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Member questions: Energy calculations, commissioning of pipework and extract system designJanuary 2019

Compiled by Steve Sansom, Information & Knowledge Manager, BSRIA

Q: When carrying out CO2 and fabric energy efficiency calculations for Building Regulations compliance purposes, is it acceptable to use midpoint U-values for elements such as wall panels?

The guidance in both Approved Documents L1A (for new dwellings) and L2A (for non-dwellings) states “U-values should be calculated using the methods and conventions set out in BR 443 Conventions for U-value calculations, and should be based on the whole element or unit.” Going by this, a midpoint U-value would not be acceptable to Building Control. Approved Documents can be downloaded from and the BRE publication BR 443 can be downloaded from

Q: Could you please confirm the maximum TVCs (total viable count) of bacteria permissible at PC (practical completion) in a closed water system? Also were there any changes in the recommended criteria for TVC at PC between the two editions of BSRIA’s Pre-Commission Cleaning of Pipework Systems, published in 2011 and.2012?

The recommended criterion for TVC at PC is less than 100,000 cfu/ml (colony forming units per millilitre) as indicated in Table 6 of BG 29/2012 (page 36). It is worth noting that during the period between pre-commissioning and practical completion the TVC should be monitored and trended in the same way as pseudomonads are, see Appendix B of BG 29/2012 (pages 88-89) for further details.

Regarding your second question, the Pseudomonad guideline for practical completion changed from 10,000 cfu/ml to 10,000 cfu/ 100 ml. A corrigenda to this effect was issued in October 2011. You may read details of these changes and others in the Preface of BG29/2012.

Q: I am being asked to design an extract system for a plant room containing an ammonia chiller. Can you tell me what standards apply and what the requirements are?

There is a standard that applies to this type of system: BS EN 378-3:2016 Refrigerating systems and heat pumps. Safety and environmental requirements. Installation site and personal protection. This is available for BSRIA members to borrow from the library. This standard requires a ventilation rate of 4 air changes per hour when the room is occupied. An emergency ventilation system is also required, with a rate of 15 air changes per hour. The standard doesn’t state whether these ventilation rates need to be provided by the same fan with two speeds, or by two separate fans, however, the system does have to be independent of other ventilation systems on the site. Having said that, it is useful to have two fans for resilience, both capable of meeting the requirements of the standard. That way, if one fan fails or undergoes routine maintenance, the system will still protect occupants.

Q: I am carrying out flushing and cleaning on a new building. Can the building’s four-pump booster set, which draws from a potable water tank, be used for flushing?

If you do it this way, there would need to be at least a double check valve downstream of the booster pump, to protect the building’s domestic water systems from contamination. Also you’d need to be able to control the booster pump set to deliver the correct flushing velocity. Furthermore, all the water that’s flushed through would have to be drained away – you wouldn’t be able to recirculate any of it. The circuits for domestic and closed water systems should be kept separated. Swapping the use of a pump between the two systems increases the risk of cross-contamination and consequently would require additional control measures such as disinfection, monitoring of deposit formation and monitoring of initiation of corrosion.

Q: I would like to get a publication that is no longer available on your website, is this possible?

A selection of withdrawn or superseded are available on BSRIA website and are free to download for members. Withdrawn and superseded publications are also available for sale to non-members for £30 + VAT in PDF format only. However, if you do not see the title you wish to acquire on BSRIA website, please do not hesitate to email the bookshop at as we may still have the item in archive and be able to give or sell you a copy.

The BSRIA Information Centre is available from 08.30 - 17.00 Monday to Thursday and 08.30 - 16.30 on Fridays.
E: T: +44(0)1344 465 571


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