Boiler Expansion Vessel — How to Check and Recharge
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →Before checking the expansion vessel charge, the heating system must be fully depressurised (drain to 0 bar) and the boiler must be cold. Working on a pressurised system is dangerous.
The expansion vessel absorbs the increase in water volume as the heating system heats up. When it fails, the pressure relief valve (PRV) discharges water every time the boiler heats up, and you may need to repressurise weekly. Checking and recharging the expansion vessel is a DIY-accessible job with the right tools.
Not sure if this matches your problem?
Use our interactive tool — answer a few questions and get a personalised diagnosis.
Most likely cause & what to check
Identify the expansion vessel — on combi boilers it is usually inside the boiler casing (a red or silver cylinder). On system boilers and cylinders, it may be a separate red vessel in the airing cupboard or adjacent to the boiler.
Turn off the boiler and allow the system to cool completely. Connect a hose to the drain cock (at the lowest point of the system, usually near the boiler) and drain the system to zero bar — watch the pressure gauge and stop when it reads 0.
Locate the Schrader valve on the expansion vessel (similar to a bicycle tyre valve). Use a tyre pressure gauge to check the current nitrogen charge pressure — it should read 1 bar (for most UK domestic systems, matching the system static fill pressure).
If the charge is below 1 bar, use a bicycle pump or foot pump with a Schrader valve connector to pump nitrogen (or air as a temporary measure) up to 1 bar. Check with the pressure gauge after each 10 pumps.
Once the vessel is charged to 1 bar, refill the heating system using the filling loop until the pressure gauge reads 1.5 bar. Turn the boiler back on and monitor the pressure as it heats up — it should rise to no more than 2.5 bar.
If the vessel is at the correct pressure but the PRV still discharges when the boiler heats up, the expansion vessel diaphragm has failed (water has permeated through). The vessel needs replacing — a Gas Safe or plumbing engineer job (£150–300 for parts and labour).
Check the vessel charge annually as part of boiler maintenance — most Gas Safe engineers do this as part of a boiler service.
Not confident doing this yourself?
Post the job and we'll match you with vetted local engineers. Free, no obligation.
🛠 Tools & materials you may need
Was this guide helpful?