he popular contemporary trend for glazed facades on residential schemes fills them with natural light and maximises city views but, on warmer days, the glazing can heat apartments to uncomfortably warm levels. If the building is constructed of concrete, this solar gain is stored as heat in the structure overnight, creating the risk that the apartments will be above acceptable temperatures in the bedrooms while residents are trying to sleep.

A sleeping environment of 26°C or above will result in disturbed rest and thus limit the health benefits of sleep. CIBSE’s Technical Memorandum, TM59, stipulates that these elevated temperatures can be tolerated for short periods and may even be raised slightly if moving air from a fan is part of the strategy for managing overheating; however, this must be done in a cost-effective and energy-efficient manner.

Design challenge

Proline was commissioned to take the building services engineering from concept through to detailed design and installation for a 20-storey residential tower with a large percentage of glazing in the facade.

In city centre locations where windows are not an option due to noise, mechanical ventilation must be considered. This usually involves an MVHR system (mechanical ventilation with heat recovery). But if the use of an MVHR alone can not control the heat gains within the dwelling – usually due to high solar gain through windows – the next stage is to introduce some form of cooling.

The introduction of cooling usually involves prohibitive costs and design challenges due to the space required by cooling units and their visual impact. The running costs and noise of a standard air conditioning system also make it an unappealing choice.

It was clear that the building we were working on was failing the overheating criteria by some distance. Attempts to control overheating using MVHRs delivering eight air changes per hour (ACH) were set to increase the build costs to such an extent that the project would have been financially unviable for the main contractor, so we set about developing an innovative alternative.

Developing a solution

Our in-house expert in building modelling for thermal performance was able to suggest some design changes to the glass specification to reduce the amount of thermal gain to the structure. However, reductions to the G-value of the glazing had to remain within the parameters for the building to pass the SAP (Standard Assessment Procedure) calculations required to pass Part L Building Regulations.

We needed to understand the scale of the problem and our ability to model the building in-house meant that we could explore ideas and potential solutions. When the building was remodelled with the revised specification, the Proline team was able to reduce the number of air changes to circa four per hour, but this was still considered onerous both in terms of build costs and end-user comfort.

The team began to experiment with the thermal model, investigating how much cooling would need to be introduced into the ventilation system to reduce the air change per hour requirement down to a more achievable two. This took the modelling process through several iterations until the team calculated that just 0.5kW of cooling introduced into the airflow of the ventilation system would enable each apartment to sit within the limits set by TM59 with a two ACH.

The next step was to formulate how that 0.5kW of cooling could be introduced into the MVHR system without compromising the space available in the apartments. A coil was developed that works with the velocity of the airflow and is chilled using cooled water provided by a district cooling chilled water main. We were able to engineer this to a size of just 400 x 200 x 200mm, which meant it could be fitted into a small space within the apartment. We then put together a full design proposal with a breakdown of install and operational cost implications, which was approved by the developer, the main contractor and the building services consultant.

Low energy, low cost

The system works by intercepting the airflow of the MVHR in the plenum, shortly after the air intake enters the apartment. The incoming air passes over the face of the coil, which has been cooled using the water from the chilled district main, before entering the living accommodation to provide on-demand heat tempering, reducing the indoor temperature by up to 6°C. The airflow is then extracted through the kitchen and bathroom via a heat exchanger.

The district main has been designed as a three-stage chiller with a buffer vessel to store chilled water, thereby reducing the potential for short cycling on the system. Chilled water flows into the cooling system at 6°C with a return at 12°C and residents will use a simple switch to activate the system for their own apartment as and when they need to.

The design is believed to be a first for the residential sector, with varied elements brought together to create a total solution. It means that this 20-storey scheme will benefit from the space efficiency of centralised plant while offering each resident control over their own indoor environment. The system is inaudible and invisible, does not reduce the living space and is simple and easy to maintain thanks to a condensation tray accessed via a maintenance hatch.

The capex cost of installing the district cooling main and individual apartment plenum cooling system has reduced the build cost by a six-figure sum as compared to the original eight ACH proposal, while the operational cost per minute will be less than the cost of boiling the kettle.