Mini-Split 101


Our brand new Mini-Split Guide is available here!  Our Mini-split expert Nick Lucas will be happy to review this information with you if you are in the market for a new mini-split heat pump.Mini Split Book Final front page

The descriptions and pictures below are designed to foster a basic understanding of the parts of your mini-split system.  This page will focus on non-ducted split systems such as the Mitsubishi units that have an outside heat pump and a wall unit (or units) and slim ducts that have minimal or no ductwork.  For heat pumps with duct work, please visit our Heat Pumps 101 page.

When you call for service, we hope they will help you to identify those parts of your system that may need service.  When you receive an invoice from us, we hope the information below will help clarify which parts and systems we worked on during our service visit.


The outside part of the mini-split system is usually powered by a double pole circuit breaker in the breaker box.  An extremely common cause of no heating or cooling is a circuit breaker in the off position.




The outside disconnect is the switch used outside to turn on and off power to the outside heat pump.  An extremely common cause of no heating or cooling is having this switch in the off position.




The outside heat pump unit houses the compressor, expansion valves, outside coil, condenser fan, and control board.





The outside unit uses a condenser fan and motor to pull outside air through the coil to extract heat in winter and reject heat in the summer.






This is a view inside the heat pump unit with its case removed showing the fan and the coil behind it.  The compressor is the black cylindrical object in the lower right.






The refrigerant flow is controlled in modern heat pumps in the outside unit with electronic expansion valves, one for each inside wall unit or slim duct. This makes sure that refrigerant flow is optimized at all times.  When the outside unit is first started up, these electronic expansion valves will tick for a few minutes as they re-index themselves.



To changeover from heating mode to cooling mode, the direction of the refrigerant must by reversed so that the outside unit gets hot and the inside unit gets cold in summer for cooling and vice versa in heating.  This is accomplished in the outside unit with a reversing valve.



rotatedIMG_1644Modern heat pumps are highly sophisticated units that depend on computer programs to operate efficiently and reliably.  The outside control board performs this task.  Unfortunately, this is also the point of greatest vulnerability to the inadequate Vineyard power grid.




The refrigerant pipes are connected to the outside unit at the service valves – this is also the point where we can connect our gauges to measure the refrigerant pressures in the system.





The line set is connected to the outside unit with flare fittings.  These were originally designed to simplify relocating the mini-split unit (a common practice elsewhere in the world where people take their appliances with them when they move).  Unfortunately, the flare fitting is the leading location of leaks in mini-splits.



The control board not only monitors the pressures but also the refrigerant temperatures going to the indoor heads using temperature sensors.





The outdoor unit power wire is 230 volts AC and is the sole source of power for the entire system.






The indoor units are powered and controlled by 4 wires from the outside unit.  One wire is ground, one wire is communication, and two wires are power.






The outside unit compressor is variable speed to precisely control the flow of refrigerant.  It is well insulated in sound proofing to reduce noise output.






The copper pipes that carry the refrigerant into the house and back out are called the “line set” .  Both lines are insulated because a heat pump moves heat into the house in the winter and out of the house in the summer line set inside house





The inside wall unit (or mini-split) has a small blower, coil, and washable filter.  It is called a mini-split because it has separate outside and inside units and it is quite small in size and capacity.





The wall unit has a washable air filter on the upper surface – return air goes into the top and supply air comes out of the bottom per fins.






The green light on the Daikin equipment should be illuminated solidly.  If it is blinking slowly, that indicates that the two or more indoor heads are not in the same mode – one is in heating and the other cooling.  The outside unit will honor the first indoor unit call that it gets (for heating or cooling) and will ignore the request for the opposite mode from the other indoor head.



The inside wall unit can be configured to sense occupancy (by detecting motion) and turn on to a set temperature.  We usually recommend that people turn these off as it may be confused by people that are asleep.





The inside wall unit senses the temperature of the room from its on-board sensor.  The remote controller has no sensing ability – it just tells the inside wall unit what to do.



The other type of inside unit that works with the mini-split platform is the “slim duct” – basically a tiny air handler.  It is designed for minimal ductwork and a small space.  But it permits ducting one unit to cool or heat two or more small rooms.





The typical method of control of these inside units is the remote control.  One remote control will operate all of the inside wall units.  A separate adapter is necessary to permit them to work with the slim duct units.  Please remember that everytime a button is pushed, ALL information  (fan speed, temperature setting, mode, etc.) is re-sent to the inside unit.



415k4HXdcpLThe other common way to control the inside units is with the Mitsubishi MHK-1 wall mounted thermostat/controller which uses a wifi adapter module for use with the wall and slim duct units. This thermostat can be seen remotely on your smart phone or computer when connected to the Honeywell Redlink gateway.




Rosenbaum - Blizzard 4During the winter, the outside heat pump unit is removing heat from the cold winter air.  This works for about 30 minutes and then frost starts to build up on the outside coil.  The heat pump recognizes this and uses the warm refrigerant to melt, or defrost, the ice and frost on the coil.  This results in a puddle of water under the unit.  This is why it is critical to keep the outside unit clear off the snow and ice so that the melted water has a place to go and does not build up and freeze inside the unit.  On a Daikin system, the thermostat will say “Standby” when the system is defrosting.   Defrost time varies depending on how the computer system understands how much frost is present and how cold it is outside.




Our goal is to be Martha’s Vineyard’s premier plumbing and HVAC shop – we appreciate the opportunity to service, install, and rescue your mini-split system!