Boiler Pressure: What It Should Be, Hot and Cold, by Brand

What boiler pressure means

Combi and system boilers work on a sealed heating system. The water in the pipes and radiators is kept under a little pressure so the pump can move it round, the boiler's safety devices can work, and air is less likely to form. The gauge on the boiler shows that pressure in bar. One bar is roughly the pressure of the atmosphere, so a reading of 1.5 bar means the water is pushing out at one and a half times that.

Older regular boilers with tanks in the loft run on an open-vent system. They have no pressure gauge to read because the tank provides a small, fixed pressure. Everything on this page applies to sealed systems.

What should boiler pressure be when the heating is off?

With the heating off and the system cold, most boilers should read between 1 and 1.5 bar. This cold reading is the one that matters, because it is the figure manufacturers give and the one you top up to. Check it first thing in the morning before the heating comes on, or after the heating has been off for an hour or two.

What should boiler pressure be when the heating is on?

With the heating running, expect the pressure to be higher than the cold reading, often somewhere around 1.5 to 2 bar. British Gas describes around 2 bar as normal with the heating on. Baxi says the needle will go up a little when the boiler heats up but should not go higher than about 2.5 bar.

Why the pressure rises when the heating comes on

Water expands as it heats. In a sealed system that extra volume squeezes the air cushion in the expansion vessel, and the pressure goes up a little. A rise of around half a bar is typical. A rise of a bar or more, especially to near 3 bar, points to a problem: usually too much water in the system, or an expansion vessel that has lost its charge. Our guide to boiler pressure too high explains how to bring it down.

Boiler pressure by brand

Manufacturers publish their own figures. They are close to each other, but check the user manual for your exact model because some differ.

Sources: Worcester Bosch, Ideal Heating and Baxi homeowner advice pages. For Worcester boilers specifically, our Worcester boiler pressure guide covers the Greenstar filling methods.

Ideal boiler pressure

Ideal Heating gives a normal cold operating pressure of 1.0 to 1.5 bar for its boilers. It says that topping up once or twice a year is generally nothing to worry about, but to get help if the pressure drops repeatedly over days or weeks, falls to zero, or needs topping up more often than that. On Ideal Logic boilers, low pressure shows as the F1 code.

Combi boiler pressure

A combi and a system boiler use the same kind of sealed heating circuit, so the normal range is the same: 1 to 1.5 bar cold on most models. One thing that confuses people is the difference between boiler pressure and water pressure at the taps. The gauge only shows the pressure in the heating circuit. The pressure and flow of your hot water come from the mains supply and have nothing to do with the gauge reading. Weak hot water with a normal gauge reading is a separate issue, explained in our combi boiler guide.

How to read the boiler pressure gauge

There are two kinds of pressure gauge on a boiler.

Dial gauges

Older boilers, and many current ones, have a round dial marked from 0 to 4 bar, usually on the front panel or underneath the boiler. Many dials have coloured zones:

• Green zone: usually around 1 to 2 bar, the normal working range.
• Red zone at the low end: below about 0.5 to 1 bar. Worcester Bosch says a needle in the red below 0.5 bar means water has been lost and must be replaced.
• Red zone at the high end: around 3 bar, where the pressure relief valve opens.

Some dials also have a second, adjustable red needle. The installer sets it to mark the correct cold fill pressure for your system, so you can see at a glance if the main needle has drifted.

Digital displays

Many newer boilers show the pressure as a number on the screen. Some show it all the time, some only when a button is pressed, and some only when there is a fault. Your user manual shows where to find it.

Why boiler pressure changes over the year

Even a healthy sealed system does not hold exactly the same reading forever. Small amounts of water are lost each time radiators are bled or a valve is worked on, and tiny weeps at old joints can go unnoticed. Air slowly works its way out through automatic air vents, and the space it leaves is taken up by water, lowering the pressure a little. That is why Worcester Bosch and Ideal both say topping up once or twice a year is normal.

Temperature matters too. A reading taken with the radiators still warm from the evening will be higher than one taken the next morning. Always compare like with like: note the cold reading at the same time of day, and you will spot a real drop quickly.

Is low boiler pressure dangerous?

No, low pressure on its own is not dangerous. When the pressure falls too far, the boiler shuts down and shows a fault code to protect itself, which is a safety feature doing its job. The inconvenience is no heating, and on a combi, often no hot water either.

What matters is why the pressure dropped. Occasional small drops over months are normal. A pressure that keeps falling means water is escaping somewhere, from a leak, the relief valve or through bleeding radiators. Water leaking inside or near the boiler can reach electrical parts, so if you see water under the boiler, turn it off and get it checked. Our guides to boiler pressure too low and a boiler leaking water cover the fixes.

When to act

After bleeding radiators, always check the gauge, because letting air out lowers the pressure. Our guide to bleeding a radiator explains why.