I think you will find the reason is that as air comes out, it is replaced by cold water from the header tank. this cold water goes directly into the boiler, the boiler being hot will not take to kindly to cold water (it may split)
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