Many modern boilers are installed as part of a sealed central heating system, the water pressure is deliberately kept high so it can be pumped to all the radiators in the home.
It’s easy to figure out if you have this type of system in your home, you’ll see a pressure gauge on the front of the boiler, usually with the numbers 1-3 or 1-4 written on it.
What is a “Normal” Boiler Pressure?
Whilst every boiler is different, gauges should typically read between 1 and 1.5 bar when cold.
The pressure may increase slightly as the water temperature rises.
Anything below 0.5 bar and above 2.75 bar would be considered well outside of the “normal” range.
Causes of Low Pressure
To put it simply, if your system is losing water, the pressure gauge will drop and you’ll need to top up the system to keep the boiler running.
How Often Should The System Be Topped Up?
Most boilers will lose a little water throughout the year and it’s normal for the system to need a “top up” once or twice per year.
If the pressure in your system is dropping below the minimum requirements on a frequent and regular basis, then chances are you have a leak or faulty part somewhere.
Most Common Reasons For Low Boiler Pressure
Here are the most common causes of low boiler pressure problems:
Faulty Pressure Release Valve (PRV)
Sealed systems have a release valve which is triggered when the pressure in the system becomes too high.
The emergency release of hot water via this valve prevents damage to the system.
It’s not uncommon for faulty valves to release water even when the system is otherwise functioning correctly.
If too much water is released, the pressure drops and you’ll need to top it up frequently to maintain a functioning boiler.
You’ll need to hire a heating engineer to check the valve, although you should be able to see the discharge pipe, they usually pass through the external wall and you may see water, damp patches, or green mould growing on or near the wall. This is the tell-tale sign of an issue with the PRV:
Faulty Expansion Vessel
An expansion vessel is a device that helps to regulate the pressure within the sealed system.
If your boiler pressure gauge is swinging from normal (perhaps around 1 – 1.5 bar) up to near 3 bars, your expansion vessel could be faulty.
This issue won’t directly cause low pressure problems but if the pressure rises too high, the release valve will dump some water to reduce the pressure.
If this happens repeatedly, so much water will be lost that the pressure in the system drops.
A heating engineer can re-pressurize the expansion vessel but if the issue continues or if it’s broken, it will need to be replaced.
Leaking Radiators or Pipework
For your boiler’s pressure gauge to drop from 1.5 bars to zero, the system would need to lose about half a bucket of water.
That may sound like a considerable amount of water but if the loss occurs over several weeks or months, you may not see any obvious signs of leaks, especially if the leak is on the ground floor, beneath the flooring.
Leaks in other areas of the home are usually easier to spot:
- damp patches on the walls or ceiling, near to the radiators or pipework.
- mould, black spots or even fungi growing on damp timbers.
- high levels of humidity or condensation caused by the water evaporating.
- unusual smells.
To reduce noise in the system and improve efficiency, some homeowners bleed their radiators on a regular basis.
This process releases any air bubbles in the radiators and pipes and is a recommended procedure.
Unfortunately, one side effect of this is a reduction in the water pressure within the sealed system.
If you bleed your radiators, double check the pressure gauge on the boiler and top up as required.
Step by Step Inspection Checklist
You can try the steps below if your boiler is repeatedly experiencing low-pressure issues, even after you top it up with water.
Don’t worry, none of the steps below involves opening the boiler cover or interfering with the gas supply.
- Check the pressure release valve discharge pipe. The pipe from this valve usually discharges outside, on the wall. Sometimes, it may discharge into the drain under the sink. If water is being discharged, you may have a faulty valve, pressure vessel or another part.
- Inspect all the radiators in the home and check the bleed valves and connectors are tight and not dripping water.
- Check all the pipework between each radiator. Look for any obvious signs of leaks such as damp patches, yellow stains, mould, algae or swollen plaster. These may be on the ceiling or even in the loft.
- Although rare, it is possible that a leaking pipe is discharging water near the cavity wall. The water can enter the space between the two walls and then disperse into the ground. You should thoroughly check the outer wall of your home, look for any signs of water damage either internally or externally.
If none of the steps above reveals the cause of the pressure drops, you can instruct a central heating engineer to check the heat exchanger, there have been reports of some faulty exchangers leaking water into the condensate pipe.
If your heating engineer reports that the boiler is working correctly, your next step is to either lift the flooring to inspect the pipework or to instruct a specialist leak detection company to check your home with a thermal camera.
Such cameras can detect the hot water as it passes through the pipes and radiators and is very effective at locating hot water leaks. As the process is non-invasive, you don’t need to lift all the floorboards or remove plasterboard.
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