Air Conditioning: Options for the Home

Our homes are now being 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. Summer temperatures are also regularly higher than they were just a few years ago, so many of us are looking for ways of keeping our homes cool, like air conditioning.

If you choose to include an air conditioning system, you will need to ensure that it suits your needs. 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 take a look at the air conditioning options available, highlighting their advantages and disadvantages.

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 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 but in the UK the humidity levels can be too high, and cross ventilation too impractical for any actual cooling.

For really effective cooling, look for refrigerant-based air conditioning units — despite the fact that they are more power hungry.

Advantages

  • Easily moved from room to room

Disadvantages

  • tend to be noisier than installed units
  • you 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.
  • evaporative coolers may not be suitable for the UK climate

Domestic Air Conditioning Options

Installed systems usually comprise an outdoor unit that has a fan in it. The fan is then connected to an indoor unit via two insulated pipes that contain refrigerant. (The pipes, including the insulation layer, are around 28mm diameter.)

The indoor units are available in various configurations:

  • The high wall unit is a bulk-head 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
  • 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 probably need a three-phase electricity supply.

Advantages

  • Wide range of shapes and sizes
  • Available in different levels of complexity, from single room air conditioners to multi-split units

Air-to-Air Systems

Systems designed and used mainly for heating are often referred to as air-to-air heat pumps and they should have an option to reverse the refrigeration cycle if cooling is required. The vapour compression cycle still offers over 300% efficiency and so it 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.

Advantages

  • More efficient form of cooling and heating than direct electric
  • Less disruptive to install on a retrofit basis
  • Useful for homes where rooms are only used occasionally

Mechanical Ventilation with Heat Recovery (MVHR)

MVHR systems are not primarily air conditioning units — they are designed to be ventilation systems.

Some MVHR units do have a micro compressor and reversible heat pump to add heating or cooling to the incoming air. These units only operate at around half an air change per hour, with the fresh air always starting at ambient outdoor temperature.

This means that this heating or cooling is only usually enough to maintain the temperature in the home rather than actively create temperature change or to serve as the primary heating or cooling system. Proper air conditioning recirculates the internal air at a faster rate to facilitate the change in temperature.

How Much Does Air Conditioning Cost?

Air conditioning systems must be installed and maintained by a registered gas engineer. The installation costs 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 hard-wired to a room thermostat. Many systems can also be integrated into third party smart control systems.

A word of caution: make sure that the systems can ‘talk’ to each other and inform the engineer early on that air conditioning is being added to avoid heating being triggered on.

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