What should my boiler pressure be?
Most modern sealed-system and combi boilers are designed to run at around 1 to 1.5 bar when the heating is cold. When the system heats up the water expands, so it is normal to see the gauge rise to roughly 2 bar while the heating is on. Always check your boiler manual, as some manufacturers specify a slightly different range.
- Below 1 bar — pressure is low. Many boilers lock out and show a fault code below about 0.5 bar.
- 1–1.5 bar (cold) — normal.
- Around 2 bar (hot) — normal while the heating is running.
- 3 bar or above — too high. The pressure relief valve may open and discharge water outside.
How to top up (repressurise) your boiler
Most combi boilers have a filling loop — either a braided silver hose with two small valves underneath the boiler, or a built-in key or lever on brands such as Worcester Bosch and Vaillant. The general process is:
- Turn the heating off and let the system cool down.
- Make sure both ends of the filling loop are securely connected.
- Slowly open the valves (or turn the built-in key) and watch the pressure gauge.
- Close the valves once the gauge reads about 1.2–1.5 bar.
- Reset the boiler if it is showing a fault, then turn the heating back on.
Only top up through the filling loop. Never remove the boiler casing or attempt repairs inside the boiler yourself — by law, work on gas appliances must be carried out by a Gas Safe registered engineer.
Why does my boiler pressure keep dropping?
A sealed heating system should hold its pressure. Topping up once after bleeding radiators is normal, but if you are topping up every few days or weeks, water is escaping somewhere. The most common causes are:
1. A leak in the system
Check around radiator valves, towel rails, exposed pipework and under the boiler for drips, damp patches, green or white crusting on copper joints, or rust marks. Small leaks under floorboards are easy to miss.
2. A passing pressure relief valve
The pressure relief valve (PRV) protects the boiler from over-pressure and discharges through a small copper pipe that usually runs outside. If that pipe is dripping, the PRV may be worn or the pressure may be going too high when the system heats up.
3. A failed expansion vessel
The expansion vessel absorbs the extra volume of water as it heats. If it has lost its air charge, the pressure climbs sharply when the heating is on, the PRV opens, and the pressure then drops once the system cools. A big swing between cold and hot readings is the classic symptom.
4. Recently bled radiators
Bleeding radiators releases trapped air, which lowers the pressure. This is expected — top up once afterwards and it should stay put.
5. Internal boiler faults
Less commonly, a leak from an internal component such as the heat exchanger or a pump seal causes pressure loss. These can only be diagnosed and repaired by a qualified engineer.
When to call an engineer
- You need to top up more than once a month
- The pressure rises above 2.5–3 bar when the heating is on
- Water is dripping from the copper pipe outside
- You can see water inside or underneath the boiler
- The boiler keeps locking out with a pressure-related error code
Our Gas Safe engineers diagnose pressure faults across Hornchurch, Romford, Upminster and Essex every week, and we carry common parts such as relief valves and filling loops on the van. Book a boiler repair, or if the pressure loss is down to an old, sludged-up system, ask about a power flush.