Bleeding radiators releases trapped air and usually causes the system pressure to fall slightly. If the gauge then keeps climbing, particularly towards the red zone, something else may be allowing pressure to build. For homeowners dealing with boiler pressure rising after bleeding radiators, the most likely causes include an open filling loop, a depleted expansion vessel or a pressure-relief valve that has started passing water.
Why is the boiler pressure rising after bleeding radiators?
A sealed central heating system contains a fixed volume of water. The pressure gauge shows how much pressure that water is exerting within the boiler and heating circuit. Many systems should read roughly 1 to 1.5 bar when cold, although the correct range varies, so check the boiler manual rather than relying on a universal figure.
When the heating is switched on, the water expands and the pressure normally rises a little. The expansion vessel inside or near the boiler is designed to absorb this increase. A modest movement, such as from 1.2 bar when cold to around 1.7 bar when hot, may be normal. A rapid increase towards 2.5 or 3 bar is not.
Bleeding a radiator does not normally create excess pressure. However, the problem often appears afterwards because the system has been topped up too far, the filling loop has not been closed completely, or bleeding has exposed an existing expansion-vessel weakness. Note the pressure when the system is fully cold and again after the heating has operated for 20 to 30 minutes. This simple comparison gives useful clues.
Quick checks you can do now
Before checking anything, let the boiler and radiators cool. Do not remove the boiler casing, loosen internal parts or work on gas components. The following visual checks do not require access to the sealed combustion area.
- Read the cold pressure: Check the gauge before switching on the heating. If it is already above the range stated in the manual, the system may simply have been overfilled.
- Check the filling controls: Make sure the filling-loop valves or built-in filling key have been returned to their fully closed position.
- Watch the gauge while heating: A large rise that happens only as the radiators warm up points towards an expansion-vessel problem.
- Look at the outside discharge pipe: This is usually a small copper pipe passing through an external wall. Drips or a wet patch below it may indicate that the pressure-relief valve has opened.
- Check for leaks: Look around visible radiator valves, pipe joints and the area below the boiler without removing any covers.
- Record the readings: Write down the cold pressure, maximum hot pressure and how quickly it rises.
Short checklist:
- Boiler cold before checking
- Filling controls fully closed
- Cold and hot readings recorded
- No boiler casing removed
- Boiler stopped if pressure approaches 3 bar
If the pressure is close to 3 bar, water is escaping heavily, or the display reports a pressure fault, switch the heating off and allow it to cool. Avoid repeatedly bleeding radiators or draining water as a routine workaround, because that introduces fresh oxygenated water and can encourage corrosion.
Could the filling loop be letting water into the system?
The filling loop connects the mains water supply to the sealed heating circuit so that pressure can be restored after maintenance or radiator bleeding. It may be a flexible silver hose with one or two valves, or a built-in key, lever or link beneath the boiler. Because mains pressure is higher than heating-system pressure, even a slightly open or faulty filling valve can make the gauge climb.
This cause is especially likely if the pressure rises while the boiler is cold and switched off. The increase may happen over a few hours or gradually over several days. Check that both external filling-loop valves are fully closed, without forcing them. If the manufacturer’s instructions say a detachable hose should be removed after filling, follow those instructions and refit any protective caps. Do not disconnect unfamiliar pipework or loosen a connection while it is pressurised.
A valve can also pass internally even when its handle appears closed. In that situation, closing it will not stop the increase and the valve may need replacement. On some combi boilers, a damaged plate heat exchanger can allow higher-pressure mains water to cross into the heating circuit. This is less common than an open filling loop, but it can produce similar symptoms and requires testing by a competent engineer.
After bleeding, repressurise only to the figure stated in the boiler instructions and do so while the system is cold. Topping up to a high reading while the water is already warm leaves too little room for normal expansion.
How an expansion-vessel fault causes boiler pressure rising
The expansion vessel is divided by a flexible diaphragm. Heating water sits on one side and a cushion of compressed air or nitrogen sits on the other. As the water heats and expands, it pushes against the diaphragm, compressing the gas cushion and limiting the pressure increase.
Over time, the vessel can lose its air charge, the diaphragm can split, or an isolation connection can become restricted. The system then has nowhere to accommodate the expanding water. A typical pattern is a normal cold reading followed by a steep rise when the heating runs. The pressure may approach 3 bar, cause the relief valve to discharge and then fall very low after the system cools.
Homeowners sometimes respond by adding more water each morning. This creates a cycle: the cold system is topped up, the heating pushes the pressure too high, water escapes through the relief valve, and the cold reading drops again. Repeated filling treats the symptom rather than the fault and can dilute corrosion inhibitor within the system.
Testing or recharging an expansion vessel requires the water side to be correctly depressurised and the vessel’s pre-charge to be measured. Access may also require removal of the boiler casing. This should be handled by a Gas Safe registered engineer where it involves the boiler. Depending on the vessel’s condition and accessibility, the engineer may recharge it, replace it or fit a suitable external vessel.
What does the pressure-relief valve have to do with it?
The pressure-relief valve, often shortened to PRV, is a safety component designed to release water if system pressure becomes excessive, commonly at around 3 bar. It does not usually cause the initial pressure rise. Instead, it reacts to a problem such as overfilling or a failed expansion vessel.
Once a PRV has opened, dirt or scale can become trapped on its sealing surface. It may then continue to drip even after the pressure returns to normal. Signs include water from the external copper discharge pipe, a pressure reading that falls when the boiler cools, or a repeated need to top up.
Do not operate or twist the PRV to test it unless the boiler manufacturer’s instructions specifically require this as part of homeowner maintenance. Manually opening an older valve can prevent it from resealing. The discharge pipe should also never be capped, blocked or redirected into a container as a permanent solution.
If the valve is passing, an engineer should first identify why it opened. Replacing the PRV without correcting an expansion-vessel or filling-loop fault may allow the same problem to return. The engineer can inspect the discharge arrangement, test the vessel, confirm that the filling valves close properly and restore the correct cold pressure.
When should you arrange a professional boiler check?
A small pressure change between a cold and hot system is expected, but arrange an inspection if the gauge repeatedly moves towards 2.5 to 3 bar, the external discharge pipe drips, the pressure rises while the boiler is off, or regular topping up is required. Stop using the heating and seek prompt advice if water is escaping near electrical parts, the boiler is making unusual noises, or the gauge remains in the red zone.
For homes in Haslemere and across nearby areas such as Bordon, Whitehill and Liphook, variations in system design are common: some properties have compact combi boilers, while larger or extended homes may use system boilers with separate vessels and controls. Embassy Gas also supports households around Alton, Farnham and the GU35, GU34 and GU9 postcode areas. An engineer will diagnose the behaviour rather than replacing parts based only on the gauge reading.
A useful fault report should include the cold and hot pressures, how quickly the reading changes, whether the filling loop was recently used, and whether water appears outside. These details help distinguish overfilling from a passing valve, expansion-vessel issue or internal heat-exchanger fault. Resolving the underlying cause should restore stable pressure and remove the need for repeated bleeding or topping up.
Next steps: For a professional diagnosis, contact Embassy Gas on (01420) 558993.