How To Reduce Your Boiler Pressure: Step by Step Guide

A boiler’s efficiency is directly linked to its pressure level. Just like in the Goldilocks fairy tale, your boiler needs a pressure level that’s ‘just right.’

Signs That Your Boiler’s Pressure Is Incorrect

Whether it’s too low or too high, there are clear signs that your boiler pressure is incorrect. The optimum UK boiler pressure for most units is around 1.0–1.5 bar when cold and may rise to 2.0 bar when the unit is hot.

There are numerous signs indicating a pressure problem.

Signs Your Boiler Pressure Is Too Low

A sure sign that your boiler’s pressure is too low is if it’s indicated on the pressure gauge, which is usually situated on the front of the unit. Typically, a reading below 1.0 bar when cold, or frequent drops to zero, will cause the following issues:

Boiler Won’t Fire Up

Most modern boilers have an automatic shut-off function, which shuts off the system if the pressure is too low to circulate water. This is to prevent the boiler from causing more damage to the system.

No Heating Or Hot Water

Similarly, another reason a boiler might shut itself off is if the pressure drops too far, and the boiler is unable to circulate water.

Air Noises In Radiators

Even a healthy boiler makes a few distinct noises when operating correctly. However, if you hear a gurgling noise while your radiators are heating up unevenly, it is a sure sign that there’s not enough water in the system, or an air pocket.

Constantly Needing To Repressurise

If you’re topping up more than once every few months, there may be a leak or a faulty expansion vessel.

Signs Your Boiler Pressure Is Too High

Typically, homeowners need to top up their boilers at least once a year, which is an easy DIY task. However, if your boiler is running at a high pressure, it can be trickier.

A sure indicator that the boiler’s pressure is too high is if the reading on the unit’s gauge is above 2.5 or 3.0 bar. Usually, at these high-pressure levels, the gauge’s needle will be in a designated red zone. Most boilers have a green “safe” band. If the needle is sitting above it, that’s a warning sign.

Boiler Makes Banging Or Gurgling Noises

High pressure can stress components and cause unusual sounds.

Water Dripping Outside

When your boiler’s pressure starts to climb, multiple pressure relief valves will release the built-up pressure. You can check if this is the case by locating a copper pipe, which will be sticking out of the wall. If water is constantly dripping from the pipe, your boiler’s pressure is too high.

Boiler Shuts Down Or Displays An Error Code

Many modern boilers lock out to prevent damage if the pressure is dangerously high. If the boiler has shut down and is displaying an error code, it is always best to refer to your unit’s manual to clarify its meaning.

Radiators And Pipe Leaks

If you’ve noticed that your radiator’s joints or valves are weeping water, this is a sign that your heating system is overpressurised.

Now that we know what to look out for to determine if your boiler’s pressure is too low or high, it’s always best to exercise caution if the system’s pressure climbs above the recommended level. If pressure continues to climb or reaches 3 bar, switch off and call a Gas Safe registered engineer.

How To Reduce Your Boiler’s Pressure

Before reducing your boiler’s pressure, remember to proceed with caution.

Remember the following:

• Do not remove the boiler casing. Anything inside is for professionals only.
• Turn the boiler off at the controls and let it cool if the radiators are hot.
• Protect your floors with towels and use gloves.
• If the gauge is touching ~3 bar or you see water dripping from a copper pipe outside (pressure relief pipe), turn the boiler off and call a Gas Safe engineer.

Once you’ve made the appropriate checks, you’ll need the following:

• Radiator bleed key (or small flat screwdriver),
• Towel,
• Bowl or bucket,
• And access to the boiler’s filling loop.

Reducing your boiler’s pressure is a simple task and can be accomplished in a few easy-to-follow steps:

Step 1: Make Sure You’re Not Accidentally Adding Pressure

It’s unlikely, but it’s always best to be sure. To repressurise a boiler or top up its water level, you need to connect it to a water main via a filling loop. Typically, the filling loop is located beneath the boiler. It is usually identifiable by a silver braided hose with one or two small black levers/knobs. Close the filling loops properly, and ensure that the levers are positioned perpendicular to the pipe (at a 90° angle to it). Some boilers have an internal filling loop that’s opened via a key or lever. Make sure that it is in the off position or the key is removed. Before working on your boiler, ensure you watch the gauge for 2–3 minutes. If pressure still creeps up, we’d suggest turning off the system and calling in a professional.

Step 2: Lower The Pressure By Bleeding A Radiator (Easiest)

To lower the pressure in your heating system, you need to bleed the excess water out through a radiator. It’s best to choose a radiator that’s located downstairs or at the lowest point in the property, as it typically releases pressure more quickly. Place a towel and bowl beneath the bleed valve, as you want to minimise your cleanup. When ready, insert the bleed key, turn slowly anti-clockwise until you hear a hiss (air), then a trickle (water). Don’t open the bleed valve fully, as you will let too much water out and will need to top up the water levels in the system. Let out small amounts of water (10–30 seconds), then close the valve. Ensure you check the boiler pressure gauge regularly. Repeat in short bursts until the cold pressure is ~1.2 bar. Turn the heating on for 10–15 minutes and recheck. If the system’s pressure is good, it should rise back to ~2.0 bar. When cool again, top up or bleed as needed to maintain a pressure of approximately 1.2 bar cold. It’s essential not to rush the process and swing from one extreme to the other. Small adjustments, recheck, repeat.

Step 3: (Optional) Use A Drain-Off Valve For Faster Control

If bleeding a radiator is slow (or you have towel rails only), you will want to locate a drain-off cock, also known as a drain tap, which is often found at the bottom of a downstairs radiator or near the boiler and its pipes. To drain the water out of the system, you will need to attach a hose to the drain point into a bucket or outside. When opening the drainage tap, do so slowly and release a little water, watching the gauge as it falls. Too fast, and you’ll need to repressurise the heating system. Watching the gauge closely, when the boiler’s gauge reads, or nearly reads 1.2 bar with a cold system (remember to let the system cool down), close off the tap tightly. Don’t use your boiler’s pressure relief valve (PRV) to drop pressure. Once opened, PRVs often fail to reseal properly and can leak. This will lead to the need to replace it.

Step 4: Power Back On And Recheck

When you’re happy with the heating system’s pressure level, turn the boiler back on. Ensure the boiler is run for 10–15 minutes and check for leaks at bleed or drain points. Expect the pressure to increase slightly as the system heats up. Turn off the system and wait for it to cool down again to confirm you’re still around 1.0–1.5 bar. If everything is in order, your heating system should be ready to get back to work.

What If The Boiler’s Pressure Keeps Rising?

For those boilers where the pressure keeps increasing, here is a quick list of common causes to check:

Filling Loop Left Partially Open

Double-check that handles are fully closed. If they are, and pressure still creeps up, the inlet valve may be faulty and require an engineer to repair.

Expansion Vessel Issue

If your boiler’s pressure level rockets when hot and drops too low when cool, and the system’s PRV discharges, it is a sure sign that you need to replace your boiler’s expansion vessel. An expansion vessel is responsible for regulating the system’s pressure level. If the vessel is faulty, it will need to be replaced by an engineer.

Faulty PRV

If your PRVs have been lifted or are weeping, it may not have resealed correctly and will need to be replaced.

Plate Heat Exchanger Pinhole

Your home’s main water supply can enter the sealed system if there is a hole in the boiler’s plate heat exchanger. This is an issue that affects combination boilers. A telltale sign that this could be the issue is if the pressure is increasing even with the filling loop shut. You will need an engineer to diagnose this issue and assist with fitting a replacement heat exchanger.

You’ve overfilled

Arguably, one of the easiest fixes is if you’ve just overfilled the system. This will require simply bleeding a little more water until you’re back near 1.2 bar cold.

However, if you need to call a professional, don’t wait for the situation to get worse. But who will assist you?

Get A Gas Safe Registered Engineer To Your Home Within 48 Hours!

It’s always best to address boiler issues as soon as possible. That’s why WarmZilla can get a qualified Gas Safe registered engineer in your local area to repair your boiler within 48 hours. If you would rather discuss your specific boiler issue with a professional, please don’t hesitate to contact our experts.