Boiler Filling Loop Types Explained
Identify the most common boiler filling loops used in UK homes
Boilers use different types of filling loops to allow the system pressure to be topped up.
The type you have depends on the boiler model, installation age, and how your system was originally installed.
Below are the most common boiler filling loop types found in UK homes, with images to help you identify yours.
Internal Filling Loop (Built-In)
What it looks like
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Usually two small levers or taps or a lever type (see below images)
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Located under the boiler casing
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Often black or blue plastic
Common notes
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Found on many modern combi boilers
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Valves are normally closed when not in use
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Overfilling can occur if valves are left open
Identification tip
If you can see two small taps or a lever directly under the boiler, you likely have an internal filling loop.
Common internal filling loop types
Choose your boiler manufacturer for step-by-step instructions:
→ How to Top Up Worcester Boiler Pressure
→ How to Top Up Baxi Boiler Pressure
→ How to Top Up Vaillant Boiler Pressure
→ How to Top Up Ideal Boiler Pressure
→ How to Top Up Viessmann Boiler Pressure
External Silver Flexible Filling Loop (Hose Type)
What it looks like
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Silver braided flexible hose
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Connects the cold mains to the heating pipework
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Has one or two isolation valves
Common notes
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Very common on new & older installs
- Can be fitted to most combi boilers
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Hose may be permanently fitted or removable
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Valves must be fully closed after topping up
Identification tip
If you can see a silver braided hose near the boiler or pipework, this is an external filling loop.
Keyed Filling Loop (Plastic Key System)
What it looks like
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Small plastic key (often blue or black)
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Key slots into a valve under the boiler
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Pressure won’t rise unless the key is inserted correctly
Common notes
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Used by several manufacturers
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Keys are sometimes removed and stored nearby
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Losing the key prevents pressure top-up
Keyed filling loops are commonly used on Worcester Bosch boilers and some Ideal models.
Follow the guide for your boiler:
Older / Non-Standard Arrangements
What this means
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Some older systems use:
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Remote valves
- Not near boiler or concealed
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Unusual pipework layouts
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Non-standard arrangements
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Important note
If you’re unsure or can’t confidently identify your filling loop, it’s best to stop and seek professional advice.
Key Takeaways
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UK boilers use several different filling loop types
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Internal, external hose, and keyed loops
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Correct identification matters before topping up pressure
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If unsure, don’t force valves or guess
👉 Not sure what to do next?
Read our guide on How to Top Up Boiler Pressure Safely