We have one of those, but we stopped using it, we got a firm in for advice on recommisioning it, and servicing cost per year and how much it would save, we didn't bother, 4 years ago it would have been around £300 to make sure it was up and running properly, about the same to service it, and it might have saved us £50 per year, it would save more now, but a photvoltaic panel would be more money saving over time.