De-aeration valves, we call them Autovents, automatically remove air and gas from radiators, which improves heating system efficiency.   

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NLB Engineering manufactures high quality de-aeration (purge) valves that are guaranteed not to leak and can fit all radiators.

Aladdin EasyFit Isolator the quick-fit plumbing isolation-valve for pressurised-pipe.

It takes 3-5 minutes to fit.  The alternatives are draining the system, or using gas or electric pipe-freezing, any of which can take between 30-50 minutes.