The high pressure alarm is the chiller fault we meet most often in summer. It usually appears on the hottest day, at the highest load, and leaves the facility without cooling exactly when it is needed. The alarm is not a fault but a protection: it protects the compressor and the refrigeration circuit from excess pressure. The real job is to find what is raising the pressure.
What is a high pressure alarm?
The high pressure alarm is a protection that activates when the refrigerant pressure at the compressor outlet and in the condenser exceeds the safety limit set by the manufacturer. When the pressure sensor or mechanical pressure switch detects the limit, it stops the compressor. Some units try to keep the pressure under control by reducing capacity before the alarm.
The limit value changes with the refrigerant. In units using R410A and R32, operating pressures are noticeably higher than with refrigerants like R134a and R513A; so the pressure figure is evaluated not on its own but together with the refrigerant's saturation temperature. In practice the value to look at is the condensing temperature: in an air-cooled unit this temperature is usually 10-20 °C above the outdoor air. If the difference goes far above that, there is a problem with heat rejection.
6 causes of high pressure
01Dirty or clogged condenser
A dirty condenser is the most common cause of a high pressure alarm. In air-cooled units, dust, pollen, poplar fluff, and oily dirt stuck to the coil fins cut off airflow. In water-cooled units, scale and sediment building up in the condenser tubes reduce heat transfer. In both cases heat cannot be rejected and pressure rises.
02Fan failure or low condenser water flow
A non-working condenser fan directly reduces the heat rejection capacity of the circuit it serves. A burnt motor, a faulty capacitor, a contactor, or a speed control device are the most common causes. In water-cooled systems the equivalent is low condenser water flow or a tower fan that is not running: a closed valve, a clogged strainer, or a faulty pump prevents the water from circulating enough.
03Excess refrigerant charge
When more refrigerant than needed is charged into the system, the excess liquid collects in the condenser and reduces the surface available for condensing. This is usually seen in units charged by estimate after a repair. The sign is a higher than normal subcooling value together with high pressure.
04Air or non-condensable gas in the system
Non-condensable gases are gases that do not liquefy in the condenser, such as air or nitrogen that got into the circuit without vacuum. They collect in the upper part of the condenser, raise pressure, and lower efficiency. They usually appear in systems put back into service with insufficient vacuum after a repair; the solution is recovering the refrigerant, vacuum, and recharging.
05Extreme outdoor temperature and hot air recirculation
When outdoor air temperature approaches the unit's design limit, condensing pressure naturally rises. What makes the problem worse is often the placement: units set too close to a wall, enclosed with panels, or placed too close to each other draw their own discharged hot air back in. This hot air recirculation can make the condenser inlet air several degrees hotter than the outdoor air.
06Faulty sensor, pressure switch, or high load
Not every alarm is a real pressure rise. A pressure sensor that has drifted out of calibration, a loose connection, or a faulty pressure switch can generate false alarms. In addition, a chilled water return temperature far above the design, meaning the unit is loaded beyond its capacity, also raises pressure. In diagnosis, sensor values are compared against an independent manifold measurement.
Does the high pressure alarm keep repeating?
Our team finds the cause by measuring pressure, superheat, subcooling, and fans on site, and recommends a permanent solution.
What can facility staff safely check?
The safe checks facility staff can do when an alarm comes are limited, but they speed up diagnosis a lot:
- Noting the alarm history on the unit's display, its time, and the outdoor temperature at that moment
- Looking from outside to see whether the fans are turning
- Checking whether the condenser coil is visibly dirty or blocked
- Checking whether there are materials, tarps, or obstacles cutting off airflow around the unit
- In water-cooled systems, verifying that the tower fan and the condenser pump are running
Opening the electrical panel, working on fan connections, attaching a gauge to the refrigerant circuit, or changing pressure settings is the job of a qualified technician. Washing the coil with pressurized water can also damage the fins; cleaning should be done with the right chemical and pressure.
Resetting the alarm again and again without finding the cause is the most common mistake. After every reset the compressor runs again at high pressure, discharge temperature rises, and the oil degrades. If this goes on, the problem can turn into a compressor failure; we described the first signs in our article on compressor failure signs.
How does a technician diagnose it?
The technician first measures the discharge and suction pressures, the superheat and subcooling values, and the condenser inlet and outlet temperatures. When these values are read together, it becomes clear whether the cause is in heat rejection, in the refrigerant charge, or on the control side. For example, high pressure with high subcooling points to overcharge, while high pressure with normal subcooling often points to a dirty condenser or a fan problem.
Next, the fan motor currents, capacitors, and speed control device are tested; in water-cooled systems the condenser water flow and the inlet-outlet temperature difference are checked. If the refrigerant charge is to be corrected, the refrigerant is weighed into the system. Refrigerant work must be done by certified personnel in accordance with the F-gas regulation.
Preventing the high pressure alarm
Most of these alarms are prevented by maintenance done before summer starts. Cleaning the condenser coils before the season, checking fan motors and capacitors, cleaning the condenser tubes in water-cooled systems, and regular water treatment are the most effective measures. Panels added later, tarps, or stored materials that cut off airflow around the unit should also be removed at the start of the season.
Regular recording is also an important measure. If the condensing temperature measured at each maintenance visit at moments close to the same outdoor temperature and load conditions is recorded, the condenser getting dirty or a fan weakening can be seen weeks before an alarm comes. In chillers connected to a building automation system, these values can be monitored automatically and a warning given when a threshold is exceeded. For the tower side, see our cooling tower maintenance article.
At YOİ we provide chiller fault and maintenance services on all brands. If the high pressure alarm keeps repeating, our team finds the cause by measurement and reports the solution and cost in writing. For details you can look at our chiller service page.
Frequently asked questions
Why is the chiller high pressure alarm more common in summer?
As outdoor air temperature rises, condensing pressure naturally rises. Problems that go unnoticed in winter, such as a dirty condenser or a weak fan, push pressure above the safety limit on hot days.
Is it a problem to reset the high pressure alarm and run the unit?
Repeated resets without finding the cause make the compressor run at high discharge temperature and degrade the oil. If the alarm repeats after one reset, the unit should be taken for a service check.
Can we wash the condenser coil ourselves?
Not recommended. Pressurized water can crush the fins, the wrong chemical can corrode the aluminum, and water can get into electrical components. Cleaning should be done by a qualified technician with the unit's power cut.
Is there a link between high pressure and compressor failure?
Yes. A compressor that runs continuously at high pressure has a rising discharge temperature, its oil degrades, and bearing wear speeds up. In the long run this is one of the most common causes of compressor failure.
Related calculators
Detailed guide: Chiller maintenance: intervals, checklist, and maintenance form
A4 form with checklist, measurement table, and signature field.Download PDFView the form