Where is my boiler filling loop and what does it look like?
Three completely different arrangements go by the same name, which is why people are told to top up their boiler and cannot find anything to top it up with.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 5 min
- Safety
- caution
Short answer
Look under the boiler for a short braided silver hose with a small valve at each end. If there is nothing there, look for a plastic key in a slot in the underside of the boiler — that is an integral filling link. If neither, follow the two pipes from the bottom of the boiler; the installer may have put the loop under a sink or in a cupboard.
Repressurising a boiler is a two-minute job once you can find the loop, and an impossible one until you can. The confusion is real: three quite different arrangements are all called a filling loop, one of them is invisible until you know to look for a key, and installers sometimes put the whole thing in another room entirely.
Safety
What you'll need
- A torch
- The boiler's installation manual, or its model number (optional)
Step by step
- Look under the boiler first.The external braided loop is the commonest arrangement: a short flexible silver hose, about the length of your hand, connecting the cold mains pipe to the heating return pipe, with a small isolating valve at each end. One valve is usually a lever, the other a slotted screwdriver type.
- If there is no hose, look for a key.Many modern boilers have an integral filling link built into the underside. A small plastic key — often white or blue, sometimes clipped to the pipework, sometimes still in its slot — pushes into a socket and turns to connect the mains to the heating circuit. If the boiler seems to have no loop at all, this is nearly always why.
- If neither, follow the pipes.The loop must connect the mains to the heating circuit somewhere. Follow the two pipes at the bottom of the boiler and look under the nearest sink, in the airing cupboard, in a kitchen cupboard, or under floorboards near the boiler. Installers put them wherever there was room.
- Identify which valve is which.One valve is on the mains side and one on the heating side. Both must be open to fill and both must be closed afterwards. If yours has only one valve, that is a non-compliant installation and worth mentioning at the next service — two valves and a removable loop is what the regulations expect.
- Check the gauge before you touch anything.Note the pressure with the system cold and the boiler off. That is your starting figure and the one the manufacturer's normal band refers to. Topping up a hot system gives a misleading reading.
- Open slowly and watch the gauge.Open the mains-side valve a little, then the other. You will hear water flowing. Watch the needle, not the clock — it moves faster than you expect. Stop within the manufacturer's cold band, which is printed on the boiler or in its manual.
- Close both valves firmly and check for weeping.Close the heating-side valve then the mains-side valve, both fully. Wipe the loop dry and check it a few minutes later. A loop weeping at a union will slowly raise the pressure and eventually push the relief valve open.
- Disconnect or remove the key.With a keyed link, turn the key back and pull it out. With an external braided loop, best practice is to undo one end so the connection is genuinely temporary — leave it hanging from one union so it is there next time. Keep the key somewhere obvious, because a lost one has to be ordered by model.
Tips
- Take a photograph of your loop, its valves and the gauge with the pressure correct. It removes all the guesswork next time.
- If you have to top up more than about twice a year, something is wrong. The loop is treating a symptom.
- Some boilers now have automatic filling built in and no accessible loop at all. Check the manual before hunting for something that does not exist.
- Write the manufacturer's cold pressure band on a label on the boiler. It is model-specific and it saves looking it up in a hurry.
If it doesn't work
No loop anywhere under the boiler
Cause: It is an integral keyed filling link, or the loop is remote — Fix: Look for a plastic key in a slot on the underside first — that accounts for most cases. Then follow the pipes to a cupboard or under a sink.
Key is missing
Cause: Lost by a previous owner or an engineer — Fix: Order the correct key by boiler model. They are inexpensive but they are not interchangeable between makes. Do not improvise with a screwdriver — the mechanism is a specific shape and forcing it damages the valve.
Loop is open but the pressure does not rise
Cause: The mains stopcock is off, the loop's own isolating valve is seized, or the filling point is blocked — Fix: Confirm a nearby cold tap runs. If mains is present and the pressure still will not rise, the loop valves need attention — engineer work.
Pressure rises then falls back within days
Cause: A leak in the system or a failed expansion vessel, not a filling problem — Fix: Stop topping up repeatedly and diagnose it properly. How fast the pressure falls is the key clue.
Loop drips after use
Cause: The valve has not fully closed, or a union has loosened — Fix: Close the valves fully again. If it continues, the union needs remaking — the loop can be replaced cheaply and is a wear item.
Questions people ask
Should the loop be left connected?
No. The water regulations expect the connection between the mains and the heating circuit to be temporary, because the heating circuit contains treated, non-potable water. Disconnecting one end after use is the correct practice.
Is a filling loop the same as the pressure gauge?
No. The gauge tells you the pressure; the loop is how you change it. They are usually near each other on the boiler, which is where the confusion comes from.
What if my system has a feed and expansion tank instead?
Then it is an open-vented system and it has no filling loop and no pressure gauge — it tops itself up automatically from the small tank in the loft. If you are looking for a loop and finding a tank, you have the other kind of system.