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Thermosyphoning freeze protection - Page 4

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Posted by daestrom on July 16, 2003, 8:57 pm

Because from what I can tell of his design, the header is horizontal, not

But his paper shows a check valve in the return line as it taps into the
incoming supply right before the dip-tube.

My point exactly.

Both pipes bare would cause them both to pick up heat from the warmer house.
Since the return line is expected to be colder, it would possibly pick up
*more* heat.  And warming the return line is against natural circulation.


Provided you only warm the riser and not both, yes.  But as we get into
these situations, I think we're straying from one of his goals of keeping
things simple and low maintenance.  Have to give the homeowner a list of
warnings and instructions of how to handle variety of circumstances.  You or
I wouldn't mind, but John Q. Public might.



Well, he did mention using a dip tube to promote stratification in the tank.
I suggested actually having the return enter the tank at the bottom (to
avoid dip-tube warming when the tank is stratified).  One penetration on
each side at the base of the tank might do it (one goes-in and one
goes-out).  Then we only have to warm the water in the pipe and a small
layer at the base of the tank.  Interestingly, with these numbers, the
return water will actually be warmer than the tank for a while.

And keep in mind, the Q00dTlb/h assumed all 16 feet of vertical run was
at the indicated temperatures.  If the return is kept insulated, that's not
too much of a problem there, but if the riser is actually being heated along
its length, not all its length will be warm and the resulting driving
head/flow will be less.

And we don't have to heat the riser all the way from 38 to 50 all at once.
If the flow is very low (due to similar densities of riser & return), then
we just have to heat the riser faster than the return (remove insulation
from only riser).  As the temperature goes above 40.44, it will slowly
buildup a dP and initiate flow.  And said flow will draw in more 38 degree
water from tank/return.  This denser water will tend to reduce/stall flow.
There will be a dynamic equilibrium where flow only increases as fast as the
riser heating can support warming the incoming water.  As the incoming water
warms, the flow will increase.

The only problem with all this is, "How long will it take to get enough flow
established to prevent freezing?"  The more bare pipe (or fins); the lower
in the overall elevation where the major heating takes place; and the colder
the return leg is kept, the faster flow will establish.  But of course, as
the house cools (if it cools at all), the less flow.

Well this is a problem with using the dip-tube return as well.  It requires
the water entering the tank from the dip tube to somehow circulate with the
upper regions.  Of course, being warmer, it would tend to 'turn-over' the
tank volume and that may be enough.  And with forced circulation one might
have a sort of header to help distribute the flow that wouldn't be as
effective with the lower flows of natural circulation.  Sort of a compromise
between *not* stratifiying the tank with forced flow, but letting it
stratify under emergency, natural circulation conditions.

Now that's an interesting idea, maybe a lift-check installed upside down so
it hangs open when the pump is off.


Posted by Nick Pine on July 17, 2003, 10:23 am

The bypass pipe might connect the collector supply and return lines just
above the tank in the basement, with your hanging check valve that only
allows flow from return to supply when the pump is running but allows slow
flow in either direction when the pump is off. To collect more house heat,
the supply and return lines might both be bare inside the house.

Steve Baer says thermosyphoning systems with thermally-equal legs are more
reliable than those with one biased leg, since they can easily flow in either
direction. A designer who decides to help water flow from A to B may design
a stuck system, if it wants to flow from B to A.


Posted by daestrom on July 14, 2003, 9:27 pm

Well that will all depends on the exact connections.  Andy was talking about
connecting the return line to the inlet so the internal dip-tube would carry
the water down to the bottom of the tank without mixing with the top water.
With a small amount of usage letting in cold (supply temperature) water at
the bottom and taking warm water off the top, I can see the bottom
temperature for the riser dropping very soon to the point where
thermo-siphoning would stop.  Have to warn the homeowner to *not* use *any*
hotwater when the power goes out.

You tell me, that was your number.

But if they were to actually *use* any hotwater when the power goes out, the
supply water will cool the bottom of the tank well below 60 before the
homeowner's shower was affected at all.  A cloudy day or two and the lower
part of the tank would be quite cool just from supply water.  But the
homeowner would get hot water from the upper, electric element.  Short
showers or just sink usage would be at upper thermostat temperature.  Yet
average tank temperature would be quite a bit lower.  And the bottom of the
tank could be close to 40 due to cold supply temperature.  Around here (NY)
our city water temp. is about 38 in the wintertime for a couple of months

So a cloudy day, take a short shower, then lose power to the circ water pump
that night.  Doesn't sound very implausible at all.  Typical weather,
typical usage, one failure.   Unless you just don't shower on cloudy days
(another warning to the homeowner?? :-)

Another thought... Exactly what kind of flow can be induced through this
circ water pump when its off anyway?  The thermo-siphon calcs assume its
similar to a length of pipe.  If it were a centrifugal pump, that would be
reasonable since the casing isn't much of an impediment when its off.  But
if it were a screw or rotory type, then it might be a serious impediment to
natural circulation.

Frankly, I doubt that insulation inside the header vs. outside would make
that much difference (UA of 0.4 vs 4).  But I suppose you have the numbers
on this.  Such a small size, one can hardly use flat heat transfer equations
anymore, may have to shift to radial calculations since the inside surface
area is so much different than the outside area.

And what about conduction of the metal in the heat pipes?  I understand the
fluid/vapor inside the heat pipes won't be circulating, but you would still
have a number of metal pipes sticking out the bottom of the header box.  Do
they have some sort of 'thermal break' in the metal construction?

And if the pump were running at night when it failed, the riser and return
temperatures would be nearly identical.  Assuming the force flow is ten
times the expected natural circulation flow, the temperature difference
would only be 1/10 of the NC delta T or 0.27 degF.  How long would it take
to build up natural circulation (i.e. a delta T)?   When there is no flow,
the water in the riser and return both cool/warm the same amount so it won't
develop a difference in temperature very fast.  Natural circulation systems
typically have the two legs of piping offset vertically to start the flow.
Putting the inlet and outlet on the same elevation may leave the system
stalled for some time.  If the return line elevation includes the tank's
internal dip tube (which is heated warmer than the riser outside tank over
the same elevation), then it's conceivable the water in the dip-tube will
actually try to rise and seat the check-valve.  This would effectively stop
NC flow from starting at all.

Given typical mid-west or new england weather, IMHO I think the system would
freeze up the first circ-water pump stops out for 8 hours or more at night.
I believe freeze protection is more complicated than this because this
design is flawed (as far as freeze protection is concerned).


Posted by andy on July 12, 2003, 1:06 am
 Disclaimer: not trying to sell tubes.......just a discussion. I hate
these conflicts of interest too but what can you do?

Nick, thanks for presenting those calculations - they were a bit over
my head...no pun intended. A few notes: the evacuated tubes are almost
always plumbed for ~3/4" tube. (Metric 22mm to be exact). This of
course would aid the thermosyphon effect. In any case the 1/2" numbers
are still very attractive.

"In short it takes heat from the stored water in the tank below and
this to the night sky "every" night.  Not very efficient at all."

Evacuated tube heat pipes are very efective thermal diodes. The losses
are quite small - especially when compared to a glazed flat plate.

"A pipe stat senses the water in the panel is
on 0C - the pumps runs"

sure but added cost and complexity - the idea here is ultimate
simplicity and elegance leading to low cost, added reliability and
ultimate consumer acceptance.

"however it does not extract heat from the tank, so a
motorised valve will have to be in place."

motorized valve?! forget it!

"Moving water is less likely to
freeze.  Another stat senses if the water is about to freeze, then a
pumps cuts out and the panel water empties into an open vented tank in
attic.  Dependent on local climate of course."

Holy smokes!! This is beginning to look like a patchwork quilt :) not
the right direction here....

"If freeze protection is the goal, keep in mind the density of water
increasing with dropping temperatures when you get to about 39 degF.
there on down to 32, the density actually decreases (i.e. 34 degree
'floats' above 39 degree water).  If your water nears 39 degF, your
thermosyphon will stop, allowing the water in upper section to

good point - i don't think this is well understood. This is why the
system would be engineered to keep the temp above 45 deg - and as nick
points out - this may not be so difficult.

Finally a frost plug may offer the last stage of freeze protection
(with ball valves at the tank on the send and return lines to stop the
water shooting off the roof - if worst comes to worst that is!)

I put this in a paper at http://www.swingsys.com/030227_DHWpaper.pdf
if you want to review and comment. Untested as of yet. Armchair theory
at best + nick's analysis..... no freezing where I live ;) The system
seems to look cheap and robust at a first glance though.....

Posted by News on July 12, 2003, 8:55 am

Pretty well standard way in commercial systems, which is coming over to
domestic.  Look at the Honeywell web site.

You can do the calcs, but I would not trust them in cold climate.  I would
have a mechanical/electrical backup to be sure.  I have seen auto frost (wax
filled)  valves that allow a panel to drain onto the roof, if it becomes
that cold.  The best is an open vented system that drains back into a tank
when no useful heat is available.  If useful heat at the collectors, the
pump takes the water out of the tank.  Simple and the best in cold climates.

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