We’ve installed and serviced a lot of ductless systems (often called mini splits) over the years. They are great in several applications and offer many benefits, including high efficiency and no duct losses. Variable refrigerant flow (VRF) systems are similar to ductless and are making gains in the world of commercial comfort.

One of the challenges with ductless and VRF systems is the need to regularly maintain each indoor unit, which requires planning for placement (access to the equipment) and care while servicing to avoid causing damage to furnishings and the indoor space itself.

We have learned a lot through trial and error, greatly improving our quality and efficiency when maintaining ductless systems. More than anything else, we have learned that techs require specific training to install, service, and maintain ductless. Putting an otherwise experienced tech with no ductless experience on a ductless maintenance call may not seem like a big risk, but we have found that it rarely goes well.

Here are my top tips for ductless maintenance.



Because we primarily install and service Mitsubishi ductless systems, we rely heavily on mylinkdrive.com for our resources. Most manufacturers of ductless equipment will either have a free and open website, a contractor portal, or an app you can reference to find installation and service instructions.

If resources are available, it is always best to have a look at them before starting to disassemble the system. This can help prevent damage to the unit as well as removing screws and parts that didn’t need to be removed for servicing. Often, you can also find servicing guidelines and checklists for confirming proper operation. Knowing this upfront can save a lot of time and hassle.



Unlike typical unitary equipment, ductless systems are generally installed in highly visible parts of the home/building. This means when you service them, anything dropped or spilled can damage walls, floors, furnishings, or even electronics.

It is a must to wear shoe covers and cover the area with drop cloths or plastic sheeting to ensure you don’t damage anything during the cleaning process. You also want to be extra careful when positioning ladders and handling tools so as not to gouge walls or furniture.



Just like every air conditioning system, cleaning the drain line and pan is one of the most important steps. In a ductless system, the drain pans are generally very shallow, so there is almost no room for error when leveling the fan coil and clearing buildup of sludge in the drain pan and drain line. When you disassemble many ductless systems, the drain pan comes completely off, so it’s a good first step to connect a wet/dry vacuum to the drain line, and flush a gallon or two of water through. Then, leave the vacuum on the drain for a few extra minutes to help dry the pan out before disassembly. Even after trying to pull everything out with the vacuum, there can still be some residual water and sludge in the pan. Make sure to have a rag in hand before dropping or removing the drain pan.

It often makes sense to remove the entire drain pan, take it outside, and clean it rather than attempting to clean it in place. Removing the pan also leaves the drain line open to blow out with nitrogen or CO2 for techs who prefer that strategy.



Many ductless systems are installed with small condensate pumps instead of gravity-fed drains. These pumps can be finicky and need to be cleaned well. In most cases, this means removing the reservoir and cleaning them carefully without damaging filter screens or floats. Once the reservoir is clean, blow out the discharge line from the pump with nitrogen or CO2 to ensure it’s clean. Don’t use any harsh chemicals, or you can easily damage the pumps.



Ductless blower wheels are notorious for dirt and gunk buildup. The blade cups are small, so it doesn’t take much to fill them up and impede the airflow. While there are some methods of cleaning the blower wheel in place, it can be tricky to do it without making a mess. I recommend removing the drain pan when possible. On many high wall units, this is done by removing the filters and front panels, removing the drain, removing the evaporator coil mounting screws on the left side, loosening the blower wheel set screw, and then pulling out the left side of the coil far enough to pull the wheel free. While this reflects a common layout with high wall units, it is recommended to check the service manual to at least confirm the process on an unfamiliar unit.

Take the wheel out, and clean it with a hose and spray nozzle without making a mess on a porch or driveway. Most manufacturers will advise against using cleaners, but we have found using a mild cleaner that is properly diluted can help break up thick soil.



The air filters should be handled gently and generally washed with only water to avoid deteriorating the filter over time with harsh chemicals. We also advise bringing the housing and vanes outdoors and cleaning them at the same time as the filter with a rag and water. We favor microfiber towels for cleaning because they do a great job of grabbing and holding dirt.



Like any good maintenance, a visual inspection is very important. Check high-voltage connection points; observe flare nut connections and caps for any signs of oil, which can indicate a leak; check the fan coil and condenser for proper levels; and ensure that the condenser can drain away condensation in heat mode to prevent ice buildup.

Wash the condenser coil — Washing the condenser coil on a ductless system is generally pretty simple, just be careful not to use aggressive chemicals, and keep all the panels on to protect the sensitive electronics. I suggest doing the condenser cleaning early in the process, so it has more time to drip-dry before run testing.

Clean the evaporator coil — In some cases, the coil may need nothing more than a wipe down with a microfiber towel and maybe a soft bristle brush. Sometimes, the coil will be dirty enough that a deep cleaning will be required. This is when we use a bib cleaning system, like the Rectorseal Desolv™ kit, to clean the coil in place without making a mess.

The bib is installed to catch water and dirt and funnel it down to a bucket below. You then use a pump sprayer and a mild cleaner to wash and rinse the coil.

Test the system — Checking the operation of a ductless system is much like a ducted system but with some key differences. First, the charge on a ductless system is dialed in down to the ounce — if you are going to connect to a refrigerant circuit, we suggest doing so with hoseless probes instead of a full gauge manifold to reduce losses. But first, ask yourself the question: Before connecting gauges, what do I expect to find by measuring pressures?

On a ductless system, the metering device is often an electronic expansion valve (EEV) inside the condensing unit, which means there is no liquid line outside of the condensing unit and often only one pressure test port. Couple this with inverter-driven compressors, variable-speed condenser fans, and blowers, and it can be difficult for techs to know what pressures/temperatures to expect using typical strategies.

Most manufacturers will tell you the only way to properly check the charge is to weigh the refrigerant in and out. For installation and major repairs, this works well, but for maintenance, this isn’t always practical. I advise checking the system without the use of a gauge manifold during maintenance and, instead, checking fan coil discharge air temperature, suction line temperature, and verifying the delivered capacity when possible.

If you do plan to connect, keep in mind that many ductless manufacturers use 5/16-inch size refrigerant access ports that will require an adapter to work with your standard ¼-inch hoses.

First, allow the system to run at least 15 minutes, and make sure the condenser coil is dry.

The outdoor suction line temperature for a properly operating ductless system in cooling mode at near Air-Conditioning, Heating & Refrigeration Institute (AHRI) conditions (80°F indoor and 95° outdoor dry bulb at 50 percent relative humidity) will usually be 20° to 30° below the indoor temperature.

The evaporator discharge air temperature should generally be 40° to 50° during cooling mode at AHRI and comfort cooling conditions and 110° to 130° during heating mode at AHRI heating conditions (70° indoor and 47° outdoor dry bulb). This means that traditional rules of thumb like 20° delta T don’t work well with ductless systems, where you can measure a 25° to 30° delta T regularly, especially during high-load conditions.

These are only guidelines and can vary from system to system. When you want a more detailed look at system performance, you can confirm the actual delivered Btus against the manufacturers’ specifications.

First, set the desired temperature well below room temperature (or above in heat mode) and then set the airflow to the highest manual setting.

Use two thermo-hygrometer (a.k.a humidity) probes — one in the intake and one in the discharge air — to calculate input and output wet bulb and dry bulb temps. Many wireless probes will automatically calculate the delivered capacity that can then be checked against manufacturer data. Be careful not to insert a temperature probe into the spinning blower wheel, or you will have a shredded probe and wheel.

If you don’t have an app-based humidity probe, you simply convert wet bulb temperature to enthalpy (h), subtract the lower enthalpy from the higher enthalpy, and then use this calculation:

Delta h x cfm x 4.5 = Operating Btuh transfer

By utilizing manufacturer data compared to actual delivered capacity, you can confirm whether or not the system is delivering the desired capacity.

These cleaning and testing methods will ensure the system is functioning to full maximum energy efficiency and capacity. This process generally takes two hours to complete. It may not be cost-effective for customers to complete twice per year, but even once per year can be enough to prevent breakdowns and keep the air fresh.

Publication date: 1/22/2018

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