What is a filling loop?
A filling loop is a short link between the cold mains water supply and the sealed heating circuit of a combi or system boiler. Opening it lets mains water flow into the heating system, which raises the pressure on the boiler gauge. Closing it cuts the link again.
That link is kept closed because heating water is not clean. It has been round radiators and pipes, often with inhibitor chemicals in it, so it must never be able to flow back into the drinking water supply. Under the Water Supply (Water Fittings) Regulations 1999, published on GOV.UK's legislation site, a sealed heating circuit is treated as a fluid category 3 risk, so the filling point needs backflow protection, typically a double check valve, and a flexible filling hose should be disconnected when it is not in use. The Worcester Greenstar 4000 installation manual puts it plainly: the filling loop should never be a permanent connection to the mains water supply.
Regular (heat only) boilers on an open vented system do not have a filling loop. They are topped up automatically from a small feed tank in the loft.
Types of boiler filling loop
There are three main types of boiler filling loop, and which one you have decides how you use it.
| Type | What it looks like | How it works | Examples in manufacturer guidance |
|---|---|---|---|
| External braided loop | A silver braided hose between two pipes under the boiler, with one or two valves or taps | Open the valves slowly, then close them and disconnect one end of the hose | Glow-worm, Worcester Bosch and many system boilers |
| Internal keyless filling link | A lever, often coloured, under the boiler | Move the lever to let water in, then return it to closed | Worcester keyless filling link (blue lever), Baxi Easyfill (green) |
| Filling key | A small port or socket under the front of the boiler, with a separate plastic key | Insert and turn the key to open, then close it and remove the key | Worcester internal keyed filling (black port at the front right) |
Some boilers use a variation of these. The Ideal Logic Combi user guide, for example, describes a filling loop with blue handles that is connected for use, then disconnected, with a cap and plug refitted afterwards.
Where is the filling loop on my boiler?
The filling loop is usually directly underneath the boiler, among the pipes and valves that come out of the bottom. Look for a silver braided hose, a coloured lever or a small black or grey socket. Glow-worm describes internal loops as sitting underneath the boiler with taps on each side.
If there is nothing under the boiler, check these places:
- The airing cupboard. On a system boiler with a hot water cylinder, the loop is often near the cylinder or the pipework feeding it.
- Behind a boiler cover or pipe shroud. Some installations hide the pipes behind a removable plastic cover at the bottom. This is not the boiler case and is fine to take off.
- In a drawer or with the paperwork. A loose braided hose or filling key is often stored away after installation.
For brand-specific locations, see our guides to Worcester boiler pressure, Vaillant boiler pressure and Baxi boiler pressure.
Take a photo of the pipes under your boiler while nothing is wrong. If you ever need to top up in a hurry, you will know which valve is which.
Using each type of filling loop
Whichever type you have, the principle is the same: with the system cold, open the loop slowly, watch the gauge rise to the normal range, then close it fully. Our guide to boiler pressure that is too low gives the full repressurising steps, and our guide to normal boiler pressure explains what the gauge should read.
- Braided hose: if the hose is detached, connect both ends hand tight first. Open one valve fully, then open the other slowly. Close both when the gauge reaches the target, then disconnect one end and refit any caps.
- Keyless link: move the lever slowly and watch the gauge closely, because keyless links can fill quickly. Return the lever fully to its closed position.
- Filling key: insert the key, turn it as your user manual describes, then turn it back and remove it.
Only use the filling controls outside the boiler case. Never remove the boiler casing to reach a valve or link; anything inside the case is a job for a Gas Safe registered engineer. Fill slowly, because overfilling can push the pressure up to the relief valve setting.
Should the filling loop be left open?
No. The filling loop should always be closed after use, and a braided hose should be disconnected at one end. There are two reasons.
- Water regulations. The 1999 regulations and manufacturer manuals treat the loop as a temporary connection, to protect drinking water from heating water.
- Pressure creep. An open or partly open valve keeps feeding mains water into the system. The pressure rises until the relief valve opens and discharges through the pipe outside. If that pipe is dripping, check the loop first, then read our guide to the boiler pressure relief valve.
To check a loop is closed, look at the valve handles. On most quarter-turn valves, the handle or screwdriver slot sits across the pipe when closed and in line with the pipe when open. Then note the gauge reading with the system cold and check it again the next morning. If it has risen with the heating off, water is still getting in and the valve needs attention.
What is a keyless filling link?
A keyless filling link is a filling loop built into the underside of the boiler, operated by a lever instead of a hose or key. Worcester Bosch says its keyless filling link comes as standard on its CDi Classic combi boilers and is available as an accessory for others, and that it is WRAS approved. Baxi's newer boilers use a green Easyfill link underneath. The advantage is that nothing goes missing and there is no hose to connect. The drawback is that the lever makes it very easy to leave the link slightly open, so always check it has returned fully to closed.
Common filling loop problems
| Problem | Likely cause | What to do |
|---|---|---|
| No hose or key anywhere | Removed after installation and lost | Buy the correct loop or key for your boiler model from a merchant, or ask an engineer |
| Drip at the hose connection | Worn or missing rubber washer | Replace the washer and tighten hand tight |
| Valve will not turn | Valve stiff from lack of use | Do not force it. Ask a plumber or heating engineer |
| Pressure creeps up with the loop closed | Valve passing water | Disconnect the hose and call an engineer |
Can I replace a filling loop myself?
An external braided loop sits outside the boiler case on the mains and heating pipework, so a competent DIYer can swap a like-for-like WRAS-approved loop kit with its backflow protection. It still means isolating the mains supply and the heating circuit, and a poor joint can flood a cupboard, so most people use a plumber. An internal keyless link is part of the boiler itself and should be replaced by a Gas Safe registered engineer.
What to do next
If you have found your loop and the pressure holds after topping up, you are done. If you are topping up every few weeks, the loop is only treating the symptom. Our guide to a boiler losing pressure shows where the water is likely to be going.