Filling loop
Checked against its sources: Updated
A boiler filling loop is the valved connection that lets mains water into a sealed heating circuit to raise its pressure.1 It is meant to be closed, and usually disconnected, except while topping up.2
- Covers
- Sealed systems, UK. The water regulation quoted is the 1999 one for England and Wales, as made.
- You
- Use it as your own user manual describes, then close it and disconnect it if it is a temporary type.
- A professional
- A valve that will not close, a loop that lets water through when shut, fitting or changing one.
1 Why it exists
A sealed system has no loft tank feeding it, so its water comes from the mains through this connection1. Water lost over time, including through bleeding, has to be replaced this way. The method itself is under repressurising a boiler.
2 Why it comes off afterwards
Heating water should not be able to run back into drinking water. Schedule 2 of the Water Supply (Water Fittings) Regulations 1999 says no supply pipe "shall be permanently connected to a closed circuit for filling a heating system" unless it has a backflow prevention device to an approved specification5. That is the 1999 text as made, for England and Wales; later amendments and the separate Scottish and Northern Irish rules were not read for this entry.
Manuals turn this into plain instructions. Worcester's 4000 says the key must be removed or the external loop disconnected afterwards, citing water regulations3. Main says the filling pipe should be connected only when repressurising4. Worcester's oil guide says the same of its link: connected for topping up, disconnected when not in use6. Some factory links are designed to stay in place, such as Baxi's Easy-Fill, which it calls a permanent filling link7.
3 Forms it takes
- Separate hose with a valve at each end. Vaillant's website describes it as silver with a flexible connection8.
- Keyed link in the base of the boiler, with a removable key3.
- Lever types, such as Worcester's keyless link and Baxi's green Easy-Fill lever37.
- Fittings set up by the installer, such as the double non-return valve line in Vaillant's 2023 manual9.
- An automatic filling accessory, on Worcester's 4000, programmed by the installer3.
On the Worcester 4000 the keyed and keyless links sit behind a clip-off lower panel, which that manual shows; the boiler case itself stays shut3. Where no Easy-Fill was fitted, Baxi says it cannot advise how to repressurise and to ask your installer7.
4 When a loop lets water through
Vaillant's website names two troubles. A filling device that is not fully closed, or that passes water when shut, makes pressure climb steadily10. It also says filling loops can leak, and to check both ends are secure1. A slow creep upward on the gauge with nothing else changing is a reason to look at the loop, then call someone; see pressure too high. Do not try to repair a faulty valve yourself.
From the Baxi channel on YouTube. Baxi shows how to tell an Easy-fill link, a rigid link and a flexi loop apart and how each one is used, and it applies to Baxi boilers with a needle gauge; those with a digital gauge have a separate video. Follow your own boiler's manual for the target pressure. Nothing loads from YouTube until you press play.
Sources
- How to repressurise a boiler, Vaillant read 3 November 2026.
- Baxi's glossary read 3 November 2026.
- Worcester 4000 manual 6720891170, p8, 2020/09, read 3 November 2026.
- Main Eco Compact Combi user guide 7716571-04, pp17-18, read 3 November 2026.
- Water Supply (Water Fittings) Regulations 1999, Schedule 2, para 24 read 3 November 2026.
- Heatslave oil guide 8716106253, p9, 03.2010, read 3 November 2026.
- Baxi Platinum guide 7697821-01, p6, 1/18, read 3 November 2026.
- Vaillant, why is my boiler losing pressure read 3 November 2026.
- ecoTEC plus operating instructions 0020308119_03, p15, 28.11.2023, read 3 November 2026.
- Vaillant, what pressure should my boiler be read 3 November 2026.