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Technical answers - heating distribution, wiring regulations and photovoltaicsMay 2012

David Bleicher provides answers to technical questions posed by BSRIA Members, including heating distribution, airtightness, the Wiring Regulations, and photovoltaics.

Heating distribution

Q. I am looking for a heating distribution system for an industrial application in which high air velocities would cause disturbance to products. I've tried radiant heating, and underfloor heating is out of the question. What else could I use?

A. There are various kinds of low velocity air distribution systems available. One which is particularly suitable for highbay spaces is textile diffusers, more popularly known as air socks. These are devices which inflate under air pressure and enable supply air to diffuse through the fabric at an even rate.

There are many benefits of air socks: as the air volume is supplied over the the entire length and area of the diffuser, the ventilation is very quiet and non-directional, which reduces local draughts. Such systems are simple, self-balancing, lighter and relatively cheaper than systems with separate distribution ductwork and terminal devices. They do, however, need to be cleaned regularly and require a sustained pressure in order to stay inflated and not flap.

Air socks require a support structure, and an unobstructed horizontal space in which to inflate.

Wiring regulations

Q. Where is diversity addressed in the Wiring Regulations?

A. There is a minor reference to application of diversity to electrical loads in the Wiring Regulations BS7671. This states that diversity may be taken into account in determining the maximum demand of an installation or part therof.

Guidance on the application of diversity can be found in IEE On-Site Guide (OSG) Appendix 1 and IEE Guidance Note 1 (GN1): Selection and Erection Appendix H. Table 1A of the OSG and Table H1 of GN1 provide guidance to determine the connected load, and Table 1B of the OSG and Table H2 of GN1 provides diversity allowances for various types of final circuits.

Relevant fluids

Q. Is cold water or grey water a 'relevant fluid' under the Pressure Systems Safety Regulations 2000?

A. Relevant fluid means steam or any fluid or mixture of fluids which is at a pressure greater than 0.5 bar above atmospheric pressure, and which fluid or mixture of fluids is a gas. A relevant fluid is also a liquid which would have a vapour pressure greater than 0.5 bar above atmospheric pressure when in equilibrium with its vapour, at either the actual temperature of the liquid, or 17.5oC.

A relevant fluid can also be a gas dissolved under pressure in a solvent contained in a porous substance at ambient temperature, and which could be released from the solvent without the application of heat.

What all this means is that water (or grey water) would not be a relevant fluid unless heated to a temperature above its boiling point.

Assessing airtightness

Q. How does a blower door measure airflow? Is it directly related to fan speed?

A. A blower door is a device for assessing the airtightness of small enclosures or buildings. In order to carry out an airtightness test, simultaneous measurements are taken of the pressure difference across the building fabric (known as building pressure), and airflow through the fan.

The blower door incorporates a flow ring on the suction side of the fan. Air passing over this ring is at a lower pressure than still air away from the fan. This pressure difference (known as fan flow) is measured and correlated to airflow.

It's important that calibrated equipment is used, as calibration parameters are needed to convert each fan pressure measurement to an airflow measurement. If these calibration parameters are not known, airtightness software will use default values, resulting in inaccurate measurements.

Photovoltaic panels

Q. How much of the energy generated by photovoltaic panels is lost in the inverter?

A. CIBSE TM25 Understanding Building Integrated Photovoltaics states that "The total balance of system loss is typically 15 per cent". However, that guidance was published in 2000 and things have got a little better since then.

Modern inverters are about 96 per cent efficient at 40-70 per cent full load, but in the UK PV panels work at low output for most of the year so an average efficiency of 94 per cent would be realistic.

It is important to size the inverters correctly for optimum annual efficiency. There can be resistance losses in the wiring, so an overall efficiency of 90 per cent is a reasonable assumption to make in calculations.


Q. Are there any regulations specific to London regarding the discharge of water from an airconditioning unit?

A. There are no restrictions on discharging condensate from cooling coils, however circulated water may have dissolved chemicals and therefore may need a permit for discharge.

For more information about the benefits of BSRIA Membership, including technical support from our expert engineers, contact:

Tel: +44 (0) 1344 465600
or email michael.doig@bsria.co.uk


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