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If you maintain a correct pressure level, you can ensure your boiler remains efficient and reliable. When boiler pressure rises to a higher level, the boiler can suffer from boiler wear, which can cause a leak or trigger the pressure relief valve.
This boiler guide explains normal boiler pressure ranges, how to check the pressure on your boiler, and what steps to take if boiler pressure too high alarm appears on your boiler system. So, bear with us for a few minutes and let us dig deeper into the details.
What is a Normal Boiler Pressure? How To Check It?
Ideally, the boiler pressure in a cold boiler should be around 1 bar and never fall to 0.5 bar or below. If you are using your boiler for heating as well, the boiler pressure usually rises to between 1.5 bar and up to 2 bar on most combi boiler gauges.
You should always check the pressure from the gauge on your boiler (manual or digital). To check the pressure, you should switch the boiler off and let the boiler cool down fully. Make sure the boiler pressure reads around 1 bar when the heating is off and the boiler is only providing domestic hot water.
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Is High Boiler Pressure Dangerous?
The excess pressure in the system can put strain on the boiler’s internal parts, like the heat exchanger, pump, and pressure release valve. This usually forces the boiler to work harder, reduces boiler efficiency, and raises energy bills.
While it’s not as dangerous as modern boilers have that safety feature, and they can turn themselves off automatically after reaching the maximum pressure range.
What are the Causes of Boiler Pressure being Too High?
Several common causes of high boiler pressure include:
- A faulty pressure relief valve that fails to open and let out excess pressure.
- A blocked pressure relief pipe can stop water from escaping and raise boiler pressure to unsafe levels.
- An expansion vessel that is waterlogged or has lost pressure cannot absorb the extra pressure from the system automatically, causing the boiler pressure to rise.
- Sometimes, recently repressurising your boiler or leaving the filling loop open allows much water from the system to top up continuously and trigger boiler pressure rises.
- High incoming water pressure in your mains can also be a cause of boiler pressure too high alerts on your boiler pressure gauge.
In rare cases, a faulty pressure gauge gives the wrong reading and makes you think the boiler pressure is too high when it is not.
How to Reduce Pressure from Your Boiler?
The process of reducing pressure from your boiler is quite simple and can be carried out through simple DIY methods. Here are some of the easiest ways to normalise boiler pressure.
Bleed the radiators
To bleed a radiator, switch off the boiler and use a radiator key on each radiator bleed valve until water emerges. After bleeding the radiators, check the pressure gauge on your boiler and ensure the pressure falls back to a safe level below 2 bar.
Release Water via the Pressure Relief Valve
If high pressure in the boiler persists above 3 bar, you may need to release pressure directly using the pressure release valve or a drain valve.
Attach a hose to the drain valve at the lowest point of the boiler system and allow some water from the system to drain until the boiler pressure falls between 1 bar and 1.5 bar.
Undo a Radiator Nut
If bleeding hasn’t done the trick, you can let more water out at a radiator lockshield (nut) valve:
- Place a towel or container beneath the valve to catch drips.
- Gently loosen the lockshield nut (usually under the valve’s cap) by half a turn.
- Watch the pressure gauge; stop once you hit 1–1.5 bar, then retighten the nut.
Use a Drain‑Off Valve
Most boilers have a dedicated drain‑off (drain‑cock) valve at the lowest point of the heating loop:
- Attach a hose to the valve and place the other end in a drain or bucket.
- Open the valve slowly and let water flow out until the boiler pressure drops to around 1.5 bar.
- Shut the valve securely, wipe up any spills, and re‑pressurise if needed.
Open the Filter Valve
Some systems include a magnetic or mesh filter with its own isolation valve:
- Locate the filter (typically near the boiler’s return pipe).
- Open the filter’s isolation/drain valve—often a small lever or knob.
- Allow debris-laden water to escape until the pressure gauge is back in range.
- Close the filter valve and top up the system if you’ve lost more than 0.5 bar.
After each method, always re‑check the pressure gauge on your boiler to ensure it’s between 1 bar (cold) and 1.5 bar (off‑peak), or up to 2 bar when the heating is active.
When to Call a Professional?
If your boiler pressure keeps rising after you reduce boiler pressure or you notice a persistent leak, contact a Gas Safe engineer immediately. Never ignore boiler pressure, dangerous warning signs, or unusual noises in your boiler system—these signs that your boiler pressure is problematic demand a qualified boiler engineer.
A heating engineer or a gas safe registered engineer can diagnose complex pressure problems, repair faulty valves, and ensure your boiler operates within the correct pressure level. When your combi boiler or system boiler shows serious pressure issues, a qualified boiler engineer ensures you fix the problem safely.
Final Thoughts
If you maintain the boiler pressure within the right range, you can keep your central heating system efficient and safe. You should always keep an eye on the boiler pressure gauge and reduce the boiler pressure when the pressure is too high.
This will prevent leaks and boiler damage. If in doubt, reach out to a Gas Safe engineer or boiler engineer to fix the boiler pressure issue and protect your home.
Frequently Asked Questions
It can be dangerous if your boiler is old and does not have safety features. In modern combi boilers, it’s unlikely for the high pressure to be dangerous, as combi boilers automatically turn off if the pressure is too high or too low. That’s how you remain safe from any sort of damage.
High boiler pressure can reduce the boiler’s performance, energy efficiency, and make it louder. If the pressure issue remains constant, it may damage your central heating system or boiler system itself.
One of the methods to reduce boiler pressure is bleeding the radiators unless they reach a safe level. To bleed the radiators, you can open the radiator bleed valve, let the water flow, and keep checking the pressure on the gauge. You can close the valve once your system has reached a safe level (1 bar normally). Opening the filter valve, the drain-off valve, or the pressure release valve can also do the job for you; you just need to choose what works best for you.
You can open the boiler’s air vent or bleed valve to release the trapped air in the heating system. To do so, you can open the valve slowly, release the air until the water pops out, and close the valve immediately.
