Out of thin air

Richard Williams continues his series on renewable energy by looking at air source heat pumps

My Burgundy cottage is over 200 years old. It has solid (one-metre thick) walls which, once warm in the winter, keep us nice and snug and, of course, cool in the summer. Burgundy enjoys a Continental climate, which makes for hot summers, cold but dry winters and changeable seasons in-between. We have recently replaced the roof, which gave us the perfect opportunity to pack in extra insulation. Now we’re debating the merits of various heating systems, among them an air souce heat pump.

ASHPs at a glance

So what is an air source heat pump (ASHP) and how do they differ from the ground source version? Essentially, they work like a fridge but in reverse, capturing the latent heat (from either the air or the ground) in a primary circuit, passing it through a heat exchanger and using it to heat water and provide heating.

The technology behind ASHPs has moved on significantly and ASHPs are now able to offer better efficiencies than their older cousins, the ground source systems. ASHPs are also quicker to install (typically only a couple of days) and do not require the significant land space that an excavated ground source system requires, making them a less expensive option, and as such a no brainer.

So how do they perform? If the outside temperature is 10 degrees (the refrigerant is at minus 50), then the potential heat gain is 60 degrees. There are small losses during the heat transfer but this is a significant amount of energy to be gained. If it gets colder, then at zero degrees I can still expect 50 degrees of gain.

So would an ASHP be suitable for our cottage? The first task was to survey the house to understand its potential energy requirements. With this completed, I was able to calculate that an 8.6kW ASHP would do the trick. I also needed to understand the local climate, allowing me to calculate the implications of really cold weather. The minimum seasonal average is zero, but I also needed to understand what would happen during really cold spells; last winter we recorded temperatures of minus 15!

Modern ASHPs have an electrical boost function which caters for such cold spells, but in general they work comfortably down to minus 10. It’s been a common comment of friends that my electricity bills will increase during these cold periods. Of course they will, but then so would my gas or fuel oil bills. This is where the clever bit about heat pumps comes in; their high efficiency. A gas or fuel oil boiler is approx 90% efficient (if it is new), whereas my heat pump will offer a CoP of 350%. It is three and a half times more efficient. Good news when I take into account the fuel bills.

I’m now sourcing a supplier and, in the next article, will look at the argument for linking my system to radiators and under-floor heating.

Richard Williams runs Burgundy Energy, an independent energy consultancy that provides advice and project management. www.burgundyenergy.com