What should boiler pressure be?

Normal boiler pressure, cold and hot

Boiler pressure is the water pressure inside your sealed central heating system, measured in bar. It is not your mains water pressure and it is not gas pressure. It describes only the loop of water running through your boiler, pipes and radiators.

The right number depends on whether the system is cold or hot, because water expands as it heats. Vaillant explains: "When water is heated, it expands. As the boiler heats the water within the central heating system, the increased temperature causes the hot water to expand, this results in a pressure rise." A rise of around 0.5 bar once the heating is on is normal.

When the system is cold

Worcester Bosch says that "When the heating system is cool, the pressure should be between 1 and 1.5 bar on the pressure gauge (the indicator needle would usually be in the green section)." Ideal Heating gives the same figure, saying your gauge "should read between 1 and 1.5 bar" with the heating off, and Vaillant says pressure "should generally be between 1 and 1.5 bars when central heating is turned off".

When the heating is running

Ideal Heating says pressure should "increase by around 0.5 bar, so it might sit closer to 2 bar while active", and Vaillant says the gauge "will usually sit between 1 and 2 bars when it is turned on". Baxi puts it slightly differently, giving 1.0 to 2.0 bar as the normal operating range across cold and hot rather than a separate cold figure, and saying that while the boiler is heating the needle rises a little but should not go higher than about 2.5 bar.

That is the whole answer for most homes. Roughly 1 to 1.5 bar cold, drifting towards 2 bar once the heating has been on a while, means nothing is wrong.

What each manufacturer publishes

How to read your boiler pressure gauge

Read it when the heating has been off for a few hours, so you are looking at a cold system. That is the reading every manufacturer figure above refers to.

Most combi and system boilers have a round dial on the front, or underneath behind the flap. Newer ones show the figure on the digital display. Worcester Bosch and Baxi both describe the needle sitting in the green section as normal.

One exception matters. If you have a regular boiler, also called a conventional, heat only or open vent boiler, you probably have no pressure gauge and nothing to top up. Vaillant describes it as "a system involving two tanks, a hot water cylinder and the boiler itself", and says homes with an open vent boiler "will also have a feed and expansion tank to maintain a consistent level of water in the heating system".

What a reading below 1 bar means

Below 1 bar cold there is not enough water in the sealed system, so radiators warm unevenly or the boiler refuses to fire and shows a low pressure code. Worcester Bosch treats below 0.5 bar as water having been lost that must be replaced.

That is a reading, not a diagnosis, and this page stops at the number. For the actual job of topping the gauge back up, filling loop by filling loop, read our step-by-step guide to raising low boiler pressure, linked below. If the gauge keeps sinking back after you top it up, that is a different problem again, and our guide to pressure that will not stay up is the one you want.

What a reading above the green band means

Where the line sits depends on temperature. Vaillant says a reading under 2.5 bar at room temperature needs no action and gives 3 bar as the upper limit. Baxi says the needle rises a little while the boiler heats but should not go higher than about 2.5 bar. Worcester Bosch treats above about 2.75 bar as the point to act on, and Ideal Heating uses the same 2.75 bar figure.

Again, that is the number rather than the fix. The steps for letting the excess out, and the reasons a gauge climbs in the first place, are in our guide to bringing high boiler pressure down, linked below.

When boiler pressure needs an engineer

Reading the gauge, bleeding a radiator and using the filling loop are homeowner jobs. These are not.

• You are topping up far more often than Worcester Bosch's once or twice a year, which points at a leak.
• There are damp patches, staining or drips near pipework, radiator valves or under the boiler. Vaillant is direct: "Do not attempt to repair loose or broken pipework joints unless you are trained to do so."
• The pressure will not rise when you open the filling loop. Vaillant says to close the filling loop immediately and call a Gas Safe registered engineer.
• The pressure is still too high after bleeding radiators, or it jumps by more than 1 bar at full temperature.
• Water is discharging from the pressure relief valve pipe outside.
• Anything that means opening the boiler. Vaillant is blunt: "Do not remove the boiler casing yourself" and "Never carry out work on a boiler yourself."
• Any gas work at all. HSE states that "Anyone employed to work on gas appliances in domestic premises must be a Gas Safe registered engineer and competent in that area of gas work."

A safety note before you touch anything

Checking a gauge, bleeding a radiator and using a filling loop are safe. Opening the boiler is not, and neither is anything involving the gas supply. If you ever smell gas, follow HSE's advice before anything else: do not smoke, put out all naked flames, do not turn any electrical switch on or off, open the doors and windows to ventilate, and shut off the gas supply at the meter control valve if you know where it is and the meter is not in a cellar or basement. Then contact the National Gas Emergency Service immediately on 0800 111 999, and leave the property if the smell is strong. Do that first, and leave the pressure gauge until later.

If your boiler has locked out with a low pressure fault code rather than simply reading low, the code tells you which instructions apply. We have written up the common ones, including the Worcester Bosch H07 low pressure warning, linked below.

Boiler pressure FAQs

Pressure dropping every week? Get it looked at.

If the pressure will not hold, that is a repair rather than a top-up. Smart Plan is a service plan, not insurance, and cover is for future breakdowns rather than a fault your boiler is already showing, so tell us about the pressure problem you have today and we will call you back. One-off repair appointments run Monday to Friday, 09:00 to 17:00. Cover is the separate product, for the next fault rather than this one: on the boiler and central heating module it pays parts and labour up to £500 in a 12-month period if your boiler is under 7 years old, or up to £200 if it is over 7. Cover-plan call-outs keep their own hours, Monday to Friday, 08:00 to 18:00, and within them we aim to get an engineer out to you within two working days. In your first 30 days a £95 call-out fee is payable in advance, and that same fee applies to faults the plan does not cover, no-access visits and early annual-service requests; the cover itself still applies right through those 30 days. Using a module starts a 12-month agreement period once you first use a service. There is a 14-day cooling-off period, but it ends the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance. From UK Boiler Company Ltd, trading since 2014.

Your reading is wrong: where to go next

• The gauge is below 1 bar: how to top the pressure back up, loop by loop
• The gauge is above the green band: how to let the pressure back down
• You top it up and it drops again: tracing where the water is going
• You have an Ideal boiler: the F1 code, the gauge location and Ideal's own steps
• Worcester Bosch H07 low pressure warning
• Boiler & central heating cover
• Worcester Bosch: topping up heating system pressure
• Vaillant: what pressure should my boiler be?
• Baxi: what should my boiler pressure be set at?
• Ideal Heating: topping up boiler pressure
• Check an engineer on the Gas Safe Register