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Drain Back Soalr Hot Water Q

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Posted by Steve S on August 9, 2004, 4:24 am
I've been helping rejuvinate a drain back solar hot water system.  The
system is now working again, but has an "unusual problem" that I would
like to get comments on.  When the water first starts flowing to the
panels, it hits the hot panels and a burst of steam is produced, with an
associated knock. (the knock is actually pretty sustainial) After the
water flow cools the panels, things work normally.

My theory is that this explosion of steam is due to the placement of the
panel temp sensor, which is placed on the top water rail.  Since these
panels are empty overnite, this means that only the copper pipe is
conducting heat to this sensor, and that its temp is probably way behind
the temp of the panels themselves.

I am curious if this location for the temp sensor is common for drain
back systems, or if putting the sensor inside the collector panel might
eliminate this problem?  Other ideas?


Posted by Anthony Matonak on August 9, 2004, 6:11 am
Steve S wrote:

I'm no expert but it sounds like you have it right to me. The idea of
the temp sensor is to turn on the pump when the panels get warmer than
the tank. Clearly the panels are getting hot enough to boil the water
so this is a heck of a lot above the tank temperature. Either the
differential controller is somehow out of adjustment (reading 300F as
90F for instance) one of the sensors is shot or the sensor is placed

You could swap out one or both of the sensors with new ones and see if
that helps. You can also test this stuff by using your own thermometers
attached at roughly the same points on the panel and in the tank. This
will tell you at what temperature differential the pump kicks in. 20F
or so should be about right. 200F difference is too much.


Posted by Ecnerwal on August 9, 2004, 12:09 pm

Sounds like your analysis is correct. Steam is definitely past the point
of warmer than the storage tank - placement almost sounds like a freeze
sensor placement, but drainback systems should not need a freeze

On the other hand, copper does conduct heat pretty well, so it might
simply be a bad sensor or mal-adjusted controller, depending on how far
the sensor is from the panel guts. I'm assuming the pipe is well

Putting the sensor in the panel itself might be a hassle, depending on
the panel construction, but would seem to make sense. Putting it very
close to the panel on the pipe might do, if things are adjusted properly
and the pipe is well insulated.

I'd probably start by pulling the sensors and comparing (resistance,
most likely) their response over a range of temperatures - if that seems
to behave, then I'd see what the controller does with the sensors
connected to a couple of pots of water with thermometers in them.

Cats, Coffee, Chocolate...vices to live by

Posted by Gary on August 10, 2004, 12:14 am
 Steve S wrote:

Not sure if this helps much, but the Tom Lane book shows several
diagrams of drainback systems with the collector temperature sensor
located as you describe (on the top header).  This position is also
mentioned in the text.  So, I think it is probably a workable location.

In installing the temperature sensor, he talks about making sure it
has good thermal contact with the header, and insulating over it, so
it sees the header temperature and not ambient.  This might be
something to check.

Assuming that a differential controller is used to turn the pump on,
he recommends that it be set to turn on when the collector temperature
is 12 to 20F greater than the temperature at the BOTTOM of  the
storage tank.  This might be something to check -- maybe try turning
the differential down on the controller.  Or maybe just replace the
controller and/or temperature sensors.
There are cheap Mulitmeters out there now that come with a
thermocouple -- you could tape the thermocouple to the header pipe
near the temperature sensor and with a bit of insulation over it, and
see what the temperature is when it turns on.  Or, use one of the
cheap IR thermometers to check the temperature along the surface
temperatures of the header and other accessible places on the collector.

I guess that since the pump turns on based on the DIFFERENTIAL between
the storage tank and the collector, that if the storage tank
temperature is already high that the collector may need to get pretty
warm to turn on the pump (especially if the differential is set to high)?

Please let us know what the solution turns out to be :-)


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