Control Systems for Air-Based Space Heating

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Unlike liquid-based solar thermal systems, air-based installations do not suffer when they are allowed to remain idle. This advantage works to your benefit when setting up control systems, as you only need to figure out how to switch the system on when it's needed and off when it isn't.

The easiest and most efficient control setup uses a differential temperature control . Place one sensor on the collector array and one inside the building. This allows the system to switch on when there is heat being generated by the sun and switch off when the building starts to overheat, while maintaining a large temperature differential between the building and sensor, which increases efficiency. If you'd like to add extra overheating protection, you can do so by installing a thermostat and relay; set the thermostat to the high limit and have it cut power to the blower when the limit is reached.

An alternative setup uses a setpoint thermostat to switch the collector on at a given temperature, but this is very inefficient and cumbersome. You're better off with the simple system outlined above.

The other essential bit of equipment for an air-based solar thermal system is a set of dampers. Surprisingly, air can thermosiphon in the same way as water, and dampers are used to prevent this. Wall-mounted collectors suffer less from this phenomenon than roof-mounted models, but it can happen in both cases. The problem is the same as with liquid-based systems: at night, the contents of the collectors (air, in this case) cool and sink, pushing hot air out of the building and replacing it with cold. The warm air in the building rises to the collector array and loses its heat.

The best dampers are motorized, with a positive seal, though you could get away with spring-loaded models as long as your blower can push them open when it runs. They should be installed on every primary duct going to a collector and set to operate on the same circuit as the blower – open when the blower operates, close when it switches off.

If you are installing a small, PV-powered air-based space heating system with wall-mounted collectors, you may be able to avoid using motorized dampers (which will use too much power). Instead, you can place a flapper damper made of hardware cloth and ripstop nylon on the lower cold-air duct. This will only work if the hot air duct is higher than the cold air duct, and the latter is both rectangular and horizontal.