Avoiding oversizing

Oversizing is an easy trap to fall into when designing climate control but can be avoided if engineers choose systems flexible enough to match building demands now and in the future, says Daikin UK’s Martin Passingham.

It goes without saying that an air conditioning system must be designed to meet the requirements of the building’s occupants. Having insufficient capacity could lead to complaints and, in the worst case scenario, result in claims against the designer.

However, during design it is not always clear how the building is going to be used, particularly for commercial premises where there is likely to be multiple tenants. For this reason, sizeable margins of safety are commonly included in heating and cooling load calculations.

Unfortunately, many systems are significantly oversized. Monitoring of Daikin VRV systems installed in the UK has shown that, on average, the buildings had a maximum demand of just 45% of the condenser capacity in cooling, with condensers operating at 20% capacity most of the time. In heating, the maximum load was 70% of the nominal heating capacity.

Oversized systems are often less efficient than they could be. While inverter-driven compressors are more efficient at part load, the peak is between 40-60% of capacity. Below this, compressor efficiency drops until the minimum inverter speed is reached, when it effectively becomes an on/off compressor. As such oversizing can lead to a system running below the most efficient operating point for most of the time.

A good start
Oversizing can be avoided if climate control is designed and specified so it meets the proposed demand and is flexible in daily operation. Choosing a system that can also be reconfigured in the future, should building demands alter dramatically, is also a good start.

Heat loss and gains within a space yet to be built or occupied can be predicted using various load calculation methods that range in accuracy from ‘rules of thumb’ through to dynamic simulation modelling software.

However, there is always margin for error, as even the most advanced methods rely on the accuracy of source data. For example, much of the available climate data in the UK comes from weather stations on airfields and is therefore unlikely to be fully representative of a building’s local climate, especially in built-up areas.

Many systems are designed primarily on cooling loads. This is not necessarily the best approach, as it assumes heating load will be satisfied; in older buildings, and poorly insulated ones, heating demand can be much higher. And, if proposed building use changes, demand may well change: a server room will have different cooling requirements than an open plan office.

Seeking advice from manufacturers can help test design assumptions and system choice. Manufacturers should challenge the design and will suggest refinements that can help reduce the risk of oversizing, while being mindful of the commercial and legal pressures the designer is under.

It is worth noting that even a small improvement in the degree of oversizing can improve efficiency and comfort. If systems are designed to have a peak operating capacity slightly above the maximum design load, the part load performance will be improved.

Challenging changes
A major issue is what happens once the climate control system design has been ‘finalised’. Changes to the design and equipment selection can often be made as a result of value engineering exercises and assumptions made by other project team members based on previous experience. While this may not directly lead to oversizing, it can have a detrimental effect on performance and efficiency.

It is in a client’s best interest, therefore, to involve the system designer throughout the project. Ideally the design will remain unaltered but, if it does change, the designer (with support from the manufacturer) should have the chance to input into the process to reduce the risk of poor performance.

One way to mitigate this risk is to choose a system that offers some flexibility. For example, a VRV system is modular, so equipment can be tailored to suit a building’s needs, allowing designs to be altered and the impact of changes to be calculated and addressed more easily. 

Installing as designed
The system designer should also supervise installation and commissioning to minimise any deviation from the original design as this can affect performance and efficiency.

Furthermore, systems should only be installed by a manufacturer-approved contractor, which should also have experience of similar installations. Retaining the company to carry out maintenance is also advisable: its intimate knowledge of the system and the building will enable it to deal with issues far quicker and easier, minimising performance issues.

Some manufacturers offer remote monitoring, fault identification and diagnosis services for their climate control systems. This will not only give early warning of any problems but also provide invaluable information to help improve building energy performance.  The monitoring can show actual demand within the building on anything from an hourly to seasonal basis.

Rectifying oversizing
There are a number of ways to deal with oversizing of an already-installed system. One is to reduce system capacity, to better match building load with its most efficient performance point. This can be achieved by downsizing or taking units out of use. The modularity of VRV systems is a big benefit, as it makes either approach feasible and should be easier, more cost effective and less disruptive than making changes to other systems.

Increasing the evaporating temperature will also make the system more efficient as it will reduce capacity and allow the system to work closer to compressor’s optimum efficiency. Evaporating temperatures can usually be raised based on in-use performance via a ‘high sensible’ setting. 
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Furthermore, selecting a system that can vary the evaporating temperatures automatically in response to the load, required capacity and weather conditions will provide greater levels of efficiency. For example, Daikin’s Variable Refrigerant Temperature (VRT) technology can improve seasonal efficiency significantly, in some cases by up to 28%.