MAILBOX: Minisplit Maintenance


Home Power has been running articles on minisplit heat pumps (MSHPs) for the last few issues. When plugged into a central power station, the actual thermal efficiency is 95%—comparable to a condensing gas furnace.

We use minisplits for well-insulated small houses and apartments. We’ve installed about a dozen over about as many years and I also live with one. But as Vaughn Woodruff’s article “Pairing Grid-Tied Power with a Minisplit Heat Pump” in HP182 shows, the great potential for solar-coupling with MSHPs side-steps most, if not all, fossil fuels.

Many of the products we buy for efficiency are rated “out of the box.” However, these efficiencies change with use. For example, an EV battery loses capacity with miles; the efficiency of an MSHP drops as lint builds up in the unit, internally blocking airflow. The air filters need to be washed periodically, but less understood is how lint can also build up inside the cylindrical fan. The indoor units placed high on the wall are pretty much built the same regardless of the manufacturer. Behind the fin tubes and electronics sits the fan. Earlier models had a problem with DC arcing from the motor to the shaft, pitting the ball bearings, so the fan had to be replaced with a motor with better grounding. I’ve pulled apart a few of these. It is a real pain—you have to disassemble the housing and louvers, and disconnect the electronics and condensate connections.

Some manufacturers, Fujitsu for one, offer a “floor-mounted” indoor unit. It still attaches to the wall but is closer to the floor. I installed one, and being able to reach the fans for cleaning by removing only a dozen fairly intuitively placed screws and avoiding having to disassemble all the unit’s workings was a pleasant surprise. This unit costs about $100 more than the high-wall-mount unit, but I found that it’s worth the small difference in upfront cost. I remain a devoted reader and hope this tip helps. Thanks for the good work.

Bill Dorsett • Manhattan, Kansas

As you have mentioned, indoor units have filters that are somewhat analogous to the lint screens in a clothes dryer. The fine screen catches larger particles to keep various contaminants—dust, pet hair, smoke particulates, etc.—from reaching the fan and the heat exchanger. Much like a clothes dryer, when this screen fills up, it reduces airflow and the unit’s efficiency. It is important to keep these screens clean by washing or vacuuming them periodically. The frequency will depend upon indoor air quality.

However, these screens don’t catch everything and, over time, the fan and heat exchanger may become coated with dust, smoke residue, or even mold. Since heat pumps cycle the home’s air, they tend to accumulate contaminants from the house that you might otherwise breathe. If mold is present or the unit needs to be deep-cleaned, there are several methods that can be used. HVAC suppliers have mold inhibitors that can be sprayed into the unit. There are also degreasers available that can be sprayed on the fan and coil without disassembling the unit. HVAC professionals also have access to low-pressure washers made specifically for deep-cleaning these units. SpeedClean’s Mini-Split Bib Kit ($90) goes around the indoor unit, keeping walls protected while the indoor unit is pressure-washed. It’s designed to channel rinse water to a 5-gallon bucket (included with kit).

Vaughan Woodruff • Insource Renewables

Comments (1)

Fred Golden's picture

Washing the outdoor coil once a year will improve it's efficiency. Check in cooling mode, the larger line should be about 50F -55F. If the unit is not cooling properly, find the leak, then repair, evacuate, recharge. These typically only hold 2 pounds of freon, it can be lost with even a tiny leak, but normally the original freon stays in the system 20+ years without leaking.

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