If your boiler pressure has dropped below 1 bar, your heating won't work properly — or at all. Repressurising takes about 2 minutes, costs nothing, and doesn't require any tools. This is one of the most common "callouts" that plumbers charge £80-£150 for, and you can do it yourself right now.
I repressurise boilers for customers almost every week. It's one of the easiest jobs in plumbing — but most homeowners don't know how to do it, so they call an engineer and pay a callout fee for what's literally a 2-minute task. Here's exactly how to do it yourself.
What You'll Need
Nothing. Seriously — no tools required. You just need access to the filling loop underneath your boiler. On newer models (Vaillant, Worcester Bosch, Baxi), there's often a built-in filling key instead of an external loop.
Step-by-Step: How to Repressurise
1
Turn off your boiler
Switch it off at the control panel or thermostat. Let it sit for a few minutes. You should never repressurise while the heating is running — the pressure readings won't be accurate and you could overfill.
2
Check the current pressure
Look at the pressure gauge on the front of your boiler. Most have a dial with a green zone (1-1.5 bar) and a red zone (above 2.5-3 bar). If the needle is below the green zone or your digital display shows below 1.0, you need to repressurise.
3
Find the filling loop
Look underneath your boiler. You're looking for a braided silver or grey hose connecting two pipes, with a small valve (or two) that you can turn. On some modern boilers, there's a built-in filling key — check your boiler manual if you're not sure. The filling loop is usually near the bottom of the boiler or on the pipes directly below it.
4
Open the valve(s) slowly
Turn the valve anti-clockwise. If there are two valves, open them both. You'll hear water flowing into the system — a rushing or hissing sound. Keep your eyes on the pressure gauge the entire time. The needle will start to rise.
5
Stop at 1.2 to 1.5 bar
As soon as the gauge reaches 1.2 to 1.5 bar, close the valve(s) by turning clockwise. Be firm but don't force them. The sweet spot is about 1.3 bar — this gives enough headroom for the pressure to rise slightly when the heating comes on.
6
Turn the boiler back on and check
Switch the heating on. The pressure may rise slightly to around 1.8-2 bar — this is normal. It should stay stable. Check again after a few hours. If it's dropped back down, you likely have a slow leak somewhere in the system.
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Pro tip: If your boiler has a built-in filling key (common on Vaillant ecoTEC and Worcester Greenstar models), you'll need to insert and turn the key rather than opening an external valve. The key is usually stored in a clip on the bottom of the boiler. Check the inside of the front panel cover for instructions specific to your model.
What If the Pressure Keeps Dropping?
If you have to repressurise more than once a month, there's a leak somewhere. Common places to check:
Radiator valves — look for drips at the connection points, especially on older radiators. Tighten gently with a spanner if you see moisture.
Pipe joints — check visible pipework in the airing cupboard, under the boiler, and where pipes enter walls or floors.
Underneath the boiler — if there's a damp patch or drip marks on the wall below your boiler, the pressure relief valve may be discharging, or there could be an internal leak.
The pressure relief valve (PRV) — this is a safety valve that releases water if the pressure gets too high. If it's faulty, it can slowly leak and cause constant pressure loss. You'll usually see a copper pipe running from the boiler to an outside wall — check if water is dripping from the end of it.
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If the pressure keeps dropping daily and you can't find a visible leak, call a Gas Safe registered heating engineer. There may be an internal leak inside the boiler itself, or the expansion vessel may need re-charging. These are not DIY jobs. A typical diagnostic visit costs £60-£120.
How Much Would a Plumber Charge for This?
A plumber callout to repressurise your boiler typically costs £80-£150 including the callout fee — for a job that takes 2 minutes and requires no parts. This is one of the most straightforward ways to save money on home maintenance: learn to do it yourself once, and you'll never need to pay for it again.
If the pressure keeps dropping and you need a plumber to find a leak, expect to pay £60-£120 for the diagnostic visit, plus parts and labour for the repair.
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Frequently Asked Questions
Between 1 and 1.5 bar when the heating is off. It may rise to around 2 bar when the heating is running, which is perfectly normal. If it regularly drops below 1 bar you need to repressurise. If it regularly exceeds 2.5 bar, the expansion vessel may need attention from an engineer.
The most common causes are a small leak in the system (radiator valves and pipe joints are the usual suspects), a faulty pressure relief valve, or a problem with the expansion vessel. If you're repressurising more than once a month, it's worth getting an engineer to find and fix the underlying issue.
Not if you stop at 1.5 bar and don't exceed 2.5 bar. If you accidentally overfill, bleed a radiator slightly to release some water and bring the pressure back down. Most boilers also have a pressure relief valve that automatically releases excess pressure as a safety measure.