Air Conditioning: The Options

We’ve become accustomed to air conditioning in our cars and offices in the summer months but can it ever be a viable option for the home? David Hilton explains what you need to know

Expert David Hilton explains the options available, how these systems are installed and the costs

When we think of air conditioning systems we tend to think of the noisy boxes wedged in the office window in some 1970s film, or the rattling unit on the balcony of a dated Mediterranean holiday flat.

However, we, as a nation, have got used to air conditioning in our cars, in our workplaces, and in many shops, so our level of expectation for personal comfort has changed.

As a result, there has been an increase in homeowners who want air conditioning at home, too. Luckily technology has moved on in recent years and air conditioning systems are now valid options for home comfort, not only for cooling but for efficient heating as well.

Further increasing the interest in these units is that our homes are constructed to be more airtight and with less thermal mass, to absorb heat. This means that more traditional methods of staying cool just don’t seem to have the same effect. (Find out more about designing out overheating on page 162.) It is therefore necessary to make sure that if we choose to include an air conditioning system, it is actually the most appropriate method of achieving the required result. This is particularly important given the running costs. Air conditioning units do have a relatively high efficiency rating but they are also dependent on electricity and therefore can have a high running cost. Here, we explore the options in more detail.

Portable Units

For occasional cooling you may consider a portable single room unit, such as an evaporative cooler or a portable air conditioner. Evaporative coolers use less energy than refrigerant-based coolers (more on which later) as they work on the principle of blowing air through a fine ‘curtain’ of water that then evaporates and introduces a cooling effect. They need to have a supply of dry air and the resultant ‘moist’ air needs to be exhausted at the opposite end of the room. This is ideal in hot,

dry climates such as Australia and Africa but in the UK the humidity levels can be too high, and cross ventilation too impractical for any actual cooling.

For really effective cooling it is best to look for refrigerant-based air conditioning units — despite the fact that they are more power hungry. The advantage of portable units is that you can move them from room to room. The disadvantage is that they tend to be noisier than installed units and you ideally need to keep the window open in order to vent the resultant heat, which can be a security risk and may result in disturbed sleep due to potential noise.

Installed Domestic Air Conditioning Options

The current installed systems usually comprise an outdoor unit that has a fan in it. This is then connected to an indoor unit via two insulated pipes that contain refrigerant. (The pipes – including insulation layer – are around 28mm diameter.) The indoor units are available in various configurations. For instance, the high wall unit is a bulkhead that is mounted on the wall and is about the size of a couple of shoe boxes. Low wall units look similar to standard radiators, while ceiling cassettes can be mounted in the ceiling in a central area of the room. More discreet slim-ducted units can be positioned in the ceiling or housed in grilled boxes above cupboards or near doors, as is common practice in many hotels.

The ducted units can also direct the air to more than one outlet through a number of ducts, with separate dampers in each duct for individual room control. These ducts must be insulated to avoid condensation.

Domestic-sized air conditioning systems are also available in different levels of complexity. Simple systems designed for a single room application will have one outdoor unit and one indoor unit. These are known as ‘single room air conditioners’. The next option is a ‘split air conditioner’ which is one outdoor unit with two, three or four indoor units. They are all on or all off, and will either all be in heating or cooling mode. These would be used in large room applications.

The third, and most flexible, domestic unit is the ‘multi-split’. This is one outdoor unit and up to eight indoor units with a maximum heating capacity of around 12kw (depending on manufacturer). Each indoor unit can be individually controlled so you could have heating or cooling in one to eight locations. Any more diversity than that and you are looking at the commercial products, and you would most likely need a three-phase electricity supply.

Systems designed and used primarily for heating are often referred to as air-to-air heat pumps and should have an option to reverse the refrigeration cycle if cooling is required. The vapour compression cycle still offers over 300% efficiency and as such is a far more efficient form of heating than direct electric. It is also far less disruptive to install on a retrofit basis.

Air-to-air systems are very often installed as secondary heating systems. Air has the specific heat capacity of one, whereas denser products have higher specific heat capacity and store up to around four times more heat. Air can therefore heat up and cool down far quicker with the same amount of energy transfer, and heating systems that target the air can have a far quicker reaction time. The primary heating system may be connected to underfloor heating, which is slow to react, so an air-to-air system can boost the air temperature by a few degrees on demand. As such, this may also be a valuable feature in homes where there is a home office or where certain rooms are used only occasionally.

Installation and Operation

Air conditioning systems must be installed and maintained by a registered gas engineer. The installation costs, as a ballpark figure, are around £1,000 per indoor unit. Therefore a single room unit can be installed for around £1,000 and a six room multi-split unit will be around £6,000, depending on the complexity of the installation.

Running costs can be difficult to quantify accurately as cooling requirements are not as specific as heating. However, the average indoor 2kw unit may cost you around 8p per hour to run. If you have six indoor units on a multi-split (12kw) then costs of around 48p per hour can be expected.

Modern systems can either have a hand-held remote control or be hardwired to a room thermostat. Many systems can also be integrated into third party smart control systems. A word of caution though: make sure that the systems can ‘talk’ to each other and inform the engineer early on that air conditioning is being added to avoid the heating being triggered on!