Air conditioning units can be extremely energy efficient, especially if used in combination with ground or air source heat pumps.
Some air conditioning systems recycle heat from within the house, capturing warmth from expelled stale air and applying it to fresh air being brought into the house.
Heat-Recovery Ventilation Systems are usually only effective in modern homes that meet eco friendly insulation regulations. Villavent (01993 778481, villavent.co.uk) is a company in the UK who supply and fit heat-recovery ventilation systems. They sell DIY kits, with prices starting at about £2,500.
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