Oversized HVAC units are expensive, inefficient and can be unreliable. Capital costs are further increased due to oversized distribution ducts and components.
The main inefficiency arises when the unit, running at part load, frequently cycles on and off. At every 'on' cycle, there is a fixed start-up period during which the unit is consuming maximum energy but providing sub-maximal heating or cooling.
Frequent cycling also reduces reliability and humidity control.
A HVAC unit should meet but not exceed the calculated sensible load, latent load and supply airflow-rate, all of which depend on the size and design of the building, the heating and cooling requirements and the outdoor climate.
To determine the appropriate HVAC unit size avoid over-estimating loads and consider:
effects of energy efficient technologies installed and strategies implemented, such as radiant chilled ceilings, displacement ventilation, advanced controls and any building-level energy efficiency measures
actual rather than rated ‘plug loads’ (the energy used by a product that is powered by an AC plug) for computers, copiers, printers and other indoor heat sources such as lighting.
These items usually run at a fraction of their nameplate (maximum) loads and usually not at the same time.
The plug load, which can be measured using a plug-in power meter, could be about 20 per cent of the maximum plug device power supply rating in some cases ventilation air required for the expected occupancy.
Some indoor spaces have occupancy substantially lower than maximal or that suggested in building codes, which makes their cooling and ventilation loads lower than expected.
There are a number of general guidelines for selecting a HVAC unit.
For example, the unit capacity should match the calculated sensible and latent HVAC loads.
Efficient units usually have lower latent capacity than standard units, and efficient buildings usually have lower sensible loads than standard buildings.
A unit's supply airflow rate is proportional to its sensible air conditioning capacity.
The nominal supply flow-rate of most units is suitable in moderate to very humid climates.
Units with higher flow rates and sensible cooling capacities are preferable in drier climates where latent loads are lower; these units can also be smaller.
High-efficiency units are usually more cost-effective in the long term, but standard units may be cost competitive in some cases (e.g. a standard unit optimised for efficient part-load operating in variable environmental conditions, such as in a temperate climate).
Water-based cooling towers are a good option in drier climates as they reduce the effective ambient temperature close to the wet bulb temperature.
However, they use large amounts of water, chemicals, pumping and fan energy, while also having potential health risks.
Evaporative pre-coolers and more sophisticated systems that use evaporatively cooled exhaust air from the building achieve a similar level of energy efficiency to cooling towers without the water consumption and health risks.
It's also important to select the right water chiller system.
When selecting a water chiller system in a new installation, or if evaluating refurbishment or replacement of an existing system, calculate the cost of energy over the new system's planned life (e.g. 25 years).
Since July 2009, the efficiency of water chillers (over 350 kWr capacity) has been regulated in Australia and New Zealand.
The minimum coefficient of performance (COP) and the integrated part-load value are stipulated.
Chillers installed before July 2009 were not subject to these minimum energy performance standards (MEPS) regulations, and so are likely to be less efficient.
Unit replacement should offer a level of energy efficiency which cannot be matched by refurbishing an old chiller.
For more information refer to AS/NZS 4776 – Liquid-chilling packages using the vapour compression cycle 2009.
About the author - This article is provided by the Energy Efficiency Exchange (eex.gov.au) which is a joint initiative of the federal, state and territory governments. It aims to support the development and implementation of energy management and energy efficiency strategies by providing quality information from respected national and international sources in one location.