Heat exchangers are a part of all indirect systems since the nonpotable collector loop fluid cannot mix with the potable water used at the tap. Exchangers are often built into storage tanks, simplifying the system by using only one pump and less plumbing. Tanks with built-in heat exchangers are much more expensive than no-frills storage tanks, which can be as simple as an electric water heater with unused electric elements.
However, no-frills tanks require an external heat exchanger and an extra DHW pump to circulate potable water from the storage tank to the exchanger. Drainback systems that have a DB tank with an integral heat exchanger also require two pumps, since the heat exchanger is not part of the storage tank.
Check valves are required to prevent reverse circulation (thermosyphon) of the collector loop fluid at night. This can cause heat loss and frozen heat exchangers in antifreeze systems. Drainback systems cannot have a check valve, which would prevent the system from draining.
Temperature & pressure relief valves are required on all water heaters and storage tanks. These safety valves are usually set to “blow off” or activate on a condition of exceeding 210°F or 125 pounds per square inch (PSI).
A pressure relief valve is used on collector loops and is activated by system pressure only, for antifreeze systems blowing off at 50 to 75 PSI. Common causes of pressure relief activation are power outages and pump and control failures.
Isolation (bypass) valves allow the solar part of the system to be isolated from the conventional plumbing system for maintenance.
Mixing & antiscald valves. SHW systems don’t have thermostats like conventional water heaters and the storage tank can reach very high temperatures. Mixing, tempering, and antiscald valves all limit water temperature in the tank by mixing in a little cold water when water temps are too high. Antiscald valves have closer tolerances than mixing and tempering valves, and usually cost about twice as much. But if a system specifies an antiscald valve, a mixing valve is not an equal substitution.
Pressure gauge. An external charge pump is used on antifreeze systems to raise the system pressure to about 15 PSI upon filling. The pressure gives the antifreeze solution a higher boiling point (about 250°F) to help limit pressure relief activation. The pressure gauge is the first place to check for problems with antifreeze systems—a reading of 0 PSI indicates that the system has a leak or the pressure relief has blown off.
Pump stations are component assemblies for antifreeze systems, containing the system pump, plus required drain, fill, check, and pressure valves. Often, they will include the expansion tank and heat exchanger. Since pump stations all have check valves, they should not be considered for drainback systems.
Although SHW systems can be complex, with several components, there’s no rocket science here, either.
Contributing editor Chuck Marken (chuck.marken at homepower.com) is a New Mexico licensed plumber, electrician, and HVAC contractor. He has been installing and servicing solar thermal systems since 1979. Chuck is a part-time instructor for Solar Energy International and the North Carolina Solar Center and works under contract with Sandia National Laboratories supporting the DOE-sponsored Solar Instructor Training Network.
SRCC • www.solar-rating.org
“Solar Collectors: Behind the Glass,” by Chuck Marken, HP133
“Solar Water Heating Buyer’s Guide,” by Chuck Marken, HP125
“Solar Hot Water Storage: Residential Tanks with Integrated Heat Exchangers,” by Brian Mehalic, HP131
“SDHW Installation Basics, Part 2: Closed Loop Antifreeze Systems,” by Chuck Marken and Ken Olson, HP95
“SDHW Installation Basics, Part 3: Drainback Systems,” by Chuck Marken and Ken Olson, HP97
“Solar Hot Water Pump Stations,” by Brian Mehalic, HP134