Control specifications

By Grant Laidlaw
We take a look at control specifications and how to compile these.

Liam asks: Hi Grant. We have been asked to put together a control specification. We do not have much experience in this aspect and it does seem to be very complex. What is a control specification and what does this involve? Any input would be appreciated.

Hi Liam, this is a fairly broad question and I will start by saying that a detailed knowledge of the operation of an air-conditioning system/plant is essential.

There are of course many different air-conditioning systems in use today and each demands a different control system. It is therefore not the purpose of this article to discuss all the different possible control systems, but rather to help the readers develop an understanding of controls and control systems and to indicate that many controls interact with each other within the specification. Neglect of any part of the control system may cause a partial or complete loss of control.

The control system specification is crucial and cannot be understated. Without a detailed control specification, the controls supplier cannot quote competitively, the plant designer cannot design adequately, the commissioning technician cannot commission, and finally, the maintenance team may not be able to diagnose system faults.

I surmise that these form some of the reasons as to why you have received a request for a control specification.

A poorly-written control specification can lead to dissatisfaction with the plant’s operation and a bad name for yourself and for the air-conditioning industry.

Please keep in mind that control systems must be kept as simple as possible at all times. A complicated system will very often result in uneconomic, unsafe, and inefficient operation.

It is only after an accurate specification of the control system is written that the correct control instruments and control system can be selected. It is also very useful in selecting the heating and cooling equipment.

It must be as complete and detailed as possible and must describe what the plant must do under all operating conditions, including faulty operation.
The following specification is a sample for a fairly typical constant volume plant. It is not necessarily the simplest or best solution, but rather includes a variety of control variations.

Included is the method of starting the plant, energising the control system, and the methods of protecting the plant and the premises.

Sample specification

For this to make sense, I will need to include a description of a plant. In this instance, let us use a plant that is a constant volume variable temperature system providing air conditioning to a store and adjacent offices. Cooling is to be provided by means of a chilled water coil.

Heating is by means of electric heaters.

The administrative offices are to be supplied with conditioned air from the plant. The minimum temperature in these offices is to be controlled by means of re-heaters in the supply air ducts to these offices.

The temperature and the humidity are to be controlled from sensors installed in the return air duct.

The temperature of the supply air must not be lower than 12°C, while the relative humidity of the supply air is not to exceed 80% while humidification is taking place. Humidification shall be by means of a steam type evaporative humidifier.

The outside air introduced must be closed off during pre-cool and pre-heat periods; that is, outside shopping hours. Indication equipment to indicate what plant is in operation or tripped, as well as instrumentation to check the plant for maintenance purposes, must be included. The usual interlocks and safeties must also be provided.

Conditions to be maintained

  • Temperature: 22°C ± 2K
  • Humidity: 50% ± 5%
  • Minimum supply air temperature: 12°C.
  • Maximum supply air humidity: 80% when humidification is taking place.

Plant operating hours

  • Monday to Sunday, 07:00 to 17:00
  • Shop hours: 08:00 to 17:00

Starting and stopping the plant

The plant shall be started and stopped automatically by means of a timing device.

The time clock shall have a weekend feature as well as a 60-hour reserve to allow for power failures. The plant shall start one hour prior to shop opening hours in order to pre-cool or pre-heat the conditioned spaces.

During the pre-cool or pre-heat period, the outside air damper shall be kept closed.

A selector switch with auto, off, and manual positions shall be provided. In the ‘off’ position, the plant will remain off under all circumstances. In the ‘automatic’ position, the plant shall be started and stopped by means of the timing device. In the ‘manual’ position, the plant shall remain on. Upon stopping the plant, all equipment shall cease operating immediately, except for the supply air fan, which shall be kept running for five minutes to cool the electric heater elements and/or to reduce the humidity in the supply air system (fan run-out).

Energising of control system

The control system shall be energised when the supply air fan is started and de-energised when it is stopped.

Temperature control

Main shopping area: The cooling and heating shall be controlled by a control system having a single temperature sensor. This sensor is to be positioned adjacent to the return air grille at a height of 1.6m from floor level.

No set point adjustment shall be provided at the sensor. On a rise in temperature, the control system shall open the chilled water valve in proportion to the deviation, provided that when the supply air temperature reaches a minimum of 12°C, the valve shall not open further.

On a drop in temperature, the control system shall activate the heaters in four equal steps. A minimum of 30 seconds shall elapse between the switching of the steps.

The electric heaters shall not operate until an air velocity equivalent to 80% of the design air velocity in the main supply air duct has been established. This shall be done by interlocking the heater contactors with the supply air fan starter and via an airflow switch installed in the main supply duct.

An overheat stat shall switch the heaters off when the temperature of the air leaving the heaters exceeds 75°C. (The overheat stat shall be provided with manual reset.)

The temperature controls shall be so adjusted that heating and cooling cannot operate simultaneously (except for dehumidification as specified later).
A dead zone of 1K shall be provided between heating and cooling.

Offices: Each office shall be provided with a heater capable of heating the design supply air quantity to the individual offices by 8°C.

These heaters are to be individually controlled by a two-step electric thermostat positioned in a representative place in the various offices.

These heaters shall also be interlocked with the main supply air fan as well as being provided with a manual reset overheat stat set at 75°C.

Set point adjustment shall be remote. A secondary adjustment with a maximum span of about 2K is to be provided at the room sensor.

Humidity control

The humidity sensor shall be positioned adjacent to the temperature sensor. No set point adjustment shall be provided at the humidity sensor.

On a rise in humidity, the humidity control system shall override the temperature controls and open the chilled water valve in proportion to the humidity deviation.

Upon a temperature drop due to the dehumidification action, the temperature control shall activate the heaters to provide reheating.

On a drop in relative humidity, the control system shall modulate the capacity of the dry steam-type humidifier in proportion to the deviation, provided that the relative humidity, two metres downstream of the humidifier, shall not exceed 80%.

The humidifier shall also be interlocked with the supply air fan and shall not operate when this fan is off or when the air velocity in the supply air duct drops to below 80% of the design air velocity. The relative humidity in the adjacent offices shall not be controlled.

Outside air damper control

The outside air dampers shall be closed outside shopping hours.

During shopping hours, the dampers shall be opened to a position that can be set by means of a potentiometer on the remote control board. This potentiometer is to be marked ‘Outside air damper, percentage open’. A minimum of 10% outside air shall be maintained.

Indicator lights

The following indicator lights shall be provided:

  • Fan on (green)
  • Fan run out (orange)
  • Fan tripped (red)
  • Heaters on [one light per step] (orange)
  • Overheat stat tripped (red)
  • Humidifier on (green)
  • Humidifier ‘tripped’ (red)
  • Outside air closed (red)
  • Filter dirty (red).


  • Temperatures: outside air, return air chilled water supply, chilled water return
  • Humidity: supply air, return air
  • Pressure: filter pressure loss
  • Amperage: supply air fan, humidifier, heaters
  • Outside air damper position
  • Supply air fan: Run/off
  • Electric heaters: On/off
  • Humidifier: On/off
  • Re-heaters: On/off
  • Temperature control point
  • Humidity control point.

Liam, this forms the basis of a simple control specification to give you a starting point and some direction. From here you will need to develop your specification. As you can see, all aspects of the control of the plant must be included as well as all safety aspects and emergency situations. This constitutes an important aspect that needs thorough attention. I hope this helps you with the development of your control specification.

Thank you for all your questions. Send your problems (and sometimes your creative solutions) to with “Solutions Page” in the subject line. You may include pictures.

SANS 10147

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