Boiler guide · Edinburgh

Why Does My Boiler Keep Losing Pressure?

A boiler that keeps losing pressure usually has a leak somewhere or a faulty part. You can safely top up the pressure yourself in most cases, but if it keeps dropping the cause needs finding.

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What the pressure gauge should show

Most sealed-system boilers, including combis, show a pressure gauge on the front. The correct reading is set by the manufacturer, and for many boilers it is roughly 1 to 1.5 bar when the heating is cold. Check your boiler's manual for its own figure. A reading that has fallen close to zero will often stop the boiler working and may show a fault code.

Common causes of pressure loss

  • A leak in the heating system: a weeping radiator valve, a pipe joint, or a leak under the floor. This is the most common cause when pressure drops steadily.
  • You recently bled a radiator: releasing air also lowers pressure slightly. Topping up usually fixes it.
  • A faulty pressure relief valve: water may drip from the discharge pipe outside, where it leaves the house.
  • A failed expansion vessel: pressure rises sharply when the heating is on, then drops back, and the relief valve may discharge.
  • A filling loop left slightly open or leaking: check the loop is fully closed at both ends.

How to top up the pressure safely

  1. Turn the heating off and let the system cool.
  2. Find the filling loop, usually a braided hose under the boiler with a valve or lever at each end. Your manual will show it.
  3. Open the valves slowly while watching the gauge.
  4. Close both valves when the gauge reaches the recommended pressure.
  5. Check the loop is fully closed and turn the heating back on.

Do not leave the filling loop open, and do not open the boiler's casing. Only a Gas Safe registered engineer should work inside a gas boiler.

When to call an engineer

  • The pressure drops again within days, which means there is a leak or a fault.
  • Water is dripping from the discharge pipe outside.
  • Pressure rises very high when the heating is on.
  • You can see water around the boiler or on pipework.

A leak on the heating pipework is a job for a plumber, and a fault inside the boiler is a job for a Gas Safe engineer. Book a boiler service or repair, or see our leak detection and repair.

If you smell gas, do not use switches or flames. Open windows, turn off the gas at the meter if it is safe to do so, and call the National Gas Emergency Service free on 0800 111 999.

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FAQs

Frequently Asked Questions

For many sealed-system boilers it is around 1 to 1.5 bar when cold, but check your manual for your boiler's own figure.

The usual cause is a leak in the heating system. Other causes are a faulty pressure relief valve, a failed expansion vessel, or a filling loop that is not fully closed.

In most cases yes, using the filling loop and following your manual. Do not open the boiler casing. If the pressure keeps dropping, call an engineer.

Low pressure itself usually stops the boiler working rather than causing danger, but a leak needs fixing. If you ever smell gas, call 0800 111 999.

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