Did You Know with the Ductless Mini Split Systems, the majority of these units have a restrictor cap tubes assembly located inside of the condensing unit?

This restrictor is on the leaving side of the condenser coil and just before the exterior service valve/port which is normally the Liquid Service Port (King Valve). This restrictor is to meter the refrigerant to the indoor fan coil unit. This means that the copper tubing that connects the condensing unit to the evaporator is part of the evaporator and not the liquid line. This copper line should be insulated as well as the suction line.

These restrictors can have a strainer in line with them to filter out any debris that could cause blockage to the restrictor. When we have a compressor failure, and the units that are equipped with this cap tube/ strainer assembly, it should be quoted to be replaced as well as the compressor, contactor, and capacitor. Many times we find after the new compressor has been replace that the restrictor assembly is the cause of the compressor failure and the unit needs to be requoted in order to get the unit to operate correctly.

When installing cap tube restrictor, you need to verify the correct length and inside diameter with the manufacture.It has been found in the past that the replacement cap tubes were too long. Installing the cap tube that is too long will create more of a restriction and cause the unit to operate with a higher than normal superheat and stave the indoor coil of refrigerant. Care should be used when soldering the cap tube restrictor and strainer assembly. You need to purge the assembly with nitrogen to prevent oxidation from forming while the soldering process is taking place. Care must be taken not to over solder the joint as to close up the end of the tube. At this time that the unit is being repaired, it is a good suggestion to add a service port can be to the discharge line for accurate discharge pressure readings.

Here are some mistakes that have been made checking and repairing these units;

·Taking refrigerant pressures from what is believed to be the high pressure side of the unit. Remember, the service port on the smaller of the two line sets is not always the liquid line.

·Over charging the unit with refrigerant. Because of the operating pressures found and connecting the gauges to the incorrect service port, it could be presumed that the unit is low on refrigerant.

·Adding liquid line driers to clean up the system and being placed in the restricted line. The manufacture warns that this can cause problems with the operation of the unit and clog the line if the drier breaks apart.

·Adding service ports and sight glasses to the restricted line. With the sight glass in this line and after the restriction, the sight glass will never show as being full until the unit is over charged.

Care must be taken as to where refrigerant pressures are taken on these units. Connecting gauges after the restrictor, on what is thought to be the liquid service port, there will be a pressure drop across the restriction device. This pressure will not be the same as the suction pressure as the refrigerant flow through the remaining copper line to the indoor coil and through possibly another restrictor built into the indoor coil assembly. This leads techs to believe that the unit is low on refrigerant and over charging the unit. Manufactures do not want the filter drier located after the restricted line as they can cause other problems down the line. It needs to be placed inside the unit in place of the strainer or in the liquid line leaving the condenser.

When charging these units in cold weather, it is best to place cardboard over the condenser coil. If this unit is equipped with condenser pressure controls (fan cycling), it is recommended to block off the condenser in order to stabilize the unit and get the pressures steady. This will help to get a correct charge on the unit.

Some of the older Carrier units have this type cap tube restrictor as well as all of the Mitsubishi and other off brand units. The newer Carriers have replaced this with an Accuator Piston assembly. This piston has an arrow on the outside of the brass body and the arrow must point toward the condensing unit. Inverting the piston or the Accuator body will cause refrigerant to flood into the indoor coil and cause compressor failure. The Accuator piston inside the brass body can be changed to set the correct amount of refrigerant to flow to the indoor coil. This should be checked at the time of start up of the unit.

The Carrier units that are equipped with this type Accuator restrictor vary with the location of the restrictor. The restrictor can be found two foot away from the fan-coil unit on the liquid line. Other 410A systems have the restrictor at the leaving side of the condensing unit. With the restrictor in this location, the line going to the indoor coil needs to be sized and insulated according to the factory specifications. We at AirTight have relocated these Accuators to just before the indoor coil because of the length and size of the line set. When relocating these restrictors, it is important to verify the operation and the superheat of the unit. The restrictor is designed to have that extra length of refrigerant line that is removed when placing the restrictor before the fan coil unit. This can change the factory perimeters of the unit.

AirTight has also replaced these restrictors with an expansion valve and the valve was placed just before the fan coil. This was done because the restrictor would not control the flow of refrigerant to the coil correctly. The factory did not approve of this because it was not tested in there facility. The unit operated correctly and more efficiently with this operation because it tracked the superheat to the setting of the valve as well as keeping the coil at 100% of its operation at all times.

In conclusion, it is important to verify the components of these Ductless Mini Split units. This will assist in ordering and installing the correct parts needed with compressor change-outs. It will also help in the correct placement of these parts at the time of the change-outs, installation of new equipment and checking for the correct refrigerant charge of the unit.