Solar Hot Water System Types & Applications: Page 4 of 4

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Inside this Article

Luke Frazer with BOS components
Luke Frazer of The Solar Collection in Talent, Oregon, with a closed-loop, forced-circulation, drainback solar hot water system.
Batch SHW
Simple batch-style solar water heaters are relatively inexpensive and great for climates with little chance of freezing temperatures.
Flat Plate Collector
Flat-plate collectors are a proven and simple technology for reliably making hot water.
Evacuated Tube Collector
Evacuated-tube collectors are touted as having better performance in overcast weather, but at additional complexity and cost.
Solar tank, showing exchanger
Some storage tanks have an integrated heat exchanger, as this cutaway shows, increasing efficiency and simplifying system plumbing.
A small electric water heater is used as a drainback tank, allowing heat- transfer fluid to drain from the collector when the system is not operating.
Circulation Pump
A circulation pump in a drainback system has to be able to overcome the head from the tank to the collector.
Collector with PV module to power pump
In a pressurized closed-loop system, one PV module can control a small DC pump.
SHW system controller
A differential pump controller uses thermal sensors to determine when the collector temperature is high enough to add heat to the SHW tank.
Flow meter
Optional flow meters show the flow rate of heat-transfer fluid through the collector loop. When placed at the top of the DB tank, they can also serve as a sight gauge to monitor liquid level in the tank.
Water Heater Relief Valve
Temperature and pressure-relief valves are mandatory safety features.
Mixing, or Anti-Scald Valve
Antiscald valves are important, since high system temperatures are possible.
Temperature Gauge
Temperature gauges can help determine the state of a system at a glance.
Solar Pump Station
Pump stations combine many system components into a single, easier-to-install package.
Luke Frazer with BOS components
Batch SHW
Flat Plate Collector
Evacuated Tube Collector
Solar tank, showing exchanger
Circulation Pump
Collector with PV module to power pump
SHW system controller
Flow meter
Water Heater Relief Valve
Mixing, or Anti-Scald Valve
Temperature Gauge
Solar Pump Station

Heat Exchangers

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.

Valves and Gauges

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

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. 

Access

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

Further Reading:

“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

Comments (1)

Fred Golden's picture

For those with a SHW heating system, and missing out on sunny days of summer, but still have a wood stove, why not add a second loop to the system.

By installing a 50' loop of 3/4" pipe behind the stove, in a area where it will warm to above 100F, you can heat that coil of water, activate the pump, and pump that water into the water heater. You would need to buy another temperature sensor for the indoor loop, and have a switch to let the controller know what sensor to pay attention to. If the SHW system is open flat plate type, then you would need it to still pump at 4F for anti-freeze protection, so a second pump and controller would be required.

The $150 - $200 in piping, insulation and $200 pump cost can be saved at $30- $50 per month (depending on water usage and fuel costs) in about 3-4 winter's use.

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