Can you afford not to do this?

Although the introduction of outside air into an occupied space is directly at odds with the requirements of sustainability, minimisation of energy usage and the permeability requirements, we all recognise that in order to dilute the polluting effects of occupancy, outside air does need to be introduced.

CIBSE Guide F asks us to match provision of ventilation and air conditioning to actual demands in the building, but we all know that demands will not only vary between design and use, but also in use. We also know through personal experience we are fundamentally lazy when it comes to the control of our personal environment. So what is required is an automatic system that modulates against demand, that can be installed at low cost and that meets the other guide requirements; servicing to occupancy pattern and activity in each zone, matching servicing to varying occupancy levels in a zone, and keeping mechanical ventilation rates to a minimum commensurate with acceptable levels of indoor air quality.

It should also be noted that humans are more productive in a well-ventilated area; our cognitive powers degrade as air quality levels decrease so there is a hidden economic advantage to employers to keep buildings ventilated properly. Olesen, at the Technical University of Denmark suggests up to 10% productivity increases which are a very short payback for a retrofit.

CO2 as a tracer

The proposal to use CO2 (Carbon Dioxide) as a tracer gas for indication of human bio-effluent output is well documented. This should not be confused with the CO2 emissions, although it should be noted the use of this methodology will reduce energy consumption against a constant supply scenario, and hence reduce emissions. The principles can be adopted at building or zone level, satisfying some of the requirements of Guide F.

Firstly let us benchmark some of the values. Outside air in an urban environment is generally at concentration levels between 400 and 450ppm (parts per million). The current global average is 380ppm. On average humans feel a difference of approximately 600ppm, but like other aspects of human physiology there are no specifics, only generalisations, perceptions, and averages. So on average, if we walk in to a room that is more than 600ppm above outside, we will feel it to be stuffy. However, we will not detect a change in value if it rises above that threshold whilst we are in occupation but our levels of concentration will still suffer.

Throughout Europe, ventilation regulation and legislation is being introduced as we seal our buildings more thoroughly. In the UK, Regulation 6 of the Workplace (Health, Safety and Welfare) Regulations states that: “Effective and suitable provision shall be made to ensure that every workplace is ventilated by a sufficient quantity of fresh or purified air”. The guidance recommends that: “Fresh air supply rates should not normally fall below 5 to 8 litres per second, per occupant.” In addition to these Regulations, the HSE occupational exposure limit for carbon dioxide in EH40 is 5000ppm. However, a more realistic recommendation has been published by Building Services Research and Information Association (BSRIA) of 800ppm over an eight hour time weighted average.

Ventilation in UK School Buildings – Bulletin 101, now an approved document as a means of compliance to the Building regulations states: “The average concentration of carbon dioxide should not exceed 1500ppm.” Generally, control levels should be set to try and maintain 1000ppm (equivalent to a dilution rate of approx 10 litres per adult per second) above which people begin to experience problems with lethargy and headaches, and the view is backed by much independent research such as that done by Myhrvold, Olsen and Lauridsen in1996.

Ventilation control

The Design Ventilation Rate which is calculated by multiplying the maximum occupancy of the relevant space by the recommended litres per second per person outside air ventilation rate for that particular application, would normally be provided continuously to the space during occupied hours. The CIBSE guides suggest the designer should modulate ventilation rates below the calculated Design Ventilation Rate if occupancy is variable or intermittent.

Occupancy based ventilation can ensure that recommended ventilation rates are maintained, yet energy savings can be derived from reducing over ventilation resulting from continuous ventilation at some arbitrary ventilation rate based on assumptions of maximum occupancy.

Enter CO2 based Demand Controlled Ventilation. The production of CO2 by people involved in the same level of metabolic activity (e.g. office work) is very similar and typically within about 15% for adults aged 16 to 65. (Children will have a different production rate of CO2 based on their size).

Simply put, doubling the number of people in a space, doubles the amount of CO2 production. As each person enters or

exits a room the CO2 production rate will change in a known and predictable increment. This may be combined with the fact that outside levels of CO2 in most urban environments remain at a fairly stable 350 to 450 range.

As a result, an indoor CO2 concentration can provide an indication of how much outside air (at a fixed and low CO2 concentration) is being used to dilute the production of CO2 by people. Demand Controlled Ventilation employs a control strategy that can take minute-by-minute CO2 levels and modulate outside air ventilation to ensure that a targeted outside air dilution rate is maintained at all times.

Making CO2 control work

It is not enough just to control and react to CO2 levels. A control strategy must be used that ensures the delivery of ventilation is responsive to changes in occupancy within the lead and lag times, and any building pressure and distribution requirements continue to be met.

During occupied hours, if it is known there is to be a level of building generated contaminants, it is recommended that some sort of base ventilation rate occur to ensure that general non-occupant related contaminants are controlled. Practice in applying DCV control systems suggests that a good rule of thumb is to provide a base rate 15% to 30% of the calculated Design Ventilation Rate.

Dilution via natural or mechanical ventilation is the only acceptable approach to be used with CO2 DCV. This means that the interpretation of any standard will not allow special systems that just filter out CO2. This is because CO2 is not being used as an indicator of air quality; but as an indicator of occupancy and the ventilation rate of the space.

Mixed gas or so-called VOC sensors cannot be applied using this interpretation because they are non-specific and react to very general changes of gases in the space. They should never be used to proportionally control ventilation as a CO2 sensor might be used for occupancy.


A carefully designed CO2 control strategy can maintain ventilation rates at all times. Even if a space is not intermittently occupied, CO2 control can ensure that a space is not over or under-ventilated for current occupancy levels. It is the only simple method of ensuring that fresh air is actually distributed to spaces in proportion to their occupancy.

CO2 control does not make a distinction between fresh air resulting from infiltration or mechanical ventilation. If an open window or leaky construction is providing plenty of fresh air, CO2 control will reduce mechanical ventilation proportionally, again providing opportunity for energy savings.

A CO2 control system operated in conjunction with a building energy management system can record and store CO2 concentrations over time. This can provide the owner with a documented record of building ventilation that can be used to troubleshoot or verify optimum
building ventilation levels.

While a CO2 system can be used to maintain ventilation rates recommended to the British Standard or ANSI/ASHRAE Standard, a control strategy can be selected to allow ventilation rates ranging from 5 litres per second per person and up.

Demand Controlled Ventilation using CO2 is not an indoor air quality cure-all. However, when applied as part of a conventional building control system, it can save energy and maintain regulatory recommended ventilation rates at all times. All types of spaces are good candidates for CO2 DCV including school classrooms (often under-ventilated) and office spaces (often over designed and over ventilated).

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