The efficiency increase is from the lowering the flue gas temperature by heating water. I'm not saying more, but spec figures are presented to make products look as good as possible. Think of cars that can do 100mph and 50mpg. They don't do both at the same time.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local