Grade d or above gcses, including maths and english would be an advantage. Mike and i are still in love with our bathrooms — and would recommend avon plumbing without hesitation! All the forum's best tips go in. Based on the efficiency calculated to an industry standard called sap , the least efficient has a rating of . and the most efficient is . the average efficiency of all the boilers analysed is .. What's the cost of replacing your boiler? Decorate your garden with.