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Technical answers - condensate piping, exterior lighting regs and ventilation of mortuariesOctober 2011

David Bleicher provides the answers to technical questions posed by BSRIA Members.

Performance Measurement Protocols for Commerical Buildings, published by ASHRAE, USGBC, and CIBSE is available on library loan to BSRIA Members

Condensate piping

Q. What material should I use for condensate piping?

It depends what you mean by condensate. The condensate from cooling coils in air-handling units and fan-coil units is cold, so PVC drain pipe is generally used. Other materials such as copper can also be used. However, the condensate from condensing boilers is hot and acidic, and would corrode many common types of metal pipe, so PVC drain pipe is generally used for this application as well.

Building regulations

Q. Is exterior lighting covered by Building Regulations?

Yes it is. It's classified as a "fixed building service", so new installations of exterior lighting need to meet energy efficient standards. Also, increasing the capacity of exterior lighting on an existing building may trigger the need for consequential improvements to the energy efficiency of that building.
The trouble is, there is nothing in the approved guidance that defines energy efficient exterior lighting for non-domestic buildings. It would be up to individual building control officers to judge whether an exterior lighting design meets the Building Regulations.

Guidance for exterior lighting on dwellings can be found in Table 40 of the domestic building services compliance guide.

Design parameters

Q. I've heard that design parameters for spaces are categorised I, II, III and IV. What do these categories mean, and where are they stated?

BS EN 15251:2007 defines the categories. For example, category II is "Normal level of expectation and should be used for new buildings and renovations". A building in this category would be expected to achieve less than 10 per cent of persons dissatisfied (PPD) with the thermal environment (a PPD of less than 10 per cent), and would have a ventilation rate of 7 litre/s per person.

This doesn't override Approved Document F, which gives a ventilation rate of 10 litres/s per person for office buildings. British and European standards can be loaned from the BSRIA library.

Venting mortuaries

Q. What design ventilation rates are required for mortuaries?

Healthcare Technical Memorandum (HTM) 03-01 Specialised Ventilation for Healthcare Premises recommends special extract ventilation systems for mortuaries, but does not give specific ventilation rates.

Health Building Note (HBN) 20 Facilities for Mortuaries and Post-mortem Room Services gives much more specific guidance on the design of ventilation systems, including a recommendation of 12 air changes per hour for post-mortem rooms. It's important to note that Control of Substances Hazardous to Health (COSHH) requires exposure to formaldehyde to be controlled, and it may also need to be monitored.

Underfloor heating

Q. Why is it a bad idea to mix radiators and underfloor heating on the same heating system?

It is quite possible (given enough wall space) to design a system with radiators and underfloor heating such that the relevant areas achieve balance at design conditions with the same flow temperature, say 45°C. The problems occur at part-load conditions when using weather compensation, which is normal practice for heat pumps to maximise their COP.

As the flow temperature is lowered, the heat output of the radiators will drop faster than that of the underfloor system. If the compensation curve is set to match the radiator output, the heat pump won't run efficiently and underfloor areas may become overheated. If the compensation curve is set to match the underfloor output then radiator areas may overheat.

Surface temperature

Q. What is the maximum surface temperature for radiators in hospitals?

The 1998 Health Guidance Note, Safe Hot Water and Surface Temperatures, available from the BSRIA library, provides the answer. This document states: "Wherever patients, residents or visitors have access, the maximum surface temperature of space heating devices should not exceed 43°C when the system is running at the maximum design output".

Water softener

Q. Our water supply is very hard and we have used a classic ion exchanger to soften it, but the exchanger is no longer working. Can you help?

The conventional base exchange system, a salt regenerated resin, will work under all circumstances, though it is not advisable to drink the water. This is why water from the kitchen cold tap should not be softened.

You can ensure water doesn't become over-softened by using a test kit to adjust the bypass ratio. This is important, as over-softening the water will waste money. To avoid this, it's advisable to choose a model that auto-detects when it needs to regenerate.


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