By Steven Wisbaum As more and more homes move to energy-efficient heating and cooling systems, heat pumps have become increasingly common. But it doesn’t end with getting your heat pump installed. Learning how to conduct maintenance on a heat pump is critical. Ductless heat pumps, also known as “mini-splits” have two main components — an outdoor compressor/condenser and an indoor air-handling unit. A conduit containing the power cable, refrigerant tubing, and a condensate drain links the outdoor and indoor units. The air handler contains an air filter, and both the air handler and the outdoor unit contain a fan and heat exchanger coils. It’s important to understand that mini-splits circulate indoor air, rather than heat or cool outdoor air that’s then released into the building, so cleaning and maintenance is required for mold prevention and optimum operating efficiency and longevity. Here’s why:The blower fan pulls this indoor air through the filters, through heat exchanger coils, and then out the bottom of the unit, which means that the surfaces of all these components will collect the dust, pollen, pet dander, cooking grease, and other debris in this air. If not cleaned regularly, this debris significantly reduces operating efficiency, which in turn increases energy consumption and operating costs, and also increases the likelihood of premature equipment failure. This debris will hold moisture, which can then grow and spread mold spores throughout a building The growth of mold can also be a problem when a mini-split is left idle for multiple days during or at the end of the summer cooling and dehumidifying season because of any moisture on the coils, fan, and drain pan. To dry out this moisture, operating manuals contain instructions to operate the system in a way that only the fan will be running without the system doing any heating, cooling, or dehumidifying. Depending on the use conditions, the air filters typically need to be cleaned every two to four months, which is relatively easy for a homeowner to do, assuming the air handler is within easy reach, but trickier if a tall ladder is needed. However, cleaning the blower fan, coils, and drain/base pan of the air handler, flushing the drain line, and cleaning the outdoor unit’s fan and coils requires a “deep clean” by a technician using a specialized pressure washer and a wash water collection system. This deep cleaning should also include a service inspection to check internal components and wiring for damage or loose connections, a test of the thermostat and controls, and an inspection of the outdoor unit for mouse nests. This combined deep clean and service inspection can cost between $200 and $300 for a system with one or two air handlers. To summarize, installing air source heat pumps in buildings that are well insulated can reduce the greenhouse gas emissions associated with fossil-fueled heating systems and save consumers money. However, to ensure these goals are achieved, and to avoid the serious health problems associated with mold, these systems need to be properly installed, maintained, and cleaned. Kate Stephenson, Jeff Forward, and Gabriel Erde-Cohen contributed to this article.