Domakihub How to Repressurize a Combi Boiler and Restore Hot Water Fast
A safe, tool-free fix most homeowners can do in under ten minutes.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 4 min read
A boiler that loses pressure is one of the most common reasons heating and hot water stop working in homes, holiday rentals, and small hotels across Greece — especially after a cold snap or a round of radiator bleeding. The video above from Which? shows the process clearly for the most common boiler types, and the steps below walk you through it in detail so you can get your system back up without waiting for a technician. Most owners can complete the job in less than ten minutes with no tools at all.
Why Boiler Pressure Drops
A combi boiler is a sealed pressurized system, which means any water that escapes — through a small leak, a valve drip, or air release when you bleed radiators — reduces the system pressure. The pressure gauge on the boiler front panel (or a digital display on newer models) will fall below 1 bar, and most boilers lock out automatically to prevent damage.
Occasional pressure loss of a few tenths of a bar per year is normal. If the gauge drops to zero frequently, or you see water pooling around radiator valves, pipe joints, or beneath the boiler itself, there is a leak that needs a plumber — repressurizing alone will not fix it.
What You Need Before You Start
Repressurizing requires no special tools. You will use the filling loop — a short braided hose or an internal key valve already fitted to your boiler. Locate the pressure gauge (it should read between 1 and 1.5 bar when the system is cold and working correctly) and find the filling loop, which is usually under the boiler or accessible behind a small panel. On some Worcester Bosch and Ideal models the filling mechanism is a built-in keyhole valve rather than an external hose.
Make sure the boiler is switched off and the system is cold before you begin. Attempting to refill a hot system can give a misleadingly high pressure reading and risks scalding if you need to access any fittings.
Steps to Repressurize Your Combi Boiler
- 1
Switch the boiler off and let it cool
Turn the boiler off at its own power switch or programmer. Give the system at least 30 minutes to cool if it has been running recently. Working on a cold system gives you an accurate pressure reading and prevents burns.
- 2
Locate and attach the filling loop
Find the filling loop — usually a short silver or grey braided hose with a valve at each end, situated under or behind the boiler. If it has been disconnected and stored, reattach both ends securely now. On boilers with an internal filling key, insert the key into its slot before proceeding.
- 3
Open the filling valve slowly
Turn one or both filling valves (depending on your boiler model) slowly anti-clockwise to allow mains water into the system. You will hear water flowing. Watch the pressure gauge continuously — the needle or digital reading will begin to rise. Never leave the valve unattended at this stage.
- 4
Stop at 1 to 1.5 bar
As soon as the pressure gauge reads between 1 and 1.5 bar, close both filling valves firmly. On most gauges the green zone marks this safe range. Do not overfill — pressure above 2.5 bar can trigger the safety pressure relief valve and release water onto the floor.
- 5
Restart the boiler and check for errors
Switch the boiler back on. If a low-pressure fault code (such as E119, EA, or F22 depending on brand) was showing, it should clear automatically within a minute or two as the boiler performs its restart sequence. Check that hot water and heating return to normal, then verify the pressure has not dropped again after an hour of normal use.
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FAQ
- How often should I need to repressurize my boiler?
- A healthy combi boiler may need topping up once or twice a year at most. If you find yourself repressurizing every few weeks, the system has a leak — common culprits include radiator valves, the pressure relief valve, or a small weep at a pipe joint. Call a certified plumber to locate and seal it.
- What pressure should a combi boiler run at?
- The ideal cold-system pressure is between 1 and 1.5 bar. When the system heats up fully, pressure naturally rises to around 1.5 to 2 bar — this is normal. Most boilers lock out below 0.5 bar and will trigger a safety release above roughly 3 bar.
- Can I repressurize the boiler myself or do I need a technician?
- Repressurizing through the filling loop is a routine owner task — no gas or electrical work is involved. However, if you cannot find the filling loop, if the pressure drops immediately after refilling, or if the boiler shows any error codes beyond a simple low-pressure fault, contact a Gas Safe (or Greek equivalent) registered engineer before attempting further work.