Heat Pump Cooling Basics


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.

  1. The indoor coil will be larger than a typical evaporator since it becomes the condenser in the heating cycle.
  2. The outdoor coil will have more refrigerant circuits than a typical condenser which limits  the pressure drop through its coil.
  3. A heat pump has a method to defrost the outdoor coil during the heating mode.
  4. A four-way reversing valve to reverse the refrigerant flow
    1. This is what determines whether it will heat or cool indoors
  5. A compressor capable of handling higher compression ratios
  6. An accumulator is sometimes installed in the suction line between the reversing valve and the compressor depending on the manufacturer
  7. 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.

reversing valve1

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.

reversing valve cooling

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.

reversing valve troubleshooting guide_

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.

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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13 Responses to Heat Pump Cooling Basics

  1. Indeed a very nice post. I am also associated with Heat Pump , Air Source Heat Pump , Domestic Heat Pump . Thanks for writing such good posts and as I have subscribed to your blog, I do expect that you will be posting nice posts like this on a regular basis.Heat Easy, Heat Pumps, Air Source Heat Pumps

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  3. dick says:

    so is a york energized in cool or heat?

  4. Robert says:

    can you measure the temperatures of the tubes entering and leaving the valve and expect similar temps. example suction line entering the valve and exiting the valve should be similar ( or within a few degrees)

  5. Frank says:

    My heat pump shuts down after the defrost cycle in cooling mode. Then it waits 4 minutes and restarts. Any idea why?

  6. Scott Prud'homme says:

    Hi Mike, I have been looking all over the Internet for an answer to what should be a very simple question, but so far have not found one. The question is: Can the signal to the Reversing Valve be left activated by the thermostat if there is no signal to turn on the Compressor? I have a thermostat that does exactly that: it turns the signal to the Compressor (on the Y terminal) OFF but leaves the reversing valve signal ON after the demand for Cooling (O-Type) or Heating (B-Type) is satisfied. I have two HVAC guys telling me two different things. One says it’s OK, the other says it will burn out the solenoid. Who’s right?

    • On York units we only activated the reversing valve in “cooling” through the ” O” terminal. Cooling is a shorter season so it saves wear and tear on the valve. We never energize in heating mode other than for the purpose of defrost. Not knowing your equipment or how it is set up, you would need to check with your local manufacturer distribution to see how they want it done.

      • Scott says:

        Thanks for answering so quickly. Your response implies that if there is no need for cooling (that is when the demand for cooling is satisfied), both the compressor AND the reversing valve are deactivated. Sorry if I provided too much info in my original post. What I really want to know is if it is OK to leave the reversing valve energized if the compressor is not running.

      • Leaving it energized is a waste of money. Leaving it energized does take a toll on the coil. It still boils down to how your manufacturer wants it energized? Is it just energized in cool like York or do they energize in heat?. If it is always energized, how does the unit go to heat if it is only supposed to be energized in cooling?
        There are too many variables here which is why I said you should check with the manufacturer as to how they want it run.

      • Scott says:

        You confirmed my suspicions. For the record, let’s assuming an O-Type heat pump. When cooling is required, the compressor and reversing valve are energized. After the demand for cooling is satisfied, my thermostat turns off the call to the compressor, but leaves the reversing valve energized, presumably because the stat manufacturer figures that it will most likely call for cooling the next time. Whether their assumption is right or wrong doesn’t matter for this discussion. When the thermostat decides that heat is required, it de-energizes the reversing valve at that time. That said, it appears as if my thermostat is wasting energy by keeping the reversing valve energized and at the same time shortening its life. Thanks for the answers and a really neat blog. I particularly enjoyed the fruit cake recipe on your Just for Fun page. 😎

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