How to change water filter if the only valve is before the boiler?
Personally, I'd just install a ball valve on either side of the filter. With those ball valves in place, replacing the filter would take no more than two simple twists to shut off the water. This SHOULD have been done originally, and it is something not difficult to do. Plumbing is not difficult to do, though I will admit that my first attempt had me fearful. You can learn it quickly enough. If not, then call a plumber. This is a quick fix that will not cost more than an hour of their time under any circumstances.