Air Water Heating Systems

Air systems are indirect water heating systems because air, circulated through air collectors and via ductworks, is directed to an air-to-water heat exchanger. In the heat exchanger, heat is transferred to the potable water, which is also circulated through the heat exchanger and returned to the storage tank. Figure 5.14 shows a schematic diagram of a double storage tank system. This type of system is used most often because air systems are generally used for preheating domestic hot water and hence the auxiliary heater is used in only one tank, as shown.

Solar air collector array

Outdoor equipment

Roof slab

Indoor equipment

Air-to-water heat exchanger

Hot water OUT

Roof slab

Indoor equipment

Air-to-water heat exchanger

Hot water OUT

FiGuRE 5.14 Air system.

Cold water IN

FiGuRE 5.14 Air system.

The advantages of this system are that air does not need to be protected from freezing or boiling, is non-corrosive, does not suffer from heat transfer fluid degradation, and is free. Additionally, the system is more cost effective because no safety values or expansion vessels are required. The disadvantages are that air-handling equipment (ducts and fans) needs more space than piping and pumps, air leaks are difficult to detect, and parasitic power consumption (electricity used to drive the fans) is generally higher than that of liquid systems.

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