TL;DR
If your boiler pressure keeps creeping above its normal range, there are three usual culprits. It could be an over-filled system, a filling-loop valve that isn’t shut tight, or a fault with the expansion vessel or pressure relief valve. Bleeding a radiator can bring it down a little. However, if the pressure climbs straight back up, it’s time for a qualified Gas Safe registered engineer — check the Gas Safe Register to find one.
What actually counts as “too high”
Most combi boilers found in Slough homes are designed to run at roughly 1.0-1.5 bar when the system is cold. The pressure relief valve is typically set to start discharging somewhere around 3 bar, as a safety limit. These figures vary between manufacturers, so always check the label or manual on your own boiler. As a rule of thumb, watch for the gauge sitting noticeably above your cold-fill range — creeping towards 2.5 bar or beyond. If it stays there once the system has cooled, you’re dealing with excess pressure worth sorting out. This is the mirror image of the pressure-loss problems covered in our guide to why your boiler keeps losing pressure, since many of the same components are involved either way.
What usually causes it
Over-filling via the filling loop. Because leaving the loop open after topping up, or adding water too enthusiastically, has the same effect: the pressure simply keeps climbing past where it should sit.
A filling-loop valve that isn’t fully closed, or is slightly weeping. Even a small leak past the valve seal lets a slow trickle into the circuit. This nudges the pressure upward over hours or days, without you noticing it happen.
A faulty expansion vessel. The vessel is meant to absorb the extra volume of water as it heats. If the internal bladder has lost its charge, or the vessel has ruptured, it can no longer take up that expansion. So pressure spikes instead.
A defective pressure relief valve. A valve stuck partially open, or with a worn seat, can vent water continuously. This can paradoxically also lead to over-filling, as someone tops the system back up to compensate.
In older Slough properties, original pipework and fittings have had longer to wear. This makes filling-loop seals more prone to this kind of slow weep. Thames Water manages the mains supply locally, but what happens inside your sealed heating circuit is a separate matter from mains pressure.
Bleeding a radiator to bring pressure down
- Turn off the heating and let the boiler cool — working on a hot system risks scalding.
- Find the bleed valve on the radiator, usually a small square-nipple fitting at the top.
- Hold a cloth or small container underneath to catch any water.
- Insert a radiator key (or flat-head screwdriver on some valves) and turn slowly anticlockwise. You’ll hear a hiss as trapped air escapes.
- Close the valve as soon as water starts dripping steadily — a snug turn clockwise is enough, don’t over-tighten.
- Check the pressure gauge. If it’s back in your boiler’s recommended cold-fill range, you’re done.
- Only bleed further radiators if needed — releasing air from too many at once can introduce noise into the pump or leave cold spots.
Never bleed a radiator while the boiler is firing, and never force a stuck bleed screw.
When bleeding a radiator isn’t enough
DIY bleeding fixes a temporary rise caused by trapped air. However, it won’t touch the underlying causes above. Book a qualified heating engineer if any of the signs below apply.
- Pressure returns to a high level shortly after bleeding or topping up
- You can see water dripping from the filling-loop valve, the pressure relief valve, or any joint even when the boiler is off
- The relief valve is discharging continuously — a stream or puddle near the discharge point
- The expansion vessel feels solid or waterlogged when you press its Schrader valve
- You smell gas, hear unusual noises, or the boiler shows an error related to pressure
A competent engineer will check the filling-loop seals, test the expansion vessel’s charge, and verify the relief valve’s set point. They’ll also confirm everything is balanced correctly. In older Slough estates this sometimes turns up ageing pipework that’s worth replacing, rather than repeatedly patching.
For this, book a Gas Safe registered engineer rather than guessing at the cost yourself — check the Gas Safe Register to find one. A reputable engineer will assess your specific system on-site, and explain what’s driving the price before any work starts. If the opposite problem sounds more familiar — pressure that keeps falling rather than rising — our guide to fixing low boiler pressure covers that instead.
Frequently Asked Questions
What should I do if my boiler pressure is too high? Try bleeding a radiator first, following the steps above, to release a little water and air from the system. If the pressure climbs straight back up afterwards, the cause is elsewhere. Most often it’s an over-filled system, or a weeping filling-loop valve, and needs a professional look.
Why does my boiler pressure rise when the heating is on? A small rise while the heating is actively running is normal. Water expands as it heats, and the expansion vessel takes up the extra volume. It’s only a concern if the pressure stays high once the system has fully cooled back down.
How do I reduce pressure on my boiler? Bleeding a radiator is the standard way to bring pressure down a little at home. If you’ve recently used the filling loop, also check both valves are fully closed. A valve left slightly open is a common reason pressure keeps creeping up.
Does it matter whether I have a Vaillant, Worcester or Baxi boiler? The causes and the radiator-bleed fix are broadly the same across brands. What differs is exactly where the filling loop and gauge sit, and the specific cold-fill figure. Always check your own boiler’s manual or data plate for that detail, rather than assuming.
Get the pressure checked properly
If bleeding a radiator hasn’t brought the gauge down, don’t keep repeating it, since that rarely fixes the underlying cause — a Gas Safe registered engineer is who to call for that (check the Gas Safe Register). If a leaking valve or joint turns out to be part of the picture, get in touch online and our general plumbing repairs work can help with that side of it. We cover Wokingham, Beaconsfield and Uxbridge, along with Slough and the wider area.
