Where Is the Boiler Filling Loop / Refill Point?

Where to look, in this order

Stand in front of the boiler with a torch and work through the places below. Most people find it in the first two. Nothing on this list involves taking a cover off, and you should not need to: the refill point is designed to be reached by the householder.

• Directly under the boiler, among the copper pipes. Look for a braided silver hose about 30cm long, or a plastic lever, knob or key slot on the underside of the casing
• Behind a clip-on or drop-down cover at the bottom front of the boiler. Some models hide the gauge and the filling point together behind a small flap that pulls forward
• In the airing cupboard next to the hot water cylinder, on some heat-only sealed systems. The loop is often near a red heating expansion vessel; a white or blue drum on the cylinder belongs to the hot water, not the heating
• In the kitchen cupboard the boiler is boxed into, low down at the back, where the pipes drop out of the unit
• In the loft, if the boiler was fitted there. The loop is usually within arm’s reach of the hatch, but not always

If you smell gas at any point, stop. Close the filling loop, open windows, turn off the gas at the meter if you safely can, avoid switches, then leave and call the National Gas Emergency Service on 0800 111 999.

How to recognise it on the common types

An external loop is the easiest to spot. It is a flexible braided hose joining two pipes, with a small valve at each end. Each valve has either a lever or a slot for a flat screwdriver. When the lever or slot lines up with the pipe, the valve is open. When it sits across the pipe, it is shut. Some external loops are left permanently attached; others are meant to be unscrewed and kept in a drawer, and you may find only two stub valves with capped ends under the boiler.

A built-in loop has no hose. On many Worcester Bosch Greenstar combis you will find a filling link on the underside: either a slot for a removable plastic key that often goes missing, or a lever you hold down to let water in. Some Vaillant ecoTEC plus combis have a small filling tap or lever tucked under the casing near the front pipes, while others use an ordinary external braided loop. Check the model name on the front panel and the user guide, which almost always has a drawing of the underside with the filling point marked.

If there is no gauge on the boiler and nothing under it that looks like either of these, you probably have an open-vented heating system. It has no refill point at all. It fills itself from a small feed and expansion tank in the loft, and a pressure fault on that kind of system is a different problem. Our loft tank ballcock guide covers how that tank refills.

How to top up the pressure safely

Do this with the heating off and the radiators cold, because the reading rises as the water warms. A top-up done on a hot system tends to end up too high once the boiler is running.

• Find the pressure gauge and note the reading. Below 1 bar is low; most boilers lock out somewhere under 0.5 bar
• On an external loop, check both ends are tightly connected, then open the valve on the mains side first and the one on the boiler side second, a quarter-turn at a time
• On a built-in loop, turn the knob or push the lever as the user guide shows. You should hear water hiss into the system
• Watch the needle. When it reaches 1 to 1.5 bar, or whatever the manufacturer states, close both valves or release the lever
• If the loop is the detachable kind, unscrew it and refit the blanking caps. Keep a cloth underneath, as a little water always comes out
• Press the reset button once if the boiler shows a low-pressure fault, then let it run and check the gauge again when the system is hot

If you overshoot, do not panic. Let a little water out of a radiator bleed valve into a cloth until the gauge drops back. Above 3 bar the pressure relief valve will open and dump water outside through a copper pipe.

Why the regulations care about that hose

The filling loop is the only point where the water in your heating, which is dirty and usually dosed with inhibitor chemicals, meets the drinking water in the mains. The Water Supply (Water Fittings) Regulations 1999 require every system to have a device that stops water flowing back from an appliance into the supply. On a filling loop that device is a double check valve, a small non-return valve at the mains end.

The guidance to those regulations treats an external braided loop as a temporary connection: it should be unscrewed once the system is filled. A disconnected hose cannot pass heating water back into the mains even if the check valve fails. If yours has been left permanently connected, shut both valves, take the hose off after the top-up and ask an engineer to check the check valve at the next service. Filling devices built into a combi are different, because they are part of the boiler’s approved design.

The regulations are also why you should never improvise a refill point with a garden hose or a washing machine hose. It bypasses the backflow protection and puts heating water into the pipe your kitchen tap draws from. Our combi boiler pressure guide explains what the numbers on the boiler dial mean before and after a top-up.

How often should you have to use it?

A sealed system that is in good order loses almost nothing. One top-up after radiators have been bled is normal. Needing it every week, or every few days, means water is leaving the system somewhere, and topping up only hides the problem while fresh water brings in more oxygen and more corrosion.

The usual culprits, in the order an engineer checks them, are a small leak at a radiator valve or under the floor, the pressure relief valve letting by through its outside pipe, and an expansion vessel that has lost its air charge, which makes the pressure swing high when hot and low when cold. If you are topping up often, work through a boiler losing pressure every day before reaching for the loop again.

The other thing to rule out is the supply. If the needle barely moves when the valves are open, check the kitchen cold tap. A half-shut stopcock or a street problem leaves the loop with nothing to push in. Why your filling loop is not working has the checks for a loop that will not let water through.

What an engineer does if the refill point is missing or broken

The common callouts are a lost Worcester filling key, an external loop that has been taken away by a previous owner, a valve that has seized and will not open, and a built-in filling valve that drips into the casing. The first three are simple: a replacement key or a standard braided loop kit is a cheap part, typically in the range of £10 to £30 at merchants, and fitting it is quick if the stub valves are sound.

A leaking internal filling valve is different, because it sits inside the casing. That is work for an engineer, not a householder. Gas work is carried out by Gas Safe registered engineers. The engineer isolates the boiler, drains down the section, fits the part and refills and repressurises the system.

If the refill point is fine but the pressure keeps falling, the real job is finding the leak. Our boiler pressure repair visit starts from the gauge and the loop and works back to the cause. The £60 call-out covers the visit at any hour, and the repair price is agreed with you on site before any work starts.