Note that you are unlikely to achieve 95% efficiency on the gas. That requires you to run your plumbing at about 20 degrees (celcius), since burning gas means s

Note that you are unlikely to achieve 95% efficiency on the gas. That requires you to run your plumbing at about 20 degrees (celcius), since burning gas means simple thermodynamics apply. At 40C you're already down to 90%, and virtually every installation I've seen actually is set to 60-80 degrees, at which point you're nearing an efficiency of only half. Hotter water means a more reactive system and smaller radiators (you'll need five times the volume for a given power output when you compare plumbing water of ~30 to 70+. You are almost certainly not installing that. Maybe you are installing floor heating, in which case you are likely OK. But most people I speak to don't realize that 1. Gas efficiency goes up as temperature differential goes down, and 2. energy transfered to water also needs to leave the system at your radiators. So size them for the effiency you want, or be disappointed with a significantly higher gas bill.