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Boiler Filling loop: What It Does and Signs It's Failing

The filling loop connects the mains cold water supply to the heating circuit so you can put back pressure the system has lost. It's either a braided hose with two valves under the boiler, a key that plugs into the bottom of the boiler, or a built-in lever. Where the filling device is fitted separately, Vaillant says the installer sets it up and should show you how to use it.

How it works

Opening the valves lets mains water, at a higher pressure than the heating, flow into the system and push the gauge reading up. A non-return valve in the loop stops heating water flowing back into your drinking supply. When you're done, both valves close and the hose comes off so the two systems stay apart.

Signs it is failing

• Pressure creeping up on its own, a sign a valve wasn't closed fully
• Drips from the valves or the hose connections
• No water flows when both valves are open
• Handle that spins freely or won't turn
• Lost key or missing hose on a keyed or detachable loop

Topping up through the loop is the first step for every low-pressure code. If you're needing it often, there's a leak or a faulty vessel or relief valve to find.

Can you replace it yourself?

Topping up is an owner job, but follow your boiler's user manual exactly: with the boiler cold, open the valves slowly, stop at about 1 to 1.5 bar, close both and take the hose off. Don't open the casing to reach an internal loop, and a leaking or seized loop is a job for a Gas Safe registered engineer. Worcester notes that some water will drip when the hose comes off, so keep a cloth handy.

Buying the right part

External braided loops are fairly standard, but keyed and lever loops are built into the boiler and use model-specific parts. Ask the engineer to fit an approved loop with a double non-return valve, the type Vaillant supplies.

Error codes linked to the filling loop

• E118Low water pressure in the heating circuitBaxi
• E119Low primary water pressure (Platinum Combi)Baxi
• F.22Water pressure in the heating system is too lowVaillant
• CE 207 / H07Heating system water pressure too lowWorcester Bosch

Last reviewed 2026-10-21.