Are you visiting from outside your region? Visit your regional site for more relevant services and pricing.

Technical answers - ground coupling, historic buildings and escape distancesFebruary 2010

'Load Calculation Applications Manual' is now available to members from the BSRIA Library.

David Bleicher provides the answers to tricky technical questions posed by BSRIA Members. Problems solved this month include Building Regulations, escape routes and ripe bananas. 

Ground-coupling performance

Q. I'm thinking of specifying a ground-coupled ventilation system. What performance can I expect?

Ground-coupled air systems are primarily used for tempering outdoor air prior to filtration, heating, and possibly mechanical cooling.

Such systems usually comprise a length of metal or concrete piping placed underground. The piping is connected to an outdoor air intake within the site boundary and the ventilation system at the other end.

During the summer, heat transfer to the surrounding ground cools warm outside air, while during the winter cold incoming air is warmed by contact with the ground, which has greater thermal stability than air all year round.

A typical system could have the following performance characteristics:

  • A cooling effect of 45 W/m2 of ground coupling area can be obtained with an outside air temperature of 32°C
  • A heating effect of 45 W/m2 of ground coupling area can be obtained with an outside air temperature of -5°C
  • Typically, outside air at a temperature of 28°C can be cooled to about 17°C.

Ground-coupling can involve the use of labyrinths run beneath a building, such as in the form of basements. These forms of ground coupling may need some form of purge cycle as they do not lose heat as well as systems that are independent of the building.

Regulations for historic buildings

Q. How is Part L of the Building Regulations applied to historic buildings?

The Building Regulations for England and Wales contain functional requirements such as "Reasonable provision shall be made for the conservation of fuel and power". These legal requirements apply whenever building work is being carried out, regardless of the type of building. However, the Building Regulations do not define reasonable provision, nor how it can be demonstrated.

The Approved Documents provide guidance on ways of meeting the Building Regulations for common types of building work. The provisions of the Approved Documents are not legal requirements, and the applicant can always choose other means of demonstrating that reasonable provision has been made.

The reason for this set up is to give applicants flexibility in how they achieve the functional requirements. The Approved Documents give general guidance regarding historic buildings, but no specific guidance on how to show compliance for any given type of building work.

The Regulations offer a basic principle: "...the aim should be to improve energy efficiency where and to the extent that it is reasonable and practically possible. The work should not prejudice the character of the host building or increase the risk of long-term deterioration of the building fabric or fittings."

The English Heritage publication Building Regulations and Historic Buildings, (revised in 2004) provides more guidance.

Carbon dioxide setpoints

Q. What level of carbon dioxide set point should be used for a demand-controlled ventilation system?

By continually monitoring the levels of carbon dioxide (CO2) in a building zone, the number of occupants can be inferred and hence an optimum level of ventilation provided. This approach assumes that indoor air pollution is mainly caused by human metabolism.

Monitored levels of CO2 within a building will comprise the total of metablic CO2 and background CO2 present in the outside air. In order to determine the level of metabolic CO2 in the internal air, both internal levels of CO2 and that in the air entering the building's ventilation system from the outside can be monitored. The difference between the two will represent the level of metabolic CO2. In practice the use of two CO2 sensors is avoided by assuming a level of background CO2 (typically 400 ppm for a city centre and rising by about 2 ppm per year globally).

BS EN 13779 provides target levels of metabolic CO2 for different categories of indoor air quality (levels of CO2 above the level of outdoor air).

Escape distances

Q. Are there any restrictions on the maximum distance of travel during emergency evacuation in a plantroom?

Approved Document B: Fire Safety (Volume 2) - Buildings Other than Dwelling Houses provides limitations on travel distance for a range of building types.

For plant rooms, the Approved Document states that the maximum travel distance within a plantroom is 9 m for one direction only, and 35 m where more than one direction is possible.

Storage facilities for Bananas

Q. I am designing a banana storage facility. Is any guidance available?

The ASHRAE Handbook Refrigeration provides guidance on the design of banana ripening and storage facilities. This includes recommended fruit temperatures for banana ripening, along with a calculation procedure to determine the correct airflow requirements.


For more information about the benefits of BSRIA Membership, including technical support from our expert engineers, contact:
Tel: +44 (0) 1344 465600
or email


A unique resource of technical and market knowledge