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Understand Heat Pumps
My goal in this tech tip is to help those who struggle to understand heat pumps get their heads around them as quickly as possible and understand some of the things a tech needs to know about them.
The basic idea of a heat pump is to use the compression refrigeration cycle to move heat in the opposite direction from what would be considered usual by making the coil that would usually be an evaporator into a condenser and the coil that would usually be a condenser into an evaporator.
This process is generally done with reversing valve or four-way valve that connects to the suction and discharge line near the compressor. These valves can redirect the flow from one coil to another, as shown in the images above. If you want a more in-depth look at how a reversing valve works, please check out this video.
Heat pumps are pretty simple, but some of their characteristics may trip up techs who are used to cooling-only systems.
Let's look at some of the unique aspects of a heat pump one at a time.
Low Voltage O & B Terminals
The Y terminal isn't really a “cooling” signal on a heat pump; it is now a circuit that energizes the compressor contactor in both heating and cooling. The shift from cool to heat is done by the reversing valve solenoid, with the most common being a 24V call on the O terminal to designate cooling. Some systems use a 24V B call for heat instead of cool, but this is far less common.
Reversing Valve Solenoid
The reversing valve solenoid is an electromagnetic coil that mounts onto the reversing valve and is generally 24V on residential heat pumps. The solenoid does not actually shift the main valve; it only shifts a much smaller pilot valve that then uses system pressure to shift the valve. The solenoid should never be energized unless it is properly mounted on the valve, or it can overheat and fail.
Two Metering Devices
Most heat pump systems will have two separate metering devices, with one being outside for heat mode and one inside for cooling mode. That is because the evaporator is inside in cool mode and outside in heat mode. In some cases, you may even find that the system has a TXV metering device inside and a piston outside. These systems can confuse some techs because they may see the piston housing outside and assume it is also a piston inside, which can lead to charging issues.
Keep in mind that each of these metering devices must have a method of refrigerant bypass in the opposite mode by either an internal or external check valve. The goal is to have properly restricted flow in one direction and unrestricted flow in the other direction.
Bi-Flow Liquid Line Filter-Drier
In a heat pump, the liquid line is always the liquid line, but the flow direction goes from outside-in during cooling mode and inside-out during heat mode. For this reason, we must use a bi-flow filter/drier on the liquid line that can filter the refrigerant in both directions.
High Head Pressure in Heat Mode
Because the indoor coil becomes the condenser in heat mode, low indoor airflow can cause really high head pressure, compressor overheating, and high-pressure switch trips. When you find abnormally high head pressure on a heat pump, always look at indoor airflow (dirty filters, coils, blowers, duct issues, etc.).
When outdoor temperatures get low enough, the outdoor coil may become icebound and require a defrost. Different manufacturers use different control strategies, but the common sequence is that system will switch into cooling mode, turn off the condensing fan, and turn on auxiliary heat where applicable until the defrost is complete. There will be a sound when the valve switches and the steam leaves the coil, so these noises can cause nuisance service calls if the customer happens to observe a defrost.
You Need Compression For The Valve to Shift
The reversing valve solenoid relies on system pressure to force the valve back and forth. If the compressor isn't running or has poor compression, the valve can fail to shift completely, resulting in a possible misdiagnosis of the valve as the issue.
Suction Pressure Drops As Outdoor Temperature Drops
Because the evaporator is outside in heat mode, the suction pressure and suction saturation decrease as the outdoor ambient temperatures decrease. This will also increase the compression ratio the colder it gets, which reduces system capacity unless other strategies are employed to increase the capacity.
I have seen many techs overcharge a heat pump when ambient temperatures are low in an ill-advised attempt to increase the suction pressure, which will only result in other issues.
Connect the Suction Gauge to the “Common” Suction Port
The large line we would normally refer to as the “suction” line becomes a vapor line because it is a high-pressure discharge rather than suction in the heating mode. To check suction pressure, you need to connect to the specially designed common suction port that connects to suction between the compressor and the reversing valve.
Obviously, this is just an introduction, but don't be afraid. Heat pumps are getting better and better and more technician-friendly all the time. Start with reading the product info on the particular unit you are working on and go from there.