Short Cycling: Find What Is Stopping the Compressor

The short version

Short cycling means the equipment runs briefly, stops, and starts again, over and over, at a rate nobody designed for. Bryan Orr at HVAC School defines it as “a condition where equipment goes on and off more than is optimal,” and he separates the origins into two: “Sometimes short-cycling occurs due to a system fault, and it sometimes occurs due to a mismatch between system capacity and load (cooling or heating too quickly).”

The trap is treating the short run time as the fault and replacing whatever stopped the compressor last. The customer pays for that twice. The condition that opened the safety switch is still there, so the same switch opens again when the same conditions return. Every extra start goes onto a compressor that can pump oil out into the system faster than it comes back on short runs, which turns a dirty condenser coil or a weak run capacitor into a compressor replacement.

Watch one full cycle and record whether the thermostat is still calling at the moment the compressor stops. A call that is still up while the compressor sits points at the compressor’s power and protection path: the safety switches, the contactor, the disconnect and the fuses feeding it, the supply wiring, and the compressor’s own overload protector. A call that has gone away points at the thermostat, the low voltage wiring, and the load in the space. Those two lists share almost nothing, and you narrow the choice with one meter reading, though that reading does not identify which item on the list is the fault.

What counts as short cycling

Cycling is normal, and short cycling is a rate, so the first decision on the call is which one you are looking at. A cooling system that runs until the thermostat is satisfied and stays off until the space warms back up is doing what it was built to do, and the run time that counts as normal moves with the load. Trane gives the spread for a properly sized system: it “may only run for 3 to 5 minutes and be off for hours in mild conditions at the start of the season, or it may run for hours with very short off cycles on hotter days.” A system that runs ninety seconds, stops while the thermostat is still calling, and repeats that four times in twenty minutes is not doing what it was built to do.

Two thermostat settings shape the normal pattern alongside the load, and both of them are worth reading before you decide anything is broken. The deadband is the gap between the temperature that starts a call and the temperature that ends it. Cycles per hour (CPH) is the ceiling the thermostat holds on how many times it will start the equipment in one hour.

Orr gives a worked example that uses both: “This means that a thermostat with a 3-degree dead-band with a cooling CPH set to 3 and a set-point of 75 degrees would come on at 76 and shut off at 74 while turning on and off a maximum of 3 times per hour.”

The pause after a shutdown is normal too, and it gets reported as a fault more often than anything else on this page. The installation manual for the Honeywell Home FocusPro P200 and S200 describes what the thermostat is doing: “The compressor protection feature is engaged. Wait a few minutes for the system to safely restart to avoid damaging the compressor.” The same manual gives five minutes as the wait in its troubleshooting section: “Wait 5 minutes to determine if the thermostat makes a Heat or Cool call.” It also ties that wait to something you can see: “If display shows WAITING FOR EQUIPMENT under temperature reading, it is in compressor delay mode to protect the system.” Read the display before you call an idle condenser normal, because the delay is a setting rather than a fixed law.

The P200 and S200 manual and the FocusPRO N100 manual both list compressor protection as installer setup 3240, set to off or to one through five minutes.

There is no minimum run time you can apply to every system. Copeland answers the question for its ZP*KB scroll compressors this way: “There is no set answer to how often scroll compressors can be started and stopped in an hour, since it is highly dependent on system configuration.”

Copeland does state why the question matters: “The most critical consideration is the minimum run time required to return oil to the compressor after startup.” The method it gives for establishing that run time uses a sample compressor fitted with a sight tube on a system built with the longest approved line set, which is a system development test rather than a service call measurement.

Copeland is equally direct about the other half of the cycle for those same compressors, and it attaches an exception to the answer: “Other than the considerations in the section on Brief Power Interruptions, there is no minimum off time because Copeland Scroll compressors start unloaded, even if the system has unbalanced pressures.” That statement covers the ZP*KB compressors the bulletin is written for, and the thermostat sits outside its scope, holding a delay of its own. Both are true at once. Neither one settles whether a system that restarts every ninety seconds is acceptable, because Copeland puts that answer back on the system configuration and the run time it takes to return the oil to the compressor.

This post covers cooling: an air conditioner, or a heat pump running in cooling. A gas furnace that fires, runs a few minutes, and drops out on its high limit is short cycling as well, and that diagnosis starts at the limit switch and the airflow across the heat exchanger rather than at the contactor.

Before you take a single pressure reading, write down five things: how long the compressor ran, how long it stayed off, whether the thermostat was still calling when it stopped, the outdoor temperature, and what the space was doing. The last one means the setpoint, the room temperature, and whether anyone had just changed either of them. Orr sorts the causes into safety controls, a loss of power, oversizing and low load, and the way the controls are set up. That five-item list is this post’s, not Orr’s, and those records are what separates his four groups once you are standing in front of the equipment.

Watch one full cycle before you touch anything

Do this on any short cycling call where the equipment is still running when you arrive. It costs you one cycle, it needs a meter and a watch, and it narrows the suspects before you spend money on anything else. If the system is off and staying off rather than cycling, go to the lockout paragraphs at the end of this section instead, because a system that will not restart at all is a different finding.

On a conventional split system, the thermostat’s cooling call reaches the outdoor unit on the Y wire at 24 volts. Craig Migliaccio at AC Service Tech gives the range for contactor coils, which “may be designed to be powered with 24-volts, 120-volts, or 240-volts,” and states that most contactors on outdoor air conditioning and heat pump units use a 24 volt coil. HVAC School describes what happens when the call arrives: “When a call for Y sends 24v to the contactor coil and energizes it, the electromagnet pulls the contacts in, creating a path for electricity to go out to the outdoor components.” The contactor is the relay that passes line voltage to the compressor and the condenser fan, and its coil runs on that 24 volt call.

The safety switches sit inside the call’s circuit rather than beside it, which is what makes the call worth measuring. HVAC School describes the path on a split system: “The Y circuit also goes in and out of the high-pressure and low-pressure switches before it even reaches the contactor coil. As with the float switch, if either of those pressure switches open, Y would break, and the system would not run.”

Migliaccio describes the low pressure switch’s half of that arrangement: “If the pressure is too low on the low side of the system, this switch will open the 24v electrical circuit going to either the contactor or the defrost board safety switch terminal.” Equipment with a control board or a compressor protection module between the thermostat and the contactor has more places for the call to stop, not fewer, so read the wiring diagram for the unit in front of you before you assume where the switches sit.

This reading takes an energized cabinet, so treat the whole cabinet as energized and leave it to a qualified person. OSHA allows the test because it is one you cannot run dead, and it limits who may run it: “Only qualified persons may work on electric circuit parts or equipment that have not been deenergized,” and those persons “shall be familiar with the proper use of special precautionary techniques, personal protective equipment, insulating and shielding materials, and insulated tools.” The control circuit is 24 volts, and the same cabinet has line voltage on the contactor’s load side and at the compressor terminals. Wear the shock and arc flash protective equipment your employer’s electrical safety program selects for this task, keep your free hand out of the cabinet, and do not let a probe bridge the contactor’s line side while you are reading its coil.

Meter across the contactor coil terminals while the compressor is running. Set the meter to AC volts and put one probe on each of the two coil terminals. You should read roughly 24 volts, and Migliaccio gives 24 to 29.5 volts as the range to expect on a 24 volt coil.

Then watch the meter at the second the compressor stops. Read the thermostat call separately, at the indoor unit’s Y terminal, because coil voltage and the call are two different measurements and the coil can lose voltage with the call still up. Four outcomes are worth recording.

The coil voltage disappears while the thermostat is still calling. Something between the thermostat and the coil opened, which is the safety string: a pressure switch, a condensate float switch, a control board, or a protection module.
The coil still reads 24 volts with the compressor stopped. Either something in the compressor’s power path is open or the compressor stopped on its own protector. Copeland’s Comfort Alert diagnostics module sends you down the power path first, listing “Check compressor supply voltage” and “Outdoor unit power disconnect is open” alongside “Check for high head pressure.” The contactor is one link in that path, and Migliaccio describes how it fails: “This melting causes the contacts not to touch properly when the contacts are supposed to be in the closed position.” Check whether the condenser fan stopped at the same moment, because on most residential units both motors run through the same contacts, and a fan that keeps turning while the compressor drops out points at the compressor rather than at the contactor.
The coil reads low or unstable rather than clean. A reading under the coil’s range points at a corroded connection, a loose spade terminal, or the transformer feeding the coil, and that reading belongs in your notes even while the compressor is still running.
The thermostat’s call goes away. The control side ended the cycle, either because the space satisfied or because the call itself is intermittent.

Find the open switch by measuring across it rather than by unwiring it. With the call still up and the compressor down, measure AC volts from one side of each safety switch to the other. A closed switch reads close to zero volts across it, because both of its terminals sit at the same potential. An open switch reads the full 24 volts, because the circuit ends there.

Work outward from the thermostat side, and the first switch reading 24 volts across it is the one that opened. Finding it is the middle of the job rather than the end of it, and the next question is whether the switch opened at its setting or failed open. Carrier’s service procedure settles that by comparing the two: read the system pressure, check the switch for continuity, and replace the switch when it sits open at a pressure that should hold it closed. A switch that tripped at its setting is reporting a condition that is still in the system.

A system that stays off rather than cycling may have latched. Orr describes the relay that does the latching: “The purpose of the lockout relay is to keep the compressor off when there is a significant fault, EVEN if the fault condition goes back to normal.” He is also clear about how often you will find that relay: “I know lockout relays aren’t COMPLETELY gone, but electronic controls have largely replaced the function of a lockout relay.” On equipment with a board or a protection module, the latch sits in the electronics instead, so read the wiring diagram and any stored fault before you go looking for a relay.

Orr gives the reason the relay exists, and that reason is this post’s subject: “a lockout relay is an old-school way to keep a compressor or other critical component ‘locked out’ so that it doesn’t ruin itself by slamming on and off when safeties open and close.”

Resetting it does not fix anything. Orr states how the reset works: “The lockout relay contacts will remain open until the power is cycled to the lockout relay coil, allowing the contacts to go back to the normally closed position.” A system you reset and then leave is a system you will drive back to.

The causes, ranked

1. The high pressure switch opening. This is a common safety trip on a cooling call in hot weather, and the run time getting shorter as the afternoon gets hotter is the pattern that gives it away. Migliaccio states what the switch is for: “Pressure switches are typically installed in heat pumps and air conditioning systems to protect the compressor from a high pressure situation.” In cooling, he ties it to the outdoor unit: “In the case of a heat pump in cooling mode, the high pressure switch is used to make sure that outdoor unit can reject heat with the outside air.”

Orr’s high head pressure causes are “Overcharge,” “Low condenser airflow; condensing fan not operating, dirty,” “High outdoor ambient temperature,” “Mixed/incorrect refrigerant or retrofit without proper markings,” “Non-condensables in the system,” and “Liquid line restriction + overcharge.” He marks the limits of that list on the same page: “This is an incomplete list designed to help you. Always keep your eyes and ears open for other possibilities.” A condenser fan that stops while the compressor keeps running produces the fastest version of this cycle, so watch the fan through a full run before you go further.

2. The low pressure switch or the loss of charge switch opening. Anything that drops suction pressure far enough will open it, and Orr’s low suction pressure causes, from that same incomplete list, are “Low on charge,” “Low airflow/load; dirty filter, dirty evaporator, kinked return,” “Metering device restricting flow too much; piston too small,” “Liquid line restriction; clogged filter/drier, clogged screen,” “Low ambient (low evaporator load),” “Extremely kinked suction line (after the kink),” and “Internal evaporator restriction.” A coil that has iced over belongs on that list as well, because ice blocks the air path and drives suction pressure down further.

Which switch you are looking at matters, because a switch on the liquid line is a different instrument from one on the suction line. Orr describes the loss of charge switch, which some residential heat pumps carry: it is “called a low-pressure switch (because it opens on fall in pressure),” and it “will only keep the system off in cases of very low charge or ‘loss of charge’ in cool mode.” A system with only that switch can run at a badly low charge without ever tripping, so do not read an unbroken circuit as proof the charge is right.

3. The compressor’s internal overload opening. The off time this one produces runs long, because the compressor motor has to cool before its protector closes again, and how long depends on what heated the motor. HVAC School separates the two ends of that range: a locked compressor will “go out on thermal overload very quickly (and come out relatively quickly),” while low suction pressure, low mass flow and a high compression ratio make it “a long time for the compressor to come out of thermal overload.” Read the off time as a hint rather than as the answer. Migliaccio describes the device: “If the motor has a hard time starting, the windings will get heated up, but this device will help protect them from overheating by shutting off the compressor when the temperature reaches a high limit.”

Confirm it with a resistance measurement, taken after the power is off and the compressor leads are disconnected. Power off at the disconnect is not the same as proven dead, and OSHA requires the proof: a qualified person “shall use test equipment to test the circuit elements and electrical parts of equipment to which employees will be exposed and shall verify that the circuit elements and equipment parts are deenergized.”

Migliaccio’s tripped-overload pattern is an open reading between common and run, an open reading between common and start, and a normal winding resistance between run and start. The pair that still reads through is the informative one, since the windings themselves are intact and the protector between them is what opened.

Cooling the compressor shortens the wait, and Migliaccio’s instruction on that comes first: “Be sure to have the power off when attempting to cool down the compressor with water.” Ty Branaman and JD Kelly at HVAC School give the line that separates a tripped protector from a failed one: “If the compressor has been properly cooled and the overload does not reset, the overload itself has failed.”

What made the motor hot is what you actually repair. High head pressure raises the current the compressor draws. So does a run capacitor that has fallen below its rated capacitance, and so does low voltage at the compressor terminals under load.

Measure the capacitor with the power off, verified off, and the capacitor discharged, because OSHA requires that “Capacitors shall be discharged and high capacitance elements shall be short-circuited and grounded, if the stored electric energy might endanger personnel.” Measuring the voltage while the compressor is trying to start and measuring the running current are both energized work, under the same qualification and protective equipment as the contactor coil reading. Compare that running current against the compressor manufacturer’s published performance data at the pressures and voltage you recorded rather than against the rated load amps stamped on the plate.

4. An intermittent call from the control side. Copeland’s Comfort Alert diagnostics module has a flash code for exactly this pattern. The code is labeled “Short Cycling,” its description is “Compressor is running only briefly,” and three troubleshooting entries print with it: “Thermostat demand signal is intermittent,” “Time delay relay or control board defective,” and “If high pressure switch present go to Flash Code 2 information.” That third entry is the one to keep in mind, because the module does not tell you on its own whether the control side or a pressure switch ended the run.

The field version is a loose or corroded low voltage terminal, a thermostat mounted where it reads something other than the room, a wire nicked by a screw, a control board, or a condensate float switch that opens as the pan fills and closes again as it drains. HVAC School describes what that float switch does either way: “We can configure the float switch to break R or Y when it trips,” where R is the wire feeding 24 volts from the transformer, and “If the float switch breaks Y instead, then power can’t access the contactor coil, meaning that it can’t pull in the contactor.”

A contactor that drops out and pulls back in repeatedly while the thermostat call sits steady points at the coil circuit, the transformer, or a loose connection ahead of it. Migliaccio gives one cause for that: “A corroded electrical connection at the spade terminal could result in low voltage on the contactor coil and the coil not closing the contacts.”

5. Capacity above the load. This is the cause where nothing on the equipment is broken, and it is the one customers argue with. Orr states the mechanism: “This can result in short run times when the load is low or when the equipment is oversized.” Mild weather produces it on correctly sized equipment, which is why a complaint that only appears in May or at night is worth checking against the calendar before you chase it.

The Building America Solution Center, a Department of Energy resource, gives the consequence a homeowner notices: “An oversized system will turn on and bring the air temperature down below the thermostat set point quickly then shut off before the system has had time to remove moisture from the air, which can cause high humidity problems in the home, especially in humid climates.” Equipment capacity is settled by a load calculation and an equipment selection procedure rather than by a rule of thumb about square feet, and telling a customer their system is too large is a conversation you should have with the load calculation in your hand.

The cycle pattern matrix

A row is a hypothesis, not a diagnosis. Record the run time, the off time, and the state of the thermostat call for at least two consecutive cycles before you use it, because a single cycle can be the tail end of something that already happened.

Run time	Off time	Thermostat still calling when it stops	Suspect	Confirm with
Short, and shorter as the day gets hotter	Minutes	Yes	The high pressure switch, on high head pressure	Head pressure right up to the trip, condenser coil cleanliness, condenser fan running through the whole cycle
Short, worse in mild weather or behind a loaded filter	Minutes	Yes	The low pressure or loss of charge switch	Suction pressure at the trip, superheat and subcooling, filter, evaporator coil, and total external static pressure
Seconds to a couple of minutes, often failing to start at all	Long, and it varies with what heated the motor	Yes	The compressor’s internal overload	Winding resistance after the compressor has cooled, run capacitor capacitance, voltage at the compressor under load
Seconds, at random, with the contactor chattering or dropping out	Seconds	Yes	The contactor coil circuit, a loose low voltage connection, or the transformer	Voltage at the contactor coil across a full cycle, and the coil terminals and spade connections
Varies, with no pattern anyone can state	Varies	No, the call itself flickers	An intermittent thermostat call, wiring, control board, or a float switch cycling with the pan	The minimum and maximum recording on a meter at the indoor unit’s Y terminal, the float switch, and the thermostat’s own display
Short, ending with the space satisfied	Until the space warms back up	No	Capacity above the load, or a thermostat reading something other than the room	Supply and return air temperatures, the thermostat’s deadband and cycles per hour settings, and where the thermostat is mounted
Never restarts	Indefinite	Yes, and the compressor never starts	A latched lockout relay or a manual reset safety that is open	Reading and recording the fault first, then correcting what caused it before you reset the latch, then checking that tools and jumpers are out and everyone is clear before power goes back on

What the extra starts cost the equipment

Frequent starts can pump oil out of the compressor, and the oil returns when the compressor runs long enough to stabilize the system. Bitzer, a compressor manufacturer, describes the sequence for its ORBIT compressors: “With frequent starts, a percentage of oil may be pumped out of the compressor. If the compressor operates for a sufficient time to stabilize the system, oil will return to the compressor.”

Copeland states the consequence of cutting the run short, for its ZP*KB scroll compressors: “Cycling the compressor for a shorter period than this, for instance to maintain very tight temperature control, will result in progressive loss of oil and damage to the compressor.” Orr’s list of the problems short cycling creates includes “Compressor oil loss and lubrication issues,” which is the consequence stated from the practitioner side without the mechanism behind it.

The loss accumulates cycle by cycle rather than all at once, and Bitzer continues the sequence: “If the operating time is very short, the oil may still be trapped in the system. If this cycle is repeated, the compressor will progressively pump out oil, resulting in insufficient oil in the sump to ensure reliable operation.” Bitzer also gives a limit for its ORBIT compressors: “Orbit compressors should be started max. 8 times per hour and remain switched on for a minimum operating time, see Operation Instructions ESB-130.” A system that has been cycling all season is a system whose oil return you have to check rather than one you can assume is still healthy.

Contactor and relay contacts wear at the rate they open and close. Orr is specific about the speed of it: “RAPID short-cycling can quickly cause contact and relay failure due to arcing and can be very damaging to motors.” Migliaccio describes the visible result on a contactor, where arcing melts the contact faces and leaves them unable to touch properly when they close. A contactor replaced without finding the cycling that burned it is a part you will replace again.

An oversized or short cycling system can satisfy the thermostat before it has dehumidified. Orr’s list gives the reason: “Poor dehumidification (it takes a while for the coil to cool down and dehumidify).” The Building America Solution Center describes the same effect from the house side, where an oversized system reaches the setpoint and stops before it has removed the moisture. A homeowner reporting that the house feels clammy even at the setpoint is describing a humidity problem, and short run times are one cause of it rather than the only one.

The same Building America pages list moisture entering the house from other directions, ventilation loads and the sources a homeowner controls: “Aquariums, large numbers of house plants, and hanging clothes to dry indoors all add significant amounts of moisture into the home.” They also state a limit on what cooling alone can do: “In hot-humid climates, the air conditioner alone may not be able to remove enough latent heat to keep relative humidity below 60%. In such cases, the HVAC designer should consider adding dehumidification.” A lower setpoint is not a substitute for finding the moisture, and neither is assuming the cycles are the whole story.

Branaman and Kelly state the rule that covers all of it: “Now that you’ve confirmed the compressor is bad, you must find out why it failed.” They give the consequence in the next sentence: “Installing a new compressor into a faulty system will only lead to another failure.”

Common mistakes

Watch a full cycle with a meter on the contactor coil, which is energized work for a qualified person, and record the coil voltage and the thermostat call as two separate readings when the compressor stops, because the coil reading on its own does not tell you whether the call is still up.
Record the run time, the off time, the outdoor temperature, and the setpoint for at least two consecutive cycles, since the pattern across cycles has more in it than any single cycle does.
Measure across each safety switch with the circuit energized and the call up when the reading cannot be taken dead, under that same qualification, so you find the switch that opened without unwiring a circuit you then have to prove you put back correctly.
Watch the condenser fan through the whole run, because a fan that stops while the compressor keeps going raises head pressure quickly and can produce a high pressure trip that looks like a refrigerant problem.
Cool the compressor with the power off when you are waiting on a tripped internal overload, which is the sequence Migliaccio publishes.
Check the switch against the pressure the system is actually running before you replace anything, since a switch that opened at its setting is reporting a fault you still have to find, and a switch sitting open at a pressure that should hold it closed is the fault.

Don’t

Replace a pressure switch because it opened, as an open switch on its own does not say whether the system tripped it or the switch failed, which is what comparing the pressure with the switch’s continuity is for.
Read a five minute pause as a fault when the thermostat shows it is holding the compressor off, because that is the delay Honeywell Home describes in the FocusPro P200 and S200 installation manual.
Reset a lockout relay and leave, since Orr describes the relay’s purpose as keeping the compressor off even after the fault condition returns to normal, and cycling power only clears the latch.
Add refrigerant because the system is cycling, since an overcharge raises head pressure toward the high pressure switch and makes the cycling worse when that switch is what is ending the run.
Apply a minimum run time you memorized, because Copeland states there is no set answer for the ZP*KB scroll compressors that bulletin covers and gives a system development test rather than a number.
Diagnose charge from pressures taken in the first minutes of a cycle, since the system has not reached steady state and a short cycling system may never get there.

Frequently asked questions

Why does my air conditioner turn on and off every few minutes?

Short run times have several causes, and they divide by whether the thermostat is still calling when the compressor stops. If the call is still up, something in the path between the thermostat and the compressor is open: a high pressure switch, a low pressure or loss of charge switch, a condensate float switch, a contactor, the disconnect or a fuse feeding the compressor, or the compressor’s internal overload. If the call goes away, the cause is on the control side or in the load: the thermostat, its wiring, its location, or equipment with more capacity than the space needs right then. Bryan Orr at HVAC School sorts the causes into safety controls, a loss of power, oversizing and low load, and the control setup.

Is it normal for the compressor to wait five minutes before restarting?

Yes, when the thermostat or the equipment control is set to hold the compressor off. The installation manual for the Honeywell Home FocusPro P200 and S200 states that “The compressor protection feature is engaged. Wait a few minutes for the system to safely restart to avoid damaging the compressor,” and gives five minutes as the wait in its troubleshooting section. That manual and the FocusPRO N100 manual both list the protection as a setting rather than a fixed behavior, off or one through five minutes, so read the thermostat instead of assuming the delay.

That delay applies after a normal shutdown, and it is not a fault. What is worth investigating is the length of the run time before the shutdown, not the pause after it.

Can low refrigerant cause short cycling?

It can, by dropping suction pressure to where the low pressure switch opens. Low charge is the first entry on Orr’s low suction pressure list, alongside low airflow and load, a metering device restricting flow, a liquid line restriction, and low ambient temperature. Not every residential system will trip on a moderate undercharge, though. Orr describes a loss of charge switch used on some heat pumps as one that “will only keep the system off in cases of very low charge or ‘loss of charge’ in cool mode,” so a system that never trips is not proof the charge is right.

Does an oversized air conditioner short cycle?

Yes, and nothing is broken when it does. Orr states that short run times result “when the load is low or when the equipment is oversized.” The Building America Solution Center, a Department of Energy resource, describes the consequence: an oversized system reaches the setpoint and shuts off before it has removed moisture from the air, which produces high humidity indoors, especially in humid climates. Rule out the safety trips and the control faults first, because mild weather produces the same short run times on correctly sized equipment.

Ruling those out does not confirm oversizing by itself. The Building America Solution Center directs the contractor to a load calculation for the home’s cooling load and an equipment selection procedure for the equipment, and comparing the two is how you confirm it.

Glossary

AC Service Tech: a practitioner training organization, cited here on pressure switches, contactors, and compressor overload testing.
Bitzer: a compressor manufacturer, cited here on oil loss from frequent starts and its ORBIT compressor start limit.
Building America Solution Center: a Department of Energy building science resource, cited here on oversizing and indoor humidity.
Comfort Alert: Copeland’s compressor diagnostics module, which reports faults as flash codes on a status light.
Contactor: the relay in the outdoor unit that passes line voltage to the compressor and condenser fan when its control coil is energized, most often a 24 volt coil on residential equipment.
Copeland: a compressor manufacturer, cited here on run time, off time, and oil return.
CPH: cycles per hour, the ceiling a thermostat holds on how many times it starts the equipment in one hour.
Deadband: the gap between the temperature that starts a thermostat’s call and the temperature that ends it.
Float switch: the condensate safety switch that opens the control circuit when the drain pan fills.
Head pressure: the discharge side pressure the compressor pumps against, which the high pressure switch watches.
High pressure switch: the safety switch that opens the control circuit when discharge pressure rises past its setting.
HVAC School: a practitioner training and reference organization cited here.
Internal overload: the protector inside a compressor that opens on winding temperature and current, and closes again once the motor cools.
Lockout relay: the relay that keeps a compressor off after a safety opens, until someone cycles power to its coil or trips a reset switch wired into that coil circuit.
Loss of charge switch: a low pressure switch, often on the liquid line, that opens only at very low charge in cooling.
Low pressure switch: the safety switch that opens the control circuit when suction pressure falls past its setting.
Rated load amps: the current stamped on the compressor’s plate, which is not the current the manufacturer’s performance data predicts at measured pressures and voltage.
Short cycling: equipment running and stopping more often than the design intends.
Subcooling: how far the liquid refrigerant sits below its condensing saturation temperature.
Superheat: how far the refrigerant vapor sits above its evaporating saturation temperature.
Total external static pressure: the pressure the blower works against outside the equipment, measured with a manometer at locations that depend on the configuration, typically just after the filter on the return side and just after the fan on a fan coil, or between the heat exchanger and the indoor coil on a furnace.
Y: the thermostat terminal and wire that carries the 24 volt cooling call into the equipment, through whatever safeties and boards sit in the path, and on to the contactor coil.

Drafted with AI assistance and reviewed by the author.

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