Boiler Pressure Too High? How to Reduce It

How to reduce boiler pressure by bleeding a radiator

Bleeding a radiator is the standard way to bring high boiler pressure down, and you are allowed to do it yourself. Worcester Bosch puts it plainly: when the pressure is too high, "you will need to bleed a radiator until the pressure gauge returns between 1 and 1.5 bar."

You need a radiator key (the small butterfly-shaped key below) or a flat-head screwdriver, a jug, an old towel, and a clear view of the gauge.

1. Turn the heating off and let the system cool. Water from a hot radiator will scald you, and a cold system reads steadier.
   
2. Find the bleed valve, the small square-ended screw at the top of one end of any radiator.

   

   
   
3. Hold the jug under the valve and put the towel down. Central heating water is black and stains carpet.
   
4. Open the valve slowly, anti-clockwise, about a quarter turn. Air hisses out, then water dribbles and the pressure falls.
   
5. Watch the boiler gauge, not the radiator. Close the valve and read the needle every few seconds; small bursts stop you overshooting.
   
6. Close the valve fully when the gauge reads 1 to 1.5 bar with the system cold.
   
7. Turn the heating back on. The gauge climbing a little as the water heats is normal.

If the needle is still high, repeat on the same radiator or another one. If it drops below 1 bar, top the system back up through the filling loop.

How to lower boiler pressure without bleeding radiators

Two other routes let water out of the same sealed loop. First check the filling loop is fully closed. That is the braided hose, or pair of small valves, under the boiler: left even slightly open, the pressure climbs back no matter how much you release.

Use the system filter's drain valve

Many systems have a magnetic system filter on the pipework under the boiler, with a small valve on its underside.

1. Turn the heating off and let the system cool. Water leaving a hot system will scald you.
   
2. Put a bucket or wide tray under the filter and cover the floor.
   
3. Open the valve a little and let a small amount of water out.
   
4. Close it as soon as the gauge reads 1 to 1.5 bar cold.

Use a drain-off valve

If there is no filter, look for a drain-off valve: a small Y or T-shaped fitting on the pipework, often near a radiator or at the low point of the run. It needs a spanner and a short hose.

1. Turn the heating off and let the system cool. Water leaving a hot system will scald you.
   
2. Put a bucket under the valve, or run a hose to a drain outside.
   
3. Open it a fraction with a spanner, watching the gauge the whole time.
   
4. Close it once the needle reads 1 to 1.5 bar cold.

What if I reduce my boiler pressure too much?

Nothing is damaged, and you are simply back to the opposite symptom. Below 1 bar the boiler may stop heating and show a low-pressure fault code, and the fix is the filling loop rather than anything on this page: topping a gauge that sits below 1 bar back up walks through each type of loop. If you find yourself topping up again within days, something is letting water out, which is pressure that will not stay up.

The number you are bringing it back to

1 to 1.5 bar with the system cold. That is the target for the job on this page, and your own manual is the final word on your model.

This guide assumes you already know your reading is high and simply want it down. If what you actually want is the full set of normal figures, cold and running, and where each maker draws the line, that is a separate read: the bar readings a healthy boiler should show.

Why the gauge climbs when the heating runs

It climbs because, as Vaillant puts it, "When water is heated, it expands." Around half a bar between cold and hot is expected, so judge the system on a cold reading: 2 bar hot may be healthy, the same 2 bar stone cold means it is overfilled. That is the difference between a normal rise and a system you genuinely need to let water out of.

For what boiler pressure actually is, and why it has nothing to do with your mains water pressure or your gas pressure, see the bar readings a healthy boiler should show.

Read the gauge cold before you start

Take the reading with the heating off and the system cold, a few hours after it last ran. That is the reading the 1 to 1.5 bar target refers to, and the one that tells you whether you are genuinely overfilled rather than just looking at hot water. On a combi or system boiler the gauge is on the front, sometimes behind a drop-down panel, and many gauges mark the normal range in green.

Regular (heat-only) boilers usually have no gauge. Most are open-vented, fed by a feed and expansion tank in the loft, so there is no sealed pressure to reduce and nothing on this page applies. Some are fitted on a sealed system instead, with an expansion vessel and a gauge on the pipework rather than the boiler. Check your manual if you are unsure.

How will I know if my boiler pressure is too high?

• The gauge sits above 2 bar stone cold, or in the red band. Vaillant's room-temperature rule of thumb: under 2.5 bar "no action is required", above it "reducing the pressure before operating appliance is advised."
  
• The needle climbs past about 2.5 bar with the heating up to temperature. Baxi's limit for a hot system is that "the needle will go up a little, but should not go higher than about 2.5 bar."
  
• Water runs from a small pipe outside, usually on the wall near the boiler. Baxi: "If it is too high, a safety valve is fitted that will discharge water to keep the pressure at a safe level."
  
• The boiler locks out on a pressure fault, or you find damp patches at pipework, radiator valves or under the boiler.

The discharge pipe is a safety device, not a pressure control. If it releases water regularly, get the cause looked at: a valve that has passed dirt often fails to reseat.

Why is my boiler pressure too high?

The system was topped up too far

By far the most common cause. Someone repressurised the boiler to clear a low-pressure fault and stopped a bit late. Water expands as the heating runs, so a system filled to 2 bar cold can reach the safety valve within the hour.

The filling loop was never fully closed

Left even slightly open, the loop keeps feeding mains water in and the pressure creeps up day after day. Check both valves are shut, and on many boilers disconnect the hose.

The expansion vessel has lost its air charge

The vessel is a sealed tank with a cushion of air behind a rubber diaphragm, absorbing the extra volume as the water expands. Lose that charge and the pressure swings hard between cold and hot. Worcester Bosch gives the test: "If pressure rises by more than 1 bar when the heating is up to temperature then the expansion vessel may require repressurising. This will require a service engineer."

Something inside the boiler is passing mains water in

Less common, but a failed plate heat exchanger or a filling valve that no longer seals feeds mains water in continuously. The giveaway is pressure rising on its own with the filling loop shut and disconnected. That needs a Gas Safe registered engineer.

When high pressure is a fault, not a setting

Reducing the pressure takes five minutes; whether it stays down tells you if anything is fixed. Book an engineer if it climbs back within days with the filling loop shut, if the gauge jumps more than 1 bar between cold and hot, if the discharge pipe keeps releasing water, or if you find a leak.

Those faults are diagnosed with the boiler open or the system drained, so none are homeowner jobs. If you would rather have the repair bill handled, our boiler and central heating cover includes 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.

Gas safety: what to leave to a registered engineer

Reading a gauge, bleeding a radiator, opening a drain valve and using a filling loop are safe to do yourself. Taking the boiler casing off is not, and neither is anything touching the gas supply. The HSE is explicit: "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." Check any engineer on the Gas Safe Register.

If you ever smell gas, deal with that first. Call the National Gas Emergency Service on 0800 111 999, open the doors and windows, and shut the gas off at the meter control valve if you know where it is. Do not smoke, use naked flames or operate electrical switches. The pressure gauge can wait.

Reducing boiler pressure: FAQs

How do I lower boiler pressure without bleeding radiators?

Use the drain valve under the magnetic system filter beneath your boiler, or a drain-off valve on the pipework. Both let water out of the sealed loop without touching a radiator. Turn the heating off and let the system cool first, and check the filling loop is shut.

Is 2.5 bar too high for a boiler?

It depends whether the system is hot or cold. Baxi says the needle should not go higher than about 2.5 bar while the boiler is heating, so 2.5 bar hot is borderline rather than alarming. Stone cold, it means the system is overfilled.

Is high boiler pressure dangerous?

Modern sealed systems are built to handle it, provided the safety valve is working. Every boiler has a valve that discharges water outside before the pressure reaches a damaging level, fitted, as Baxi puts it, to "keep the pressure at a safe level". A valve that has passed dirt can fail to reseat, so if the discharge pipe drips constantly, get it looked at. The usual cost of ignoring high pressure is water damage, not an explosion.

Why does my boiler pressure go up when the heating is on?

Because heated water expands and the sealed loop has nowhere to put the extra volume except into pressure. Around half a bar between cold and hot is normal. More than 1 bar is the sign Worcester Bosch flags as an expansion vessel that may need repressurising by a service engineer.

How much water do I need to let out?

Usually very little, and one radiator is enough. A sealed system responds quickly to a small change in volume, so the method matters more than the amount: open the valve a fraction, count a few seconds, close it and read the gauge.

Related guides and sources

• The gauge is below 1 bar instead: how to top the pressure back up, loop by loop
  
• You only want to know what the reading should be: the normal bar figures, cold and running
  
• You bring it down and it climbs back, or drops away again: tracing pressure that will not stay put
  
• Yours is an Ideal boiler: the F1 code, where Ideal put the gauge, and Ideal’s own steps
  
• How to bleed a radiator
  
• Boiler and central heating cover, or ask us about your boiler
  
• 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
  
• HSE: domestic gas safety questions