| Differential Controls |
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In active systems, when the collector is hotter than the tank, the pump circulates fluids. But when the tank is hotter than the collector, the pump must be shut down. That's the job of the differential control. This can actually be accomplished by a couple of different devices. A differential thermostat compares the temperature of the collector and tank and switches off and on accordingly. A PV pump is simpler: It comes on when the sun comes out, and as the sun gets brighter, the pump runs faster. When the sun goes down, the pump stops. Differential controls are not needed in batch or thermosiphon systems. Isolation Valve (Solar Bypass) This type of bypass valve can be placed to isolate the solar tank in case of a problem or to bypass the backup water heater if the solar water heater can meet all the needs of the household. The valve is used to direct the flow either through or past the solar tank or backup heater. When used to isolate the solar tank, one or more are placed in the incoming and outgoing lines to the water tank. The valve(s) still allows the backup water heater to remain in service. An isolation valve may be in a three-valve configuration, or a three-port, two-valve. Tempering Valve A tempering (or mixing) valve goes at the end of the system, before the faucet, to prevent scalding burns. If the water is too hot, the valve opens to mix in cold water. The preferred temperature can be set by the user directly on the valve. Backup Water Heater This heater runs on electricity, natural gas, propane, or even wood fuel. It ensures hot water whether the sun shines or not, boosting the temperature of solar preheated or unheated water to the preferred temperature settings. The backup can be a tank or tankless heater. |
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