Refrigerant flow instability in heat pumps has been acknowledged for many years. This hunting problem in solar-boosted heat pumps may be attributed to inappropriate thermostatic expansion valve use, long evaporator tubes, and rapidly changing solar conditions. Control devices such as electronic expansion valves can be used to tackle the problem satisfactory. However, the price of the electronic expansion valve prohibits the use of it especially for a small heat pump such as household water heating system where heating capacity does not exceed 10 kW. Therefore, in general, a simple thermostatic expansion valve is used extensively in such systems. Experimental work from this study showed that hunting can be minimized by using a slightly under-sized capacity orifice in the TEV. However, higher degree of superheating is the side effect of this modification. It was also found that allowing some degree of hunting (DOH) by using an oversize orifice in the system may save up to 5% of energy input.
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