What is a Boiler Filling Loop?

A boiler filling loop is a simple but essential part of most modern combi and system boilers in the UK. It’s the flexible hose (sometimes with a braided metal cover) and valves that allow you to top up the central heating system pressure from the mains cold water supply when the pressure drops too low. 

Understanding how your boiler filling loop works can save you money, prevent bigger problems, and help you decide when something is safe to do yourself or when you need a professional.

In this guide, I’ll explain everything you need to know about filling loops — what they look like, why pressure drops, common problems, and step-by-step fixes.

What Is a Boiler Filling Loop?

A boiler filling loop is the connection between your mains cold water pipe and the sealed heating system circuit. Because combi and system boilers are “sealed systems”, they don’t have a feed-and-expansion tanks in the loft like older open-vented setups. 

Instead, when water is lost through small leaks or bleeding radiators, the system pressure falls and the boiler may show a low-pressure fault (e.g. E118 on Vaillant, F22 on Worcester Bosch, or just a flashing low-pressure symbol).

The filling loop lets you safely re-pressurise the system, usually to between 1 and 1.5 bar when cold.

There are three main types you’ll see in UK homes:

• External filling loop (flexible hose with two valves — usually supplied with the boiler)
• Built-in filling loop (hidden under or integrated inside the boiler)
• Filling key type (plastic key inserted into a special valve — common on Ideal and Baxi)

Why Does Boiler Pressure Drop and Need a Filling Loop?

Boiler pressure drops for completely normal or problematic reasons. Common reasons include:

• Bleeding radiators (air comes out, a tiny amount of water follows)
• Minor leaks on radiators, valves, or pipework
• Automatic air vents or pressure relief valve (PRV) weeping
• Recent boiler or radiator work
• Very rarely — a leak inside the boiler (heat exchanger or expansion vessel issue)

When the pressure gauge falls below about 0.5–0.7 bar, most boilers lock out and stop working until you top the system up again using the filling loop.

How to Use a Boiler Filling Loop Correctly (Step-by-Step)

Using a filling loop is straightforward and safe if you follow these steps:

1. Make sure the boiler is switched off and cooled down.
2. Locate your filling loop (usually below or beside the boiler).
3. If it’s an external loop, make sure both ends are securely attached.
4. Slowly open valve on the mains side (usually the one with the blue or black cap).
5. Slowly open valve on the boiler side.
6. Watch the pressure gauge rise — aim for 1–1.5 bar when cold.
7. Close valve tightly, then the other.
8. Disconnect the flexible hose if it’s an external type (to comply with water regulations).
9. Check for drips and wipe the valves.

Never leave an external filling loop permanently connected — it’s against UK water bylaws.

Common Boiler Filling Loop Problems and Causes

The Filling Loop Is Leaking Constantly

A filling loop that drips or leaks even when both valves are closed is one of the most common issues.

Causes and how to fix each one:

Cause 1 – Worn or damaged washer/seal in the valve

The rubber washers inside the double-check valve or isolating valve perish over time.

Fix: Turn off the mains water, open both valves to release pressure, then unscrew the leaking valve and replace the washer (kits cost £2–£5 from plumbing merchants or online). Tighten firmly but not excessively.

Cause 2 – Valve not fully closed or cross-threaded

People sometimes don’t close the valves fully or they get knocked.

Fix: Simply turn both handles clockwise until finger-tight. If the handle spins freely, the spindle may be broken — replace the whole valve (£15–£25).

Cause 3 – Limescale buildup in hard-water areas

Common in London, South East, and Midlands.

Fix: Dismantle the valve, soak in descaler (e.g. Fernox F3 or kettle descaler) for 30 minutes, rinse, and reassemble with new washers.

Boiler Pressure Keeps Dropping After Using the Filling Loop

You top up to 1.5 bar and within days or weeks it’s low again.

Causes and fixes:

Cause 1 – Leak on the heating system (radiators, pipes, towel rail)

Fix: Look for damp patches under pipes, white salt marks, or hissing. Tighten compression joints or replace olive if leaking. Small radiator valve leaks can often be fixed by draining the radiator and replacing the gland seal.

Cause 2 – Pressure relief valve (PRV) discharging outside

You’ll see water dripping from the 15mm copper pipe outside the house.

Fix: The PRV may be passing because system pressure is too high (>3 bar) or the valve seat is dirty/damaged. Try gently tapping the valve with a spanner. If it continues, a Gas Safe engineer must replace it.

Cause 3 – Faulty expansion vessel

The internal air cushion has lost pressure or the diaphragm has split, so the system can’t cope with expansion when hot.

Fix: This is not a DIY job. A Gas Safe engineer needs to re-charge or replace the vessel.

Cause 4 – Leak inside the boiler (heat exchanger or diverter valve)

Fix: Only a Gas Safe registered engineer can open the combustion chamber and repair this.

You Can’t Get Any Water Through the Filling Loop

Nothing happens when you open both valves.

Causes and fixes:

Cause 1 – Double-check valve stuck or blocked

Fix: Close both valves, remove the flexible hose, and gently poke a small screwdriver or piece of wire into the double-check valve to free it. Reassemble and retry.

Cause 2 – Mains stopcock or isolating valve under the boiler is turned off

Fix: Trace the blue-handled valve on the cold mains pipe under the boiler and turn fully anti-clockwise.

Cause 3 – Built-in filling valve failed (common on Vaillant ecoTEC and Worcester Greenstar)

Fix: Requires a new filling valve assembly — Gas Safe job.

How to Prevent Filling Loop and Low-Pressure Problems in the Future

• Have your boiler serviced every 12 months by a Gas Safe engineer — they will spot weeping joints and re-charge the expansion vessel.
• Fit magnetic system filters (e.g. MagnaClean or Fernox TF1) to catch sludge and reduce corrosion.
• Bleed radiators once a year and top up slowly.
• Never leave an external filling loop permanently connected.
• Install a small leak detector or water shut-off device if you’re a landlord or go away often.
• Add inhibitor (Fernox F1 or Sentinel X100) every 5 years or after any drain-down to protect the system.

When You Absolutely MUST Call a Gas Safe Engineer

You should never attempt these repairs yourself:

• Any work inside the boiler combustion chamber
• Replacement or re-charging of the expansion vessel
• Suspected heat exchanger leak (sweet smell, black staining, or kettle-like noises)
• Pressure relief valve replacement
• Repeated pressure loss with no visible external leak
• Filling loop is built-in and has failed

Working on gas appliances or sealed combustion chambers without Gas Safe registration is illegal and extremely dangerous.

FAQs

Is it illegal to leave the external filling loop connected all the time in the UK?

Yes — Water Supply (Water Fittings) Regulations 1999 require a double-check valve and that the temporary hose is removed after use to prevent backflow contamination.

Can I use a permanent filling link instead of a flexible hose?

Yes, many engineers now fit an approved permanent filling link with an extra isolating valve. It’s neater and still complies with regulations.

My new boiler doesn’t have a visible filling loop — how do I top up the pressure?

Many modern boilers (e.g. Viessmann Vitodens 050, Worcester 2000/4000) have a filling key or lever hidden under the boiler. Check your manual or look for a white plastic slot or blue lever.

How often is it normal to have to use the filling loop?

Once or twice a year after bleeding radiators is fine. More than that usually means there’s a leak that needs fixing.

Will repeatedly using the filling loop damage my boiler?

No, as long as you only add the correct amount (1–1.5 bar cold) and the water contains inhibitor. Adding fresh oxygenated water without inhibitor every few weeks will cause corrosion.

Stay warm, The Boilers2Go Team