting.

that seems very unlikely, given the number of things that have to go wrong
simultaneously. how does the tank get to 60 f? an average homeowner would
notice 60 f showers, so he needs to be on vacation without having drained
the pipes first, and it has to be very cold outdoors for a long time, and
the power fails or the backup water heating system fails or the circ pump
fails or its cloudy for several days or the house becomes unheated.

with inside insulation, the header needs (40-(-10))0.4 = 20 btu/h to avoid
freezing at -10 f. that might come from 32' of bare 3/4" indoor pipe with
6.3 ft^2 of surface and 9.4 btu/h-f of conductance to t (f) house air, with
(t-40)9.4 = 20, so t = 42 f. 

steve baer suggests they make the header pipe soft copper... 


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