"Christian McArdle" wrote in message
...
The CORGI registered man fitting the boiler for us is saying that we must
have a bypass.

Only the boiler manufacturer can
specify if the boiler needs a bypass.

The makers specify a minimum flow. How you do that is your business. An
auto by-pass is only one way, but can be problematic it fails and lowers
efficiency raising the return temperature. Most fitted, like 90% of them,
are incorrectly set anyway. Best avoided.