If space for collector loops is an issue then air source may be the answer as no ground works are required.  Whilst not quite as efficient as ground or water source heat pumps they are still an attractive alternative, usually around 300% efficient. 
Siting the unit away from bedroom windows or quiet rooms is advisable as they do make some noise (between 45 and 60 decibels).  It is possible to house them in an outbuilding which alleviates some of the noise issues.

If you require further technical information, please email us.