How to Solve Short Cycling in Computer and Server Room Air Conditioners

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As summer temperatures in the UK continue to rise, with 2019 seeing a record breaking 38.7C in Cambridge, how well will your server room air conditioners or datacentre cooling systems cope with a heat wave this year? Not only are summer temperatures rising. Many IT installations post Brexit, are also looking to deploy ever more powerful servers and storage devices to expand their facilities and prepare for expansion, 5G and Edge computing.

Your existing air conditioning systems may be unable to cope with new IT power demands and/or rising summer temperatures. Older AC units are less energy efficient and your installation may be prone to issues such as short cycling. Now is the time to review how you cool your server room or datacentre environment and decide whether to retrofit additional air conditioning capacity or upgrade to a more suitably sized and energy efficient AC unit. There are several types of air conditioner available but first it may be useful to explain how air conditioning works and provides cooling to your critical IT systems.

How Server Room Air Conditioners Work

Air conditioners provide temperature-controlled air flow and can generally provide both cooling and heating functions. The process relies on two Laws of Physics.

  • Combined Gas Law: which defines the relationship between pressure and temperature and states that as you heat up a gas, its pressure increases (e.g. rubber tyres heat up as they are pumped with air) and when you depressurise a gas, its temperature falls (e.g. an aerosol can cools as pressure inside is released as the nozzle is depressed).
  • 2nd Law of Thermodynamics: which states that heat naturally flows from hotter to colder bodies. Heat can only be transferred from a colder to a warmer body through the application of external work.

For reference the Combined Law combines:

  • Boyle’s Law: which states that the pressure-volume product is constant.
  • Charles’s Law: which shows that the volume is proportional to the absolute temperature.
  • Gay-Lussac’s Law: which states that pressure is proportional to the absolute temperature.

Sever room air conditioning units are generally of the split system type and consist of an indoor unit and an outdoor condenser unit. The indoor unit may be wall mount, ceiling mount or ceiling suspended or floor standing. The indoor unit extracts heat from the ‘local’ air and for this reason is often described as a ‘heat pump’. The heat is transferred to the outside air via the condenser unit and in between will be connecting pipework.

Air Conditioner Refrigeration Cycles

In terms of the physics, the air conditioners pressurise and depressurise their refrigerant gas to absorb or release heat and so decrease or increase the temperature of the air they supply into the room. This is a thermodynamic cycle often referred to as the ‘refrigeration cycle’ and is the same process whether the air conditioner is cooling an office, server room or datacentre environment.

Most server room air conditioners are air-source systems which refers to the place where the heat is expelled via the condenser unit i.e. the outside air. Other types of heat pump include ground-sourced heat pumps and water-source heat-pumps and these are principally used for heating a building rather than cooling a specific area.

How the Refrigeration Cycle Works

The refrigeration cycle is one that almost everyone relies on in their home, whether they have a small fridge or larger fridge/freezer combination. The same process applies to office and server room air conditioners and air-source heat pumps.

  1. Inner Coils and Cold Refrigerant: part of an air conditioner is known as the evaporator coil or evaporator core. This is the part of the system where the extremely cold refrigerant absorbs heat. The evaporator coil is inside or near to the air handler where the blower fan is and is the source of the cold air ‘blown’ into a room. As air flows over the cold coils, heat from the air is transferred to the extremely cold refrigerant inside and the air temperature typically drops by around 20degC. This part of the process follows the 2nd Law of Thermodynamics which says that heat naturally flows from a warmer body to a cooler body. As the refrigerant absorbs heat, it changes from a liquid to a vapour. The warmer refrigerant is then transferred to the compressor for the second stage of the refrigeration cycle.
  2. Outer Coils and Compressed Refrigerant Vapour: the compressor is normally located within the external condenser unit and increases the pressure and temperature of the refrigerant. The refrigerant needs to be warmer than the outside air temperature to satisfy the 2nd Law of Thermodynamics, that heat flows from warmer to cooler bodies and in this instance to the outside air. The cooler the outside air, the more energy is used by the compressor to raise the refrigerant pressure. When the refrigerant temperature is higher than that of the outside air, it flows into the condenser coils.
  3. Heat Transfer to The Outside Air: once the refrigerant is pressurised sufficiently it will be hotter than the outdoor air. A condenser fan blows the hot outdoor air over the condenser coils and heat is removed from the refrigerant. This again reflect the 2nd Law of Thermodynamics. As the refrigerant loses thermal energy, it condenses back into a liquid form and is pumped back to the indoor unit coils.
  4. Cooling the Refrigerant: the refrigerant leaving the condenser must be cooled further in order to absorb heat from the indoor air. An air-source split-system normally uses a thermostatic expansion valve to depressurise the refrigerant, causing it to expand and its temperature to drop further. The cooler refrigerant then flows back into the evaporator coils inside the indoor unit.
  5. Air Handler: the air handler blows cold air into the room and the refrigeration cycle starts again. Cold air falls and warm air rises. For critical applications like a server room, it is therefore important to ensure that cool air is supplied into the front of a server rack and the warm air is allowed to rise and follow a return path to the indoor unit for collection and return to the external condenser.

The average refrigeration cycle lasts around 20 minutes but sometimes the process can be interrupted, and the air conditioner suffer from what is known as ‘short cycling’. In this situation, the air conditioner compressor turns ‘on’ and ‘off’ in short bursts and the cooling (or heating process) is interrupted.

Air Conditioner Short Cycling Causes

Short cycling is normally not an issue for a correctly sized and maintained air conditioner. The problem can have several causes:

  • Dirty Air Filters: air filters help to clean the air that circulates within the air conditioning system and are a consumable item that should be regularly cleaned and replaced. The purpose of an air filter is to provide protection from the air-borne bacteria, dirt, dust, mould, metal particles and pollen. Over time these will build-up on the air filter leading to a ‘clogging’ effect, restricting air flow and causing the coils to freeze. Dirty air filters also cause the air conditioning system to work harder, leading to high energy usage and running costs.
  • Refrigerant Leaks*: refrigerant can leak from cracks and poor joints within the system. If the refrigerant is too low, the compressor will shut-off to protect itself. A low refrigerant will still react to heat and pressure and so expand, potentially enough for the compressor to start-up, leading to a cycling effect.
  • Oversized Air Conditioning System: an air conditioner must be correctly sized for the cooling load and room size. Oversizing can lead to the room cooling too quickly and unevenly, resulting in short-cycling and temperature swings within the room. This situation can occur for example if an air conditioner is placed within a small room or cupboard with a single server rack cabinet. The room cools quickly but not consistently. To assist air flow and remove the problem of short-cycling it may be necessary to put ventilation slots into the room door, which may seem counter-intuitive.
  • Thermostat Location: air conditioning systems are cybernetic thermostatically controlled systems. Most thermostats are placed in a central location away from air vents, doors and windows to allow for even-room temperature readings. If the thermostat is too close to the air conditioner, short cycling will occur. The reverse can occur if the thermostat is located at the rear of a server cabinet i.e. a place heated by warm exhaust air and therefore forcing the air conditioner to work harder.
  • Faulty or Damaged Thermostat: the thermostat itself may be faulty or damaged and should be considered for replacement. It is unlikely that a thermostat will go out of calibration for any other reason. The installation of additional server room temperature monitoring can provide baseline measurements against which to test whether the air conditioning thermostat is working correctly.
  • Airflow and Coolant Flow: blockages within system components can lead to cooling/heating issues and short-cycling. Pipes and coils can become blocked through dirt, particulates, bacteria and mould.

The list provides a summary of the main causing of air conditioner short cycling but is not exhaustive. For some sites further onsite investigation may be required to determine the cause and apply a corrective action.

Note* under the F-Gas Regulations 2006 it is mandatory for air conditioning systems to be checked at least annually if they hold 3kg or more of HFC refrigerant. For more information see: https://acrib.org.uk/help-and-advice/legislation-overviews and http://www.gluckmanconsulting.com/wp-content/uploads/2014/12/IS-5-Stationary-Air-Conditioning-and-Heat-Pumps.pdf.

Types of Server Room Air Conditioning Systems

Our cooling team often comes across five types of split-system air conditioner in IT, computer and server room type environments. These include:

  • Wall Mounted: this is probably the most installed type of air conditioner within a server or computer room and the most efficient when there are two or more racks to cool and/or a larger IT floor space. The systems can be installed in a parallel arrangement to provide redundancy and standby-duty cycling and share an outside condenser. The wall mounted arrangement is generally more energy and air flow efficient, allowing cooled air to drop down to the front of the server racks and return path from the rear of the racks to rise up and via the ceiling to the air conditioner.
  • Ceiling Suspended: this type of air conditioner is more suited to general office and buildings including restaurants and hotels. Within a computing environment, the air flow is not as efficient as a wall mounted AC unit even though the unit has air handler louvres to control direction. Potentially there is also the risk of an overhead ceiling suspended air conditioner freezing and leaking moisture into the top of server racks.
  • Floor Standing: may be more appropriate for larger server rooms and datacentres where the unit is used as a Computer Room Air Conditioner (CRAC) unit. Floor standing units can be useful when there are ‘hot-spots’ within a computer or server room that cannot be served cool air from a ceiling suspended unit i.e. in an L-shaped room.
  • Free Cooling: here the wall mounted, ceiling suspended or floor standing AC unit cooling is supported by a free cooling outdoor unit during colder months. This allows the refrigerant-base cooling system to operate at a reduce capacity as and when required, saving energy and ‘wear and tear’ on components including the compressor unit.
  • Concealed AC units: to deliver cold air to where it is needed most within the server or computer room, down the side of server racks and where possible through under floor areas.
  • Portable: portable AC units can be useful when it is necessary to quickly deploy an emergency cooling system. Either the main air conditioning system has failed, it is under maintenance or repair, extra cooling is required (during a heatwave) and/or there is no AC unit installed. The issues we have with this type of system is its capacity which is generally low, and their inefficiency which arises from the fact that they are portable. The system will have a flexible hose which will need to be attached to a window or vent for hot air to be released to the outside. There may also be a condensation tray for collecting water vapour that will require regular emptying.


Whilst existing server room air conditioners may have been able to cope with their cooling loads, it is surprising how many are not considered as critical infrastructure devices. Our cooling projects team often come across many AC units that are more than 5 years old and either poorly maintained (if at all) or poorly specified and installed in the first place.

Some cooling issues can be easily resolved during a service or maintenance visit and with widely available spares and manufacturer certified engineers, our cooling team can and does support several different AC unit manufacturer brands. For other issues, a complete system replacement is required and with rapid advances in AC unit technologies and energy efficiency, this can lead to not only reduced running costs, but a more resilient and future-proof air conditioning system.

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