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radiant heat design with solar thermal collectors - Page 3

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Posted by SJC on October 5, 2006, 2:30 am
I know closed systems have to be purged of air, but usually the
discussion on closed versus drainback is that closed takes less
circulation pump power. The question is how much less. If the
same dimension system is closed and takes 40 watts for the pump,
does a drainback with equal dimensions take 80 watts, or..?

Posted by Jeff on October 5, 2006, 2:42 pm
SJC wrote:

   The head on these sytems is nearly identical. The weight of the
falling water balances that rising. You do need a larger pump because
you have to overcome the initial head each morning, rather than when

   It's a bit of back and forth because you need the head capacity but
you may not need the flow from that pump.

I've heard variable speed pumps, but I imagine there are other ways to
handle that.



Posted by Robert Scott on October 5, 2006, 5:42 pm

What sort of a drainback system are you talking about?  I thought that drainback
systems were essentailly like the Thomason trickle-down system where the weight
of the falling water is all wasted in the trickling process.  If the pipes are
enclosed so as to benefit from the weight of the falling water, then how does
the air get into the top of the system when the pump turns off?

Robert Scott
Ypsilanti, Michigan

Posted by Anthony Matonak on October 5, 2006, 6:10 pm
 Robert Scott wrote:


A very small hole in the pipes somewhere or a valve that opens
when the pump stops.


Posted by daestrom on October 5, 2006, 9:57 pm

Well, a valve shut while the pump is running can help with the siphoning of
fluid on the return path.  But depending on where in the system your 'small
hole' is, it may break the siphon on the return side.  In that case, your
pumping against full head all day long.

Depending on just how high the collectors are in relation to the
pump/drain-down tank, and how hot the fluid/water gets, you may not be able
to rely on the return-line siphoning enough anyway.  For example, if the
collector is 20 ft above the pump/tank (2-story house), and the plain water
gets to about 170F, then you're asking for trouble.  The water will be on
the virge of 'boiling' as it comes over the top and will flash inside the
piping where the pressure is low due to gravity and the height above the

Closed systems can be pressurized *above* atmospheric to avoid this.


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