A filling loop is the part of your boiler that lets you top up the system pressure when it drops. There are three common types in UK homes, and which one you have determines exactly how you use it. If your boiler is showing low pressure or has locked out, this is the part you'll need.
What a filling loop does
Your central heating system (the boiler, the pipework, and the radiators) is a sealed loop of water. The same water circulates around all your radiators, gets heated by the boiler, and circulates back. None of that water is connected to the mains supply during normal operation. It's deliberately isolated.
The filling loop is the only way to add fresh mains water to that sealed system. It has two valves: one to control the flow of water from the mains, and a second (called a double-check valve) to stop the heating system water flowing the other way back into the mains. When both valves are open, mains water flows in and the system pressure rises.
The three types of filling loop
Before you do anything, it's worth identifying which type yours is. The location is roughly the same for all three (under the boiler, near the pipework), but they look and operate differently.
External filling loop
A flexible braided hose with two valves, usually silver. Connects to a pipe coming out of the boiler at one end and to the mains water supply at the other. Often supplied loose with the boiler when it was installed, sometimes hung up next to the boiler for occasional use.
This is the most common type on system and regular (heat-only) boilers. Some older combi installations also use this style.
Important: External filling loops must be disconnected after use. This is required under the Water Supply (Water Fittings) Regulations 1999 to prevent any chance of contaminated heating system water flowing back into the mains. Leaving an external loop connected, even with the valves closed, is a regulatory breach.
Internal (integrated) filling loop
Built into the underside of the boiler. There's no separate hose to connect or disconnect. Instead you'll find a small lever, knob, or switch on the boiler itself, often colour-coded blue or black. Common on most modern combi boilers including Vaillant ecoTEC, Worcester Greenstar, and Viessmann Vitodens models.
Because internal loops are integrated and have built-in backflow prevention, they stay in place permanently. There's nothing to remove after use.
Filling key type
A small plastic key is inserted into a slot on the underside of the boiler to enable filling. Without the key inserted, the valves can't be opened. Found on certain combi boilers as a halfway point between external and internal types. The key is usually supplied with the boiler and should be kept somewhere safe near it.
Why your boiler needs the right pressure
Modern combi boilers are sealed systems that need a specific working pressure to function safely. Most run between 1.0 and 1.5 bar when cold, rising slightly when the boiler is hot. There's a pressure sensor inside the boiler that measures this constantly.
If the pressure drops below around 0.5 bar, the sensor tells the boiler to shut down. It's a safety feature: a low-pressure system can damage the boiler's heat exchanger if it's allowed to run dry. The boiler will display a low pressure fault code, and won't fire up again until the pressure is back where it should be.
This is when you need the filling loop.
How to top up your pressure
The exact procedure varies between boilers, but the principle is the same. Always check your boiler manual for model-specific instructions.
- Turn the boiler off and let it cool down. A cold system gives an accurate pressure reading and is safer to work on
- Find your filling loop and identify which type you have (external hose, internal lever, or key-operated)
- Check the pressure gauge on the front of the boiler. If it's below 1 bar, you need to top up
- Connect or activate the filling loop. For external loops, screw both ends to the valves and tighten. For internal loops, locate the lever or switch. For key types, insert the plastic key into the slot
- Open both valves slowly. You'll hear water flowing into the system
- Watch the pressure gauge rise. As soon as it reaches around 1.2 bar, close both valves. Don't go above 1.5 bar cold
- If you have an external loop, disconnect it. This is a legal requirement under UK water regulations. Internal loops stay in place
- Reset the boiler if it's locked out, usually by pressing the reset button on the front panel
If you can't find your filling loop: Some installations have it tucked behind a cupboard panel or built into the boiler in a way that isn't obvious. If the underside of your boiler is hidden by trim or pipework, check your boiler manual for a diagram, or look online for the make and model. If you genuinely can't find it, call us and we'll talk you through where to look on your specific boiler.
Why your pressure keeps dropping
Topping up the filling loop is a normal occasional job, perhaps once or twice a year on a healthy system, usually after radiators have been bled or following a particularly cold spell. If you find yourself topping up the pressure every few weeks, something's wrong.
Common causes of recurring pressure loss:
- A leak somewhere in the system. Often hidden under floors, behind radiators, or at pipe joints. A pressure test can locate it
- A failed expansion vessel inside the boiler. The expansion vessel absorbs pressure changes as the water heats and cools. When it fails, the pressure swings cause the relief valve to dump water
- A leaking pressure relief valve, sometimes caused by a failed expansion vessel above, sometimes by debris stopping it from closing fully
- A leaking automatic air vent on top of the boiler, which can leak slowly enough that the source isn't obvious
- A faulty pressure sensor. The system pressure may actually be fine, but the sensor is reading low. A diagnostic visit can confirm this
- A leaking filling loop itself. The rubber washers in the double-check valve perish over time and can drip even when both valves are closed
When to call an engineer
If you're confident topping up the filling loop yourself, that's fine. It's safe and most owners do it without issue. But:
- If you can't find your filling loop
- If nothing happens when you open the valves (often a closed mains stopcock or a stuck double-check valve)
- If the pressure won't hold after topping up
- If you're topping up more than once or twice a year
- If you can see water leaking from anywhere
Call. The £99 call-out covers diagnosis and the first hour, and most pressure-related issues are sorted within that window.
Related advice
If your boiler is showing a low pressure fault and you'd like to read more about why pressure drops happen, the boiler repairs page covers the typical causes and the typical cost to fix each one.
