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radiant heat design with solar thermal collectors

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Posted by Eric Yancey on October 4, 2006, 1:48 am
 
I've been designing a solar supplement to the heating system for my shop.
The shop is currently heated by radiant heat tubing in a 4" concrete slab,
powered by a standard water heater.

After some discussion in this NG regarding my ideas for a heat exchanger
(thanks to all who responded), I intend to approach the project in several
phases.

Any advice/suggestions regarding my plan would be greatly appreciated.  I
only intend to implement phase II if the shop gets too cold at night and is
too cold in the early morning.

Phase I

Diagram:
http://www.yanceywoodworks.com/solar_shop1.jpg

Simply take the hot water heater out of the picture and circulate a glycol
mix through both the floor and solar collectors.  There will be no
thermostat, and the circulating pump will be photovoltaic.  If the sun
shines, the heat gets transferred to the floor.  One pump, one check valve
(to prevent thermosiphoning), one pressure tank, a few gauges, very simple.
It will be very similar to my current setup only the solar collectors will
effectively replace the water heater.


Phase II

Diagram:
http://www.yanceywoodworks.com/solar_shop.jpg

Add the water heater back in to supplement the solar collector.  In this
phase a heat exchanger will be added to transfer heat from the water heater
to the floor loop.  When this is occuring, the fluid will not be circulated
through the solar panels, but rather through a short circuited loop that
only includes the heat exchanger and the floor.  I believe I can accomplish
this via check valves and circ pumps.

I intend to put the shorter loop from the water heater on a timer so that it
only runs at night.  Therefore there shouldn't be any risk of having both
loops operating at the same time.


Phase III

I may add a heat dump in early spring if I think heat buildup is going to be
an issue.  The alternative is of course to just drain the system for the
summer.

Thanks,

Eric




Posted by SJC on October 4, 2006, 5:12 am
 
 You could put in a differential temperature controller that is used in
most solar thermal designs. That way, when the sun shines, the PV will
have the power but will wait for the collectors to get hot enough to start
circulating and when you do not want the heat you can just turn off the
controller.



Posted by Eric Yancey on October 4, 2006, 1:46 pm
 Thanks!  What is a typical lag time between receiving enough power to run
the PV pump and the time that the collector will be warm enough to supply
heat?  Obviously there are a lot of factors at play here.  If the outside
ambient temp is 30F and it is a clear day, would it be reasonable to expect
a 30 minute or so lag time?  If I lose a small amount of heat due to
premature circulation (ha!), I can live with that.  However, if it's a 2
hour lag time that would probably be unacceptable.

thanks again!

Eric




Posted by Jeff on October 4, 2006, 2:28 pm
 Eric Yancey wrote:


   The differential controller will adjust that depending on collector
temp and tank temp.

Simpler would be a snap thermostat on the collector that switches on the
pump when the collector has warmed up. You get what you pay for, but
it's all retrofittable. I think Schooner has written a bit about this
lately.

   Obviously there are a lot of factors at play here.  If the outside


Posted by Jeff on October 4, 2006, 5:44 am
 Eric Yancey wrote:


Are you worried about water freezing in the slab?

If not, why not run a drain back (to get the water out of the collectors
at night) and run the water heater in series with the solar collector.
This way the collector is still efficient under adverse conditions  and
the water heater raises the temp (when needed) and adds thermal
storeage. You can shunt the collector when it's not adding heat. You'll
need more valves (and a small tank) but less pumps. Also, no heat
exchanger as there is no potable water.

Also, I think you'll want to temper the water a bit before running it
through the slab.

   Just my take on this.

   Jeff


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