Difference between a condensing unit and a condensing boiler

A condensing unit is a device that controls temperature by compressing a refrigerant from a gas state in to a liquid state, by moving it through a system of condenser coils. The term condensing unit normally relates to cooling systems, whilst condensing boilers relate to heating systems. However the term may used interchangeably to describe a component part of both a heating or a cooling system, for example heat pumps which are used primarily for heating, contain condenser units (also referred to as compressors), whilst air conditioning units used for cooling also contain condensing units. Units specifically used for refrigeration cooling may also be referred to as cold chain condensing units.

According to the Non-domestic building services compliance guide, 2013 edition, published by HM Government, a condensing boiler is: '… a boiler that offers a high energy efficiency by recovering heat from the flue gases. This is achieved by increasing the heat exchanger surface area, which recovers extra sensible heat whenever the boiler fires. The boiler becomes even more efficient when system water temperatures are low because the larger heat exchanger area promotes condensation, allowing much of the latent heat to be recaptured. Standard losses (when the boiler is not firing) are low, and part load performance is very good. In multiple-boiler systems, condensing boilers can be used as the lead boiler.'

According to the UK government guidance A guide to air conditioning inspections in buildings from September 2024 intended to cover any air conditioning system where refrigeration is used to provide cooling for the comfort of the occupants of the building: 'Air conditioning system’ means a combination of all components required to provide a form of air treatment in which the temperature is controlled or can be lowered; and includes systems which combine such air treatment with the control of ventilation, humidity and air cleanliness. (As defined in The Energy Performance of Buildings (England and Wales) Regulations 2012 (as amended)).'

In the Sustainable Energy Authority Of Ireland the Accelerated Capital Allowances Eligibility Criteria describes condensing units as 'Air-Cooled Condensing Units products are specifically designed to provide cooling to other equipment and systems that incorporate evaporators (and associated expansion valve control systems). Air-cooled condensing units are factory-assembled units that consist of one or more compressors, an air-cooled condenser and interconnecting pipework. They may include liquid receivers, filter driers, oil separators, shut-off valves and related controls, and a weatherproof housing.' It also describes compressors as 'Refrigeration compressors are products specifically designed to raise the pressure, temperature and energy level of a refrigerant vapour by mechanical means, as part of a vapour-compression, economised vapour-compression or transcritical CO2 refrigeration cycle. Economiser packages consist of a refrigeration compressor, an expansion device, and an economiser that is capable of increasing refrigerant sub-cooling and refrigeration cycle efficiency.'

The Illustrated Guide to Mechanical Building Services, Third Edition (BG 31/2017), by David Bleicher, published by BSRIA in 2017, states in relation to condensing boilers: ‘The flue gases (otherwise known as products of combustion) from a conventional boiler consist primarily of carbon dioxide (CO2) and water vapour, plus other gases such as oxides of nitrogen. The water vapour holds a significant amount of energy, known as latent heat. In a condensing boiler, the flue gases are cooled before being discharged, so the water vapour condenses (turns into liquid water). The latent heat released is used to preheat water returning to the boiler, and so condensing boilers are more energy-efficient than conventional ones. In order for this condensing process to work, the return water must be below a certain temperature. For this reason, and also that the flue gases are cooler and less buoyant, condensing boilers can’t always directly replace non-condensing boilers in existing buildings.’

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