... Notes on Solar Water Heating (SWH) ...
Florida schools recommend:
- Aim South or at least within 45 degrees of South.
- Piping should be drainable (preferably from the equipment room),
avoiding air and liquid traps. The designer should specify slopes &
- Use reverse return piping instead of balancing valves for balancing.
- Pair a relief valve with the collector isolation valve.
- Use automatic (not manual type!) air vents on system high points
except for drainback systems.
- Draindown systems (not other types) need a vacuum breaker to let
air back in.
- Temperature relief valves should be located at points of maximum
- Minimize distances between collector and solar storage tank,
thence to backup tank, thence to point of usage/load. Heat loss
is proportional to distance.
- Insulate the piping and cover the insulation. Foam wraps break
down in the sun even when painted so wrap pipes with aluminum
tape or a metal jacket.
- The (hot) sensor on the collector should be at the collector
exit, preferably inside the collector box.
- The (cold) sensor on the supply piping should be at the tank
(bottom) exit pipe.
- Install and label, but do not connect, spare hot and cold
sensors: very helpful after failures.
- Select a controller that displays sensor temperatures, has extra
input ports, and has a "pump running" indicator (e.g., a light).
Provide a fluid flow rate meter. Every heat exchanger needs
temperature sensors on input and output (show the log mean temp
- Wire it up carefully; 30% of installations required rewiring by FSEC.
- A vertical axis tank stratifies water by temperature so the
coldest gets heated, which is the most efficient way.
- Size the solar tank to provide 65\% of a typical day's hot water
load. Provide 1-2 gallons per square foot of collector. Too big
or small creates inefficiencies.
- Insulate the tank and piping, including between solar and backup
tanks, and even the cold water supply piping. Piping heat losses
can be significant. Insulate valves, circuit setters, heat
exchangers, and even pumps.
- Label pipes with fluid type and direction. Label valves, etc.,
that are covered by insulation.
- Re-check the check valve orientation and direction, a common error.
- Hot water circulation loops should be avoided if possible (major
heat losses), or else controlled by timers (with backup
batteries!) to eliminate unneeded hours of operation but to
operate when needed.
- Provide a bypass valve to feed cold water directly to the backup
heating system when the solar system is temporarily shut down.
Label it clearly so it will not be left open after the shutdown
- Don't undersize the heat exchanger in a closed loop system,
thereby reducing efficiency. Oversize works better than
undersize in this case.
- Freeze protection.
- 1/3 of visited Florida school systems had freeze damage.
- Recirculation freeze protection is little protection.
- Use draindown or drainback design or an antifreeze system.
- On draindown systems, put the automatic drain valve in a
- Draindown and drainback systems require an operator to
periodically check that valves and tanks are still
working, and that collectors are refilling after a freeze
- Operations and Maintenance requires a operator trained to tell
when the system is and is not working, and an O\&M Manual for
that specific system describing exactly how that system works and
with instructions for operation and maintenance. Don't accept
the system without such a manual.
- Periodic (semi-annual or at least annual) inspection should check
the collector, piping, insulation, valves, sensors/instrumentation,
and the controller, and should measure overall system efficiency.
Copyright © 2000-2007,
Thomas C. Veatch. All rights reserved.
Modified: November 23, 2007