Filling Loops: Topping Up a Sealed System Safely
The filling loop is the flexible braided hose and double check valves used to top up a sealed heating system's pressure from the mains.
The filling loop is the flexible braided hose and double check valves used to top up a sealed heating system's pressure from the mains. It should be disconnected after use, and a loop that won't seal is a common, low-drama fault worth fixing properly.
What it does
Two valves at each end let you connect the mains supply to the heating circuit temporarily, refill lost pressure, then isolate and disconnect it again.
How it fails
The check valves inside wear and let mains water trickle through even when 'closed', slowly overpressurising the system, or the loop is left connected and forgotten.
Repair or replace
A loop with worn check valves is replaced as a unit — it's not economical or reliable to repair the internal valves individually.
Why filling loops cause repeat call-outs
- Pressure creeping up days after it was last topped up
- Loop left permanently connected after a refill
- Valves that won't fully shut off, letting mains pressure through
- External loops corroding where they've been left exposed
Who does the work
- Gas Safe registered engineers — registration 554328
- Worcester Bosch accredited installer
- John The Plumb Ltd, company registration 10026483
Fix a filling loop that won't behave
Tell us whether pressure rises after you've disconnected the loop and we'll check whether the valves are actually sealing.
Questions we get asked
Should I leave the filling loop connected?
No, it should be disconnected after topping up pressure — a permanently connected loop is one of the most common causes of creeping pressure.
Can I top up pressure myself?
Many homeowners do this safely by following the boiler manual, but if pressure keeps dropping or rising afterwards that points to a fault worth having checked.
Is a filling loop Gas Safe work?
The loop itself is a plumbing fitting, but where it connects into a combi boiler's internal pipework, related work is usually done alongside Gas Safe registered servicing.