Your boiler must be at the right operating pressure for the heating system to work properly.

If when the boiler is on and it loses pressure or if the pressure is too high it may cause the boiler to shut down.

Maintaining your boiler at the correct pressure helps to make your heating system more energy-efficient and keeps costs down.

 

What Is Boiler Pressure?

Boiler pressure refers to the pressure of hot water running in your sealed central heating system.

This is different to water pressure, which is the pressure of the water coming through the taps.

 

How Do We Measure Boiler Pressure?

Boiler pressure is measured in bars. A bar is a unit of pressure defined as 100 kilopascals (kPa). kPa is approximately the pressure exerted by a 10 g mass resting on a 1 cm2 area.

A bar is about equal to the atmospheric pressure on Earth at sea level. The bar was introduced by Norweigan scientist Vilhelm Bjerknes, often regarded as the founder of modern weather forecasting.

 

What Is The Rule Of Thumb For Boiler Pressure?

You should always check the manufacturer’s operating instructions, but as a rule of thumb the following applies:

    • Heating on – maintain boiler pressure between 1.5 and 2 bar
    • Heating off – main boiler pressure between 1 and 1.5 bar

Most boiler manufacturers will recommend that the ideal operating pressure is 1.3 bar.

The boiler will usually operate below 1 bar quite adequately, but if it drops below 0.5 it will normally be set to stop operating.

Your boiler pressure should not increase by more than 1 bar above its optimal working pressure when you turn the heating on. If it does, this could indicate a problem.

 

How To Tell Your Boiler Pressure

Most combi and system boilers operating in a sealed system have a boiler pressure gauge. This is generally visible on the front of the control panel.

Many boiler pressure gauges will be marked with a green and a red zone to indicate the correct pressure is being maintained or that you have a problem.

The pressure indicator can be frustratingly well hidden on some models so consult the manufacturer’s instructions if you can’t see it.

If the pressure gauge is stuck or does not appear to be working it probably needs replacing. A registered engineer should be called to do this.

NB: On open vent systems there won't be a pressure gauge.

 

How To Increase And Reduce Boiler Pressure

The basics:

  • To increase boiler pressure – top up with water
  • To reduce boiler pressure – bleed your radiators;
  • Check your boiler pressure once a month
  • Always use a registered engineer for any boiler repairs

The following steps for increasing a boiler’s pressure are generally:

  • Turn off the boiler
  • Let the system cool down;
  • Locate the filling loop – it’s a short grey hose often curved in a U shape with a valve at either end. You can find it either directly under your boiler or close by;
  • Keep an eye on the pressure gauge and open both of the valves. You should be able to do this by hand, but on older models, you might need to use a screwdriver;
  • As water fills the system you will hear a hissing noise;
  • Once the pressure gauge reaches between 1 and 1.5, close both of the valves one by one;
  • Switch your boiler back on;
  • Check if your radiators heat up. If not, repeat. If you can still feel no heat, contact a registered gas engineer.

Why Is The Boiler Pressure Higher When The Heating System Is On?

When you turn the heating on, the water in the pipes and radiators heats up and expands. This water expansion causes the pressure to increase in the whole system.

Boilers are fitted with a pressure relief valve to regulate jumps in pressure.

Recognise a boiler fault when the heating is on

If you do see a rise up by 1 bar or your pressure needle moves into the red section above 2 bar this could signal a fault with the pressure relief valve or the expansion vessel.

These types of faults will require an authorised and regulated service engineer to check them.

What Happens If The Boiler Pressure Is Too High Or Too Low When The Heating Is On?

If you have low boiler pressure the entire system could trip and your radiators will start to cool quite rapidly depending on room temperature.

If the boiler pressure is too high you could strain the system to failure and have a burst pipe leaking somewhere in the house.

How Often Should I Check My Boiler Pressure?

Check your boiler pressure once a month to make sure it’s not too high or too low.

You should also check the pressure after bleeding your radiators in case it has dropped below 1 bar.

It’s a good idea to keep an eye on the boiler pressure just before autumn and winter starts so that your boiler is in top shape for the colder months.

How Often Should I Need To Increase Boiler Pressure?

You should only have to increase the boiler pressure once or twice a year.

But, if your boiler keeps losing pressure this could mean that the water from the system is leaking, for example, one or more of the radiators has sprung a leak.

If you notice that your boiler pressure is too high, it may be that you have recently topped up the boiler pressure by too much.

It may also mean that you have forgotten to close the pressure relief valve after topping up or accidentally knocked the valve and loosened it.

Leaving the filling point open all the time and is seriously not recommended. It can result in water coming out of the relief valve, the corrosion inhibitor might become diluted and ultimately the relief valve will sooner or later fail if mains pressure is maintained.