Hybrid Car – More Fun with Less Gas

What overtemp control method for solar hot water heating? - Page 2

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Posted by Ron Rosenfeld on October 4, 2003, 12:48 pm

It may need more power.  I was on the grid at the time and really didn't
measure things closely.  The pump for the PV panels was the smallest of the
pumps, though.  I don't recall it's size.

-- ron  (off the grid in Downeast Maine)

Posted by offgridman@cs.com on October 5, 2003, 11:43 am
As I understand, being a completely filled system there will be no head
pressure for the pump to overcome, it will only have to overcome the
frictional losses in the system. Hence a small mag drive DC pump should
work. Please correct me if I am wrong.


Ron Rosenfeld wrote:

Posted by Ron Rosenfeld on October 5, 2003, 12:48 pm

I really don't know enough about the topic to do other than describe they
system I lived with back in the 1980's.  I don't even recall the size of
the circulating pump.  But if a drain-down system is simpler (or cheaper),
you might consider that there will be, in general, sunshine when the
circulating pump is being used.  And that could be used to generate PV
energy to run that circulating pump.

-- ron  (off the grid in Downeast Maine)

Posted by daestrom on October 5, 2003, 5:46 pm

The question comes in when talking about 'drain-down' systems, and exactly
what one means by that.

One form of 'drain-down' uses a simple check-valve on a 'tee' at the top of
the collector.  One side of valve is connected to the 'tee', the other is
open to the atmosphere.  When running, the pressure inside the system keeps
the valve shut, when off, gravity 'pull's the water down into the drain-down
tank and the check-valve lets air into the collector to break the vacumn

When the pump first starts in this type of system, it must pump water up to
the roof against gravity.  This requires more 'head' developed than in a
closed-loop system.  Once the loop is full, the pump must still maintain a
positive pressure at the check-valve/vent point, otherwise, air would keep
being sucked in.  So the pump has to always pump against this static head of
water up to the roof.  Higher head requirement =>> more pumping power

Another system uses a solenoid at the vent location.  Open the vent when you
want drain-down, shut it and start pump for operation.  Pump must still be
able to develop head to push water up to collector, but once the return leg
is flooded and the solenoid shuts, gravity on the return leg can help aid

In a closed system, the pressure is maintained by an expansion tank.  The
pump then only has to overcome the losses of flow through the piping.  Lower
head=>> less pumping.  BUT, now you need to manually drain in cold weather,
or use anti-freeze.


Posted by offgridman@cs.com on October 6, 2003, 11:37 am

Exactly as I thought, I have limited PV available and had planned on
using a closed system with a pressure tank, and a antifreeze mixture in
the system.  I already own a March circulation pump. It is not capable
of generating more than about seven foot of head  pressure. Not enough
in my case to get to the roof.

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