Diversion Loads in Solar Thermal Space Heating

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With any solar-powered heating system, whether it's for the domestic hot water supply or space heating, a diversion load can be essential to safety and the system's lifespan. Pressurized solar thermal systems retain solar fluid in the solar loop at all times (except during maintenance), so they need somewhere to dump excess energy when they're not in use. That's where a diversion or shunt load comes in.

A diversion load sits between the equipment that uses the solar thermal installation's energy and the collector array, on the return circuit. The heat generated by the collectors is passed to the solar fluid, which travels to the heat exchanger equipment. There, it divests itself of the heat that will be used, then travels on into the diversion loop. Here, it dumps any excess heat before returning to the collector array.

This effectively ensures that the solar fluid never gets too hot and breaks down, shortening its life.

Most diversion loops are automatic, though manual versions still exist because the automation technology wasn't always easily available. The shunt itself is usually made of Pex or copper tubing, which is not insulated and is buried outside. Assuming the ground is averagely moist, one foot of one-inch Pex per square foot of collector or one foot or one-inch copper tube per two or three square feet of collector is reasonable. Very dry soil lowers the shunt efficiency, so you'd have to oversize the loop.

There are alternatives to the buried pipe loop. For example, you can circulate the solar fluid through a finned radiator or finned baseboard tubing placed above ground in a cool, shady, windy spot – you'd need about one foot of finned tubing per every four square feet of collector. You could also direct the fluid into a pool heater or a hot tub, if you have one that will use all the excess heat to provide a little luxury. The excess energy is basically free heat in non-heating months, so use it wherever you can... be creative!

Modern controllers often come with an automatic shunt loop switch as the norm. It's pretty easy to configure the loop: install an aquastat on the return line leading to the collectors and set it to below 180°F (82°C). When the limit kicks in, the aquastat can set off a zone valve (usually a 24V AC affair, so you'll need to run it some power or step down the normal power using a transformer placed after the aquastat to avoid unnecessary consumption) or a diversion pump. Point the valve or pump to the shunt load and you're done.

If you want to use a manual system, which is not recommended, you would install ball valves on the solar loop and on the shunt loop. To divert the flow, you simply open one and close the other. It is vitally important to always have one valve open or the solar fluid will not circulate and will risk overheating, which not only reduces its life but also risks damaging the system. You can install a manual system around an