Apart from the legal sides of working on a gas appliance, sticking the restrictor in is something that needs to be done just right, or it can start endless problems with some boilers. I would really advice to leave this bit to the installer.
"FLUE RESTRICTOR RING (horizontal flue)
To ensure maximum efficiency of the appliance,
it may be necessary to fit one of the supplied
flue restrictor rings to the appliance flue outlet
(see tables below).
Total flue length Restrictor required
Less than 1.2 metre 82.5mm diameter
Less than 3.0 metres 88mm diameter
Less than 4.5 metres not required"
Mine is less than 3m but more than 1.2m, so I need the 88mm restrictor.
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