What is a heat pump? It is an air conditioner that contains a valve that allows it to alternate between heating and cooling.
Yes — it is an AIR CONDITIONER. Some times we forget that fact — but a split system heat pump is also a year-round comfort system. In the summer it draws heat out of your home to keep it cool, and in the winter it draws heat from outside air into your home to keep it warm. Outside air always has heat in it — even at very low outdoor temperatures.
Like a central air conditioning system, it includes a compressor, a fan, outdoor coil, indoor coil and a refrigerant. The efficiency of Heat Pumps is rated using SEER (for cooling efficiency) just like any other air conditioner but it also has an HSPF (for heating efficiency — Heating Seasonal Performance Factor). A Heat Pump uses electricity as its power source.
There are some differences between a standard air conditioning system and a heat pump system.
- The indoor coil will be larger than a typical evaporator since it becomes the condenser in the heating cycle.
- The outdoor coil will have more refrigerant circuits than a typical condenser which limits the pressure drop through its coil.
- A heat pump has a method to defrost the outdoor coil during the heating mode.
A four-way reversing valve to reverse the refrigerant flow
- This is what determines whether it will heat or cool indoors
A compressor capable of handling higher compression ratios
An accumulator is sometimes installed in the suction line between the reversing valve and the compressor depending on the manufacturer
a bi-directional filter/drier to allow flow in both directions
Since we are into the cooling season, we’ll look at how the heat pump works in cooling. The main thing to be aware of is, does the manufacturer energize the reversing valve in heating or cooling? Most manufacturers energize the reversing valve in cooling (but you really need to know on your specific piece of equipment). The thermostat sends a Y , O, & G call to the indoor unit to turn on the blower and the Y & O continue out to the outdoor unit. There it is usually attached to a defrost board (control board) that then energizes the compressor contactor with the Y signal and energizes the reversing valve with the O signal. The “O” function (reversing valve in cooling or in heating) is determined by the thermostat and how it is set up.
“Y” is simply a call for compressor — could be heating — could be cooling. You need to remember this. The determination for mode of operation is the presence or absence of an “O” signal from the stat. This determines whether you are going to heat or going to cool. This will energize the reversing valve to go into cooling.
When looking at a reversing valve, the single port on the bottom is always the discharge line from the compressor. The center port of the 3 on top is always the suction line to the compressor. The right port goes to the outdoor coil and the left port goes to the indoor coil. This is universal with all manufacturers. When the solenoid on the valve is energized, the “slide” inside the valve moves to direct the discharge from the compressor to the outside coil. It also lines up the port from the indoor coil with the suction port to the compressor.
You can also see the pilot valve is “blocking” the small tubing on the right side of the valve forcing the hot discharge gas to shift the internal valve to the left to activate the cooling mode.
As long as you know that the valve is in the cooling position, the cooling sequence is the same as any other air conditioner. You will still need to verify either superheat or sub-cooling. Verify proper air flow, do all the other checks you would do for any air conditioner.
You do need to keep in mind that the safeties (pressure switches and temperature switches) are part of the defrost control but they will function just as any safety does through any control board to protect the equipment.
One problem most techs have is determining if the reversing valve is working or not. Of course electrically, we need to have the proper voltage at the solenoid coil. If the voltage is there, then it is a question of whether the valve is shifting properly internally. Word of caution — NEVER “rap” on the valve body because you could “dent it” causing the internal slide to stick. One good way to see if the slide is “free to move” is to use a strong magnet. Simply hold it up against the body and move it back and forth. you will be able to “feel” if the slide is free and moving. If you don’t have a magnet, you can also diagnose a reversing valve by the “touch” method. Attached is a chart I have had for many years that make diagnosing reversing valves pretty easy.
CAUTION: Keep in mind that you will be working on the unit while it is operating and you will possibly touch a HOT discharge line. Be careful when doing these checks.
Once you know that your “Y” is present to energize the compressor and that the “O” is present to energize the reversing valve and it is working, the remainder of your refrigerant side diagnostics are the same for a heat pump as they are for any other air conditioner.