There are a range of products that have come online in recent years that promise to reduce heating costs but the most effective, and innovative, seem to be based on recovering already generated heat. A few years back I met a retired heating engineer who had invented a system that he ended up installing in his own self-build project which recovered the latent heat in used shower water (I think he set up some sort of divert from the waste).
The one that seems like it is beginning to firmly establish itself is the Gas Saver. Known in the business as Passive Flue Heat Recovery, this device fits onto the flue of a regular condensing boiler and recovers much of the heat energy from the flue and recycles it to boost the heating of the Domestic Hot Water supply. It’s unusual in at least three respects – firstly, it comes with some recommendation from the Energy Saving Trust (despite their faults, the one body with enough clout to independently test/trial products that claim to be ‘green’) and secondly, it counts towards a SAP assessment (which in itself gives it some more gravity). Thirdly, it’s passive, which means that it doesn’t rely on an additional energy source (e.g. electricity) to power it.
It costs around £500, saves between 35-40% of your energy costs (for domestic hot water rather than your total energy bills) and therefore should pay back in less than 5 years (assuming a saving of £96 on a hot water component (20%) of a typical energy bill (£1,200, growing by 12% a year). The only downside is that it seems to be limited to certain boiler makes. Find out more here