Since 2005 the vast majority of new boiler installations have been condensing boilers which are far more efficient than boilers using older technology.
You can find out at the Home Heating Guide website how boiler efficiency is rated for boilers from a large range of boiler manufacturers.
However, even new 'A' rated condensing boilers do not give you maximum boiler efficiency as they still lose energy via the flue, that you may notice in excessive 'pluming' that resembles steam.
The addition of GasSaver enables households to recapture wasted heat and use it for hot water production, which gives you greater efficiency and lower fuel bills.
In answer to the question how efficient is my boiler? one answer may be in the boiler efficiency table, but gas boilers only reach their highest efficiency levels with GasSaver installed.
GasSaver has been shown in independent tests to deliver up to 97%+ boiler efficiency and to reduce the energy required for hot water production by 37%.