filling loop?

depends.on many boilers it is located on copper pipework.it is a silver braided hose connected to either one or 2 isolation valves .1 of these contains a special double non return valve to prevent water contamination.
the filling loop (temporary filling loop) joins the pipe containing cold drinking water to the dirty water in the radiators return pipe.when the pressure gets low in many modern boilers they stop working and the pressure needs to be topped up by opening one valve then gradually opening the other until pressure of approx 1 to 1.5 bar cold then close one valve then other.if you have put too much pressure in you need to make sure both valves off then bleed radiator until pressure low enough.
usually the isolation valves have black plastic handles(like washing machine valve handles) they are shut when at right angles to copper pipe (default) and open when in line.
some bosch boilers and baxi boilers have built in filling loop.
sometimes filling loop is in airing cupboard with remote pressure gauge if boiler is downstairs.
water regulations stipulate that loop should be removed and capped off between fills.this is laborious and you generally find the loop connected for the life of the boiler.

jonh
February 2014