How to Top Up My Water Pressure

🔧 How to Top Up My Heating System Water Pressure: A Step-by-Step Guide

If your radiators are cold at the top, your boiler is showing a low-pressure warning, or you're hearing strange noises in the system, it might be time to top up your heating system's water pressure.

Don’t worry — this is a common issue in sealed (pressurised) central heating systems, and it’s usually quick and safe to fix yourself. In this guide, we’ll walk you through how to check and top up your boiler pressure step by step.

In this example, we're using an Ideal Logic Combi c30 boiler, but the steps are the same for most modern combi boilers.

📉 What Is Heating System Pressure?

Modern combi and system boilers use pressurised sealed systems to circulate hot water. These systems rely on a set level of water pressure (usually 1.0 to 1.5 bar when cold) to operate efficiently.

If the pressure drops too low, the system may:

  • Fail to heat your home properly
  • Lock out with a fault code
  • Cause damage over time due to air entering the system

🛑 Before You Begin

Check the following:

  • Your boiler’s manual for the correct pressure range
  • Whether you have an external filling loop or an integrated filling loop
  • Ensure the system is cool (topping up when hot can give false readings)

⚠️ Warning: Only attempt this if you’re confident. If your pressure keeps dropping or your boiler is leaking, call a Gas Safe engineer — there may be a fault in the system.

📊 Step 1: Check Your Boiler Pressure Gauge

Look at the pressure gauge on the front of your boiler or nearby pipework.

  • The needle should be in the green zone (usually 1.0–1.5 bar when cold)
  • If it's below 1.0 bar or flashing red, you'll need to repressurise the system

🧰 Step 2: Locate the Filling Loop

There are two common types:

🔁 External Filling Loop

  • Usually a silver flexible braided hose with two tap handles or screw valves at each end
  • Located underneath the boiler, on pipework nearby or in an airing cupboard

🔧 Built-In Filling Loop

  • Found on many combi boilers
  • Small black or blue tap handles underneath the boiler
  • May only have one valve or lever

💧 Step 3: Open the Filling Valves

For an external filling loop:

  1. Turn both valves slowly to open — you should hear water flowing
  2. Watch the pressure gauge rise
  3. Once it reaches 1.2–1.5 bar, turn both valves back off firmly
  4. Remove the filling loop if instructed in your manual

For a built-in filling loop:

  1. Turn the lever or knob anti-clockwise slowly
  2. Stop once the pressure reaches the recommended level
  3. Turn the lever off (clockwise) until tight

⚠️ Don’t overfill — excess pressure can damage the system and trigger the pressure relief valve.

🔁 Step 4: Reset Your Boiler (if required)

Some boilers require a manual reset after topping up pressure. Look for a reset button on the control panel or consult your boiler’s manual.

🔍 Step 5: Monitor for Pressure Drops

Check the pressure again over the next few days:

  • If it drops again, you might have a leak, faulty expansion vessel, or pressure relief valve issue
  • In that case, contact a Gas Safe Registered engineer

✅ Ideal Pressure Levels

  • Cold system: 1.0 to 1.5 bar
  • Hot system: Up to 2.0 bar
  • Never exceed: 3.0 bar — the safety valve will open to release pressure

🧼 Pro Tip: Bleed Radiators After Topping Up

Topping up the system can sometimes introduce air. After repressurising:

  • Bleed your radiators using a radiator key
  • Then check pressure again and top up if needed

🛠 When to Call a Professional

Call a Gas Safe engineer if:

  • Pressure drops repeatedly
  • You notice leaks from pipework or boiler
  • Boiler keeps locking out despite pressure being OK
  • You’re unsure how to safely operate the filling loop

🔚 Conclusion

Keeping your boiler at the correct pressure helps it run safely and efficiently. Topping up the water pressure is a simple task for most homeowners — just follow the steps carefully and never force anything.

Need help with persistent pressure issues or want a system health check?

👉 Contact Gas-Go Heating — your trusted local Gas Safe engineers. We offer expert boiler servicing, repairs, and full system diagnostics.

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