Service and regulation

Filling loop

A short connection, often a braided silver hose with a valve at each end, that lets mains cold water into a sealed heating system to raise its pressure. It is opened briefly to top up a low gauge and then closed again, and on some boilers it is built in as a lever or a keyed fitting rather than a separate hose.

Also called filling link, fill loop, charging loop

Direct answer

What is the filling loop on a boiler for?

The filling loop is the valved connection between the mains supply and a sealed heating system, opened briefly to raise a low pressure gauge back into the normal band and then closed so mains water cannot keep feeding in.

Filling loop: how it works

A sealed system runs at a set pressure so water reaches the top of the pipework and the boiler sensor reads enough head to fire. Bleeding radiators, small evaporation losses, and minor weeps lower that pressure over time. Opening both filling-loop valves lets mains water, which is at a higher pressure, flow in until the gauge rises. Closing the valves again isolates the two systems so the heating circuit holds its own pressure and no heating water can flow back toward the drinking supply.

Where you meet the term

  • On a low-pressure fault code, where the manufacturer instruction is to top up to the stated band with the filling loop before anything else.
  • After bleeding radiators, when releasing trapped air has dropped the gauge below the normal cold figure.
  • In a plumbing check where a loop left slightly open has slowly pushed the cold pressure up until the relief valve started to weep.

How to check it

  1. Repressurise slowly to the marked band. With the boiler off and cool, open both loop valves gently, watch the gauge rise to about 1.5 bar, then close both valves fully. Restart and clear the code as the manual directs. Owner check
  2. Confirm the loop is closed and, on fixed hoses, disconnected. A technician checks that both valves are shut and that a detachable loop is removed after use, since a loop left connected and cracked open raises pressure and risks backflow. Technician
Read this twice

Filling loop: what it is not

  • Leaving the filling loop open is not a way to hold pressure up. It masks a leak and slowly overpressures the system until the relief valve discharges.
  • A filling loop does not cure a system that keeps losing pressure. Repeated top-ups within days point at a leak or a failed expansion vessel that needs finding.

Filling loop questions

How do I know if my filling loop is open or closed?

A lever sitting in line with the pipe is open, and one turned across the pipe is closed. After topping up, both valves should sit across the pipe so no mains water keeps feeding in.

What pressure should I fill my boiler to?

Most sealed systems want about 1 to 1.5 bar cold, but the boiler manual gives the exact figure. Fill to the middle of that band with the system cold, not hot.

My pressure keeps dropping after I top it up. Why?

Either there is a leak, or the expansion vessel is flat so the gauge sinks as the water cools. A heating engineer needs to find which, rather than the loop being opened again and again.

Where this definition comes from

Written from manufacturer support article material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.

  • How to fix high boiler pressureBaxi · manufacturer support articleBaxi guidance on high boiler pressure: a stated water pressure range of 1 to 2 bar with the gauge needle expected to sit in the green area, a needle creeping into the red described as pressure that needs attention, water dripping from an external copper pipe attributed to the pressure relief valve having operated, three causes given as a radiator removed for decorating and the system overfilled on reinstatement, excessive topping up after low pressure, and a filling loop left open; and a bleeding procedure of switching the heating off, letting radiators cool, opening each valve anticlockwise until hissing stops and water appears, closing it clockwise, and checking the gauge for about 1.5 bar, with an engineer called if pressure stays high afterwards.
  • Boiler pressure too high: how to fix itIdeal Heating · manufacturer support articleIdeal Heating guidance on high boiler pressure: a normal range of 1 to 2 bar with the system at rest sitting around 1 to 1.5 bar, and pressure above 2.75 bar described as too high; the statement that high pressure is unlikely to be dangerous because the system should have a pressure relief valve that operates and shuts the boiler off, with the caution not to rely on that valve alone; causes given as filling loop valves left open, identified as open when aligned with the pipe and closed when perpendicular, overfilling after a previous low-pressure episode, and a faulty or damaged expansion vessel which the guidance says not to inspect or repair without a professional; four methods of releasing pressure in order of ease as opening the filter valve near the boiler, using a drain-off valve in the pipework, bleeding a radiator, and undoing a radiator nut; and the direction to a Gas Safe registered engineer where pressure keeps going too high or too low.