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Fixing Your Boiler Pressure: Increase or Decrease Pressure

The pressure on your boiler can fluctuate for different reasons. Whether you have high or low boiler pressure, we explain here the reasons for the increase or decrease, and how to re-pressurise your boiler.

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Simply answer a few multiple choice questions about your existing boiler and we’ll give you a fixed price for a new boiler including all parts and installation[28]

What pressure should my boiler be?

Your boiler's pressure is usually displayed on the boiler's built-in pressure gauge, and should read around the 1 bar mark when you are not running any hot water and the central heating is not turned on. Therefore, 1 bar is a low-pressure setting.

Why is my boiler pressure high?

Once the boiler begins to heat water, the heated water expands and causes the pressure gauge to increase. With an in-built pressure relief valve, boilers are well-set to control these quick leaps in pressure so you don’t need to worry if the boiler shows an higher pressure than you expected when you are both using the central heating and running hot water. Once you have finished using the heating and hot water, the pressure should lower back to safe levels almost immediately.

How to lower the boiler pressure:

  1. Check that the pressure relief valve is tightly closed. When you re-pressurise your boiler you'll need to turn the valve or filling loop to pump water into the boiler, which can be easy to forget to close. If you haven’t tampered with the boiler’s pressure recently, it is possible that you may have accidentally knocked the valve or it has loosened over time.
  2. Bleed your radiators to reduce the pressure in your central heating system and therefore in your boiler. Keep bleeding the radiator until the boiler pressure read on the display has restored back to safe levels.

If your boiler pressure still persists to be too high, there could be one of two issues; Either you may need to re-pressurise the expansion vessel, or you could have a fault with the filling loop. However, you will not be able to do this yourself and will need to call a Gas Safe engineer. Even though the pressure may still be irregular, the pressure relief valve should prevent the boiler from overpressurising so you will still be safe to carry on as usual as long as you are not experiencing loud, unusual noises coming your boiler and your boiler is still functioning properly.

Why is my boiler pressure low?

Your boiler pressure is too low if your pressure gauge is showing that your boiler pressure is below the 1 bar mark. It’s important that you increase the boiler pressure to its normal state as soon as possible.

How to increase your boiler pressure:

The boiler must be switched off and completely cool before starting to re-pressurise.

To increase the boiler pressure, you will need to find the filling loop. This is a flexible hose found underneath your boiler with a small valve at each end. Then make sure both ends of the hose are secure and tightly attached to the two valves.

By using a flat headed screwdriver, you can open the valves by their tap handles. This will then allow the cold mains water into the system, you should hear this filling up the system.

The pressure will then increase so keep a close eye on the pressure gauge until it reaches a 1.5 bar mark; you can then close off both valves, one at a time. As your boiler is now at the desired pressure, feel free to switch the boiler back on, and follow by resetting the boiler.


If you still have any concerns regarding your boiler pressure, get in touch with our experts on 0203 805 4581 .

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[28] Exclusions apply. See www.help-link.co.uk/fixed-price-quote-faqs/