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Ensuring the Promise and Avoiding the Perils of Ductless Air Source Heat Pumps

Contributors: Kate Stephenson, HELM Construction Solutions; Jeff Forward, Forward Thinking Consultants, Gabriel Erde-Cohen, We Clean Heat Pumps

In buildings that are very well insulated, ductless air source heat pumps (aka “mini-splits) are becoming increasingly popular as a cost-effective and energy-saving alternative to heating systems that use oil and natural gas. And they can also be operated for both cooling and dehumidifying, often at a lower cost than conventional air conditioners and dehumidifiers. 


However, while most Vermonters are aware of the importance of properly cleaning and maintaining wood stoves, dryer vents, and oil/gas boilers and furnaces, because mini-splits are relatively new, there’s increased risk of improper operation and maintenance, which not only reduces efficiency, but also increases the risk of expensive repairs and/or early replacement, as well as the spread of mold.


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.

Cleaning and maintenance for mold prevention and optimum operating efficiency and longevity: 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. 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. And 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 equipment failure. This debris will also 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, due to moisture retained on the coils, fan and drain pan. To dry out this moisture, operating manuals contain instructions to operate the system so that only the fan will be running without the system doing any heating, cooling or dehumidifying.

Depending on the use conditions, the air filters may need to be cleaned every two to four months, which is relatively easy 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, testing the thermostat and controls, and inspecting the outdoor unit for mice nests. This combined deep clean and service inspection can cost between $200 and $300 for a system with one or two air handlers.


There are now a handful of companies in Vermont that offer this combined deep cleaning and service inspection, including We Clean Heat Pumps; Fresh Air Solutions, Energy Co-op of Vermont, Heat Pump Services, Mansfield Services, and Benoure Plumbing Heating and Air Conditioning.  


Proper installation: Because these appliances are still a relatively new technology in our region, and many builders and installers are still learning “best practices”, it’s critical that mini-splits are installed properly to avoid preventable repair issues, minimize the risk of mold growth in the air handlers, avoid creating rodent access points into a building, and to minimize the risk of damage to the outdoor unit from mice, snow and ice. Improper installation can also impede some important deep cleaning procedures. For example, air handlers should be installed perfectly level to ensure condensation drains properly into the drain pan and the condensate drain line. The condensate line also needs to be installed so that the exit end is accessible for vacuuming out any blockages. And if “hard plumbed”, there should be a “clean out” to facilitate this cleaning process. The holes drilled through outside walls also need to be properly sealed after installation of the refrigerant and condensate lines to prevent access to rodents. If a mini-split is to be used for heating, the outdoor unit should also be installed on a pedestal to avoid being impacted by snow, or the ice that forms from condensation dripping off the coils. A pedestal also reduces the potential for access by mice, and for leaves and other debris being pulled in by the fan. A roof or cover may also be needed to protect the outdoor unit from falling icicles and snow dumps.


Protecting outdoor units from mice damage: Additional preventative strategies should be employed to help reduce the likelihood for expensive repairs due to mice nesting in the outdoor compressor/condenser and damaging its wiring and circuitry. These strategies include placing the unit on a pedestal, using steel wool or a rodent-proof filler to close off potential access points in the protective metal casing (but NOT anywhere close to the circuit board), conducting a yearly internal inspection for mice nests, and if needed, using a rodent deterrent product.


Considering the need for a back-up heating source: Although cold climate heat pumps are specifically designed to produce heat in cold temperatures, their efficiency decreases as the outside air temperature decreases.  Some newer models can generate heat at temperatures of -200 F and/or contain an internal back-up heating element, but this is not always the case. Therefore, adequate consideration should be given to the installation of a back-up heating source to ensure occupant safety and protect plumbing infrastructure during deep freeze events, and/or when a mini-split becomes temporarily inoperable due to electro-mechanical failure. 


Summary: Installing air source heat pumps in buildings that are well insulated can reduce the greenhouse gas emissions associated with fossil-fuel 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. And adequate consideration should be given to the need for a back-up heating source.

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