The Glitch
Overview: Given all the recent interest in solar
domestic water heating, Joe Wrenchturner decides to have a go at it. He orders
some goodies through the Internet store Solar-Is-Us and installs them according
to the schematic below.
Exercise: Can
you spot a few “minor” details that should be changed before the sun shines on
this installation?
The Fix
1. All closed-loop piping circuits with a
heat source must contain a pressure relief valve and an expansion tank. Without
them, you’re going to have a mess when the weakest piping component in the
circuit lets go with superheated glycol ready to blast out.
2. The collector circuit must
include a check valve to prevent nocturnal cooling caused by cool glycol in the
collectors descending down the supply pipe as warm glycol from the heat
exchanger rises up the return. Trust me, this can dump an entire tank’s worth
of heat back to the atmosphere in a few short hours.
Note the check valve has been
mounted between a pair of purging valves in the Fix schematic. Add glycol
solution to the circuit using the left-side valve and remove purging flow
through the right-side valve. The differential pressure associated with purging
keeps the swing check backseated and eliminated the need of installing a ball
valve.
3. The PONPC (Point Of No
Pressure Change) still applies in solar circuits. Mount the circulator to pump
away from the expansion tank location.
4. Although the Glitch system
may operate, it’s not going to perform well with the collector temperature
sensor mounted on the inlet rather than the outlet of the collector. This
sensor should be as close as possible to the collector outlet. Mount it tight,
and be sure it’s wrapped with insulation.
Also, be sure that no moisture
can get into the wiring splice between the collector sensor and the cable back
to the controller. Moisture leads to corrosion, corrosion leads to resistance
change, and resistance change in a thermistor sensor circuit leads to
inaccurate temperature information and poor control.
5. NEVER, NEVER, NEVER install a
solar domestic water heating system without an anti-scald rated thermostatic
tempering valve between it and the plumbing fixtures.
6. I would recommend a
microbubble air separator in the collector circuit, and generously size the
expansion tank to accommodate a bit of extra glycol solution to make up for
eventual air elimination.
7. Put a good-quality float-type
air vent ― one that’s rated for “solar duty” ― at the
top of the system. Set it on top of a ball valve that can be closed after the
system is deareated. The latter protects the vent mechanism from potentially
high temperatures (think 350+ ºF) when the collectors stagnate. It also allows
for easy servicing of the vent if needed.
