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HVAC system sizing: make the right choices

In our experience, many commercial buildings are fitted with heating, ventilation and air-conditioning (HVAC) systems that are incorrectly-sized to the point that they have a negative impact on equipment efficiency, running costs and the comfort of occupants. Here's a few tips on how to make the right choices about HVAC sizing.

Don’t rely on ‘rule-of-thumb’ calculations

Modern HVAC systems must be designed based on precise understanding of the heating and cooling loads on a space - estimations simply won’t do. Ensure the maximum efficiency and reliability of your HVAC plant by obtaining accurate equipment load data and using it to select the exact size of equipment for your needs.

Don’t deliberately oversize

The mistake many people make when sizing an HVAC system is to assume the worst case scenario by deliberately choosing an oversized system. The idea behind this is to allow for any deviation in the design capacity and that an oversized system will cover all requirements. Not only are oversized systems more costly to install, but all buildings are different and it’s crucial that HVAC systems closely match the needs of the building occupants.

Don’t undersize, either

On the other hand, if a HVAC system is designed below capacity, it will have to work harder and longer to satisfy demand, which will inevitably lead to early component failure.

Use variable-speed drives

Constant starting and stopping accelerates wear and tear which will inevitably lead to premature failure, so include a variable-speed drive for smooth, controlled operation. Variable-speed drives allow excellent part-load performance and ensure your system is only working as hard as it needs to. Speak with a drives supplier who will help you size your equipment.

Have flexibility in mind

Consider possible future expansion by generously-sizing main pipework and ductwork to allow capacity to be redistributed if needed.

If all this seems like more than you can fit into an already-packed schedule, have your system sized by experienced engineers such as Gibbons. You’ll enjoy the peace of mind that it’ll be done right first time, without the need for remedial action later. For more information, call Gibbons’ HVAC Services Manager Andrew Knight on 07850 204915 or email

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