Glossary · Boilers and boiler types
What is a filling loop?
A filling loop is the short flexible hose, with a valve at each end, that connects the cold mains supply to a sealed central heating system so the water pressure can be topped up. Under water regulations it should not stay connected, so many are removed or stored detached after use.
Key points
- It links the cold mains to the heating pipework, but only while you are topping up the pressure.
- The Worcester user guide shows a cold-system target of 1 to 1.5 bar. Your own boiler's guide gives the figure that applies to you.
- Both valves must be closed again when the pressure is right, and the Worcester guide says the hose must be removed after use to meet water regulations.
- Only add water when the system is cold. Needing to top up often points to a leak or a fault.
- If the loop looks different to the one in your guide or cannot be found, the guide says to contact your installer.
How a filling loop works
A sealed heating system has no loft tank to top it up. The water inside is under pressure, which you read on the gauge on the front of the boiler. Over time that pressure can drop a little, for example after you bleed a radiator. The filling loop is the way to put water back.
It is a short flexible hose with a shut-off valve at each end. One end connects to the cold mains and the other to the heating return pipe. Open the valves and mains water flows into the system and the gauge rises. Close them and the filling stops.
Because it connects drinking water to heating water, it is controlled by water regulations. The Worcester installation manual says a sealed system must be filled using a WRAS approved filling loop.[2]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions Water Regs UK explains that where a fill point has a flexible hose, it is good practice for the hose to be completely disconnected and removed when not in use, although detaching one end on the right side of the backflow device may be acceptable.[3]Water Regs UK, When can a filling loop be used? The Worcester user guide is blunt about it: to comply with the Water Authority regulations you must disconnect the external filling loop after re-pressurisation.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: User instructions
What the manufacturer’s guide describes
The user guide for the Worcester Greenstar 24/28/30 combi shows the best cold pressure as between 1 and 1.5 bar, and says to check regularly that the pressure is maintained.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: User instructions For topping up, it describes a householder procedure with an external loop, in outline:
| Stage | What the guide says |
|---|---|
| Before starting | Make sure the isolating valves are shut, then fit the loop hand-tight to both valves |
| Filling | Open one valve a quarter turn, then slowly open the second until you hear water flow and the gauge rises |
| Stopping | Close the valve at 1 to 1.5 bar, then close the other valve |
| Finishing | Check both valves are shut, remove the hose and replace the blanking caps |
The guide also says that if the gauge reads over 1.5 bar after over-filling, you should bleed one radiator until it falls back to between 1 and 1.5 bar.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: User instructions The installation manual adds that the system should only be topped up when the appliance is cold.[2]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions The table is a summary of one model’s guide and not a substitute for your own. Boilers with a built-in loop or a different arrangement have different instructions, and the guide says that if your loop does not look like the one shown, or you cannot find it, you should contact your installer.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: User instructions
When topping up is a warning sign
One top-up after bleeding a radiator is routine. Frequent top-ups are not. The same guide says that if water leaks are found or excessive bleeding is needed, a service engineer must be contacted to inspect the installation.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: User instructions Our pages on low water pressure and boiler leaking water cover the symptoms, and a pressure relief valve that discharges points the other way, to pressure that is too high. If the cause is inside the boiler, it is gas appliance work and an engineer’s job. BASI handles these under boiler repair.
Example
Mark in Keighley bleeds the radiator in his spare room and sees the gauge drop below the range in his guide on a cold system. He reads the user guide for his own boiler, follows its filling steps and closes the valves when the gauge reaches the range it gives. He removes the hose and replaces the caps. Two weeks later the gauge is low again, and a second top-up is followed by a third. That pattern is the cue to call an engineer, not to keep filling.
What people get wrong
Leaving the filling loop connected is the most common error. It feels handy, but the Worcester guide says it must come off after use to meet water regulations, and a permanently open link between mains and heating water is the thing the rules are written to prevent.
The second error is filling a hot system, or filling past the top of the range and leaving it. The installation manual says to top up when the boiler is cold, and the user guide shows how to bring an over-filled system back down by bleeding a radiator.
The bottom line
A filling loop is the temporary hose that lets you put water back into a sealed heating system. Use it only as your own boiler’s user guide describes, fill to the pressure range it gives, close both valves and take the hose off afterwards. If the pressure keeps dropping, stop topping up and ask a Gas Safe registered engineer to find the cause.
Common questions
What is a filling loop used for?
It adds mains water to a sealed heating system so the pressure on the boiler gauge returns to its normal range. It is connected, used and then closed off again.
What pressure should I fill to?
The Worcester Greenstar user guide we checked says 1 to 1.5 bar on a cold system. Boilers differ, so follow the figure in your own user guide or ask your installer.
Should the filling loop stay connected?
No. The Worcester guide says to disconnect the external filling loop after re-pressurising, to comply with water regulations. Water Regs UK says a hose is best fully disconnected when not in use.
Why does my boiler keep losing pressure?
Repeated top-ups point to water escaping somewhere, such as a leak or a fault on the system. The manufacturer says to contact an engineer if leaks are found or the pressure keeps changing.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Worcester, Bosch Group. Greenstar 24/28/30 RSF combination boiler: User instructions. Checked 11 November 2026. ↩
- Worcester, Bosch Group. Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions. Checked 11 November 2026. ↩
- Water Regs UK. When can a filling loop be used?. Checked 11 November 2026. ↩