HVAC Air Conditioning Refrigeration Efficiency

This guide, written by an industry independent expert, explains in depth how you can determine what refrigeration capacity is needed and what type of appliance is best suited to avoid unnecessary energy consumption before purchasing an air conditioner (e.g. a split unit).

Anyone wishing to purchase an air conditioner or a single room air conditioner (e.g. a split air conditioner or a compact room air conditioner) will often first wonder what cooling capacity is required for the respective application. After all, it would be really bad to buy a device that does not prove to be sufficiently powerful in the end. On the other hand, you don’t want to be unnecessarily high dimensioned, i.e. invest more money and possibly even increase your energy consumption.

Unfortunately, one cannot rely on the fact that every specialist, for example from an air-conditioning specialist, can clarify such things completely and correctly. Even those who should definitely be able to do this often lack knowledge and technical understanding. In addition, there are possible conflicts of interest: Those who would like to sell you a device will be reluctant to explain to you how you could avoid this investment. After all, even if you have read this article, you as an interested layman can form an opinion about which points are to be considered with an adequate clarification of the situation.

We concentrate here on the air conditioning of residential and office rooms; in other cases (e.g. in trade and industry) special circumstances may exist which we cannot cover all of here.

A well-founded estimate of the heat load occurring

A number of factors can contribute to the overheating of a room – first of all the heat loads, i.e. the input of heat from various sources. Of course, it is important to understand how important these are. The following sections will give you a good overview:

Solar radiation through windows

When sunlight enters the room through large windows without sun protection, this is often the most important factor for the heat load. A good part of the energy from sunlight (including heat radiation) enters the room through glass panes and is largely converted into heat when it hits objects in the room.

The full solar radiation per square metre (with an unclouded sky) roughly transmits 1 kW (kilowatt). However, this is reduced by approx. 13% if the window is not perpendicular – for example, by a deviation of 30°, and by half at 60°. (This is why roof windows on the south side contribute considerably more than vertical windows, especially in summer, and roof windows on the north side contribute much less.) The result must be multiplied then still with the energy permeability of the window, which can lie e.g. with modern triple glazing with approximately 0.6 (corresponding to 60%). Altogether one will “harvest” thus per square meter with reasonably vertical incidence often approximately 0.5 kW warmth, which can be in the winter a very welcome contribution to the heating, in the summer however often an undesired high heat load is. Several large windows can produce several kilowatts – which can quickly overwhelm a smaller split air-conditioning unit and even more a compact room air-conditioning unit.

Of course, one should first of all think about adequate sun protection. Very important: An inner sun protection (i.e. on the inside of the window) can effectively avoid glare, but it hardly contributes to summer heat protection: All radiation absorbed by the window is converted into heat, which largely remains in the room. (Only blinds that reflect strongly from the outside could help to some extent.) On the other hand, external solar shading can easily keep more than 80% of the unwanted heat out without blocking all incident light. This means that the problem is already greatly alleviated.

By the way, sun protection is completely ineffective if it is not used! One should remember not to operate blinds, for example, only when the room has already become unpleasantly warm.

Heat conduction through walls and roofs

The heat input through heat conduction in vertical walls is usually not too big a problem in summer – even with not so good thermal insulation, because the temperature differences between inside and outside are not very high in summer. This aspect becomes more important with roofs – with pitched roofs as with flat roofs. A tiled roof often becomes very hot when exposed to full sunlight – under certain circumstances significantly more than 60°C. This results in a large temperature difference between the tiles and the ceiling, so good thermal insulation is really important.

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