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radiant heat design with solar thermal collectors - Page 5

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Posted by SJC on October 8, 2006, 5:03 pm
"A lot of drain-back systems do exactly this.  The pump has enough head to
maintain pressure at the top of the 'U' at slightly over atmospheric
pressure, and a small float valve mounted at the high point lets air out of
the system until water is pumped up that high."

I am confused on the terminology here.

Draindown systems use a valve:


(see item 2 on the drawing)

And Drainback systems use a drainback reservoir:


(see item 7 on the drrawing)

Unless Thermomax is completely wrong on this, it needs clarification

Posted by daestrom on October 8, 2006, 9:22 pm

Perhaps I added to your confusion.  I have been using the term 'drain back'
and 'drain down' interchangebly for the type of system that Thermomax is
calling a 'drainback' system with a tank to hold the water drained from the
system until the next sunny day.  Sorry about that.

The type that uses a reservoir is what I've been talking about.  The pump
has to have enough head to get water to the highpoint so it can go 'over the
top' and fall/siphon down the return line.  If the return line is kept
flooded, the weight of water in it can aid in pumping the water through the
system.  But this aid comes at a price.  The absolute pressure at the top of
the system is lower than atmospheric pressure.  The other option is to use a
larger pump and have it maintain the pressure at the collector higher so
that it is always at least atmospheric pressure.


Posted by Solar Flare on October 11, 2006, 2:16 am

Posted by Solar Flare on October 11, 2006, 2:21 am
 Both systems use a drainback reservoir. One is an added tank and one
is the main tank.

Also a vacuum break valve was forgotten on the so-called "drainback
system". If the air cannot get in, the liquid cannot get out.

I do not have a vacuum break valve on my system. The reservoir and
storage tank has a return pipe into it above the water level and drain
down or drain back happens naturally by siphoning. The source line to
the pump comes off the bottom of the tank and the return goes to the
top without any seal. This could be dangerous but my plan is to
install a smaller pipe inside a larger one for the vacuum break, into
the top of the reservoir/storage tank

Posted by nicksanspam on October 5, 2006, 6:10 pm

A small hole in the return pipe above the nighttime water level?


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