This 10-minute article by David Hilton, writing in Homebuilding and renovating magazine provides some valuable information and dispels some myths, as we look to phase out gas boilers in line with recent Government strategy. If you are considering specifying an air source heat pumps as part of a new extension, you might think about improving levels of insulation (and airtightness) for the existing property.
There is rarely a better opportunity to upgrade the performance of your home as when you are getting building work carried out. Consider an insulated screed floor with underfloor heating for the the extension and (where possible) in adjoining rooms. This is not the cheapest solution but does provide a larger heating surface over traditional radiators; the system can run at lower temperatures, reducing running costs..
Consider a combined system providing hot water (this requires a smaller internal heat pump that uses heat from a room or extract ventilation to heat water. Such a system requires a heat store cylinder, usually fitted in a cupboard.
As a rule, renewable strategies are optimised when several technologies are combined on a project. Heat pumps and solar PV panels, are complementary and can be combined to provide much of the energy to run the system, reducing reliance on the grid.
Nu-heat provide a variety of solutions. Refer to their website for additional technical information on the subject.