💧 Boiler Pressure Keeps Dropping: Why & How To Fix
Last updated: February 2025
You top up the boiler. Pressure gauge back to 1.2 bar. Three days later? It's dropped to 0.5 bar again. Boiler locks out. You top it up again. Repeat.
This isn't normal. There's a leak somewhere. Here's how to find it and what it costs to fix properly.
What Boiler Pressure Actually Is
Your central heating system is a sealed loop. Water goes from boiler → radiators → back to boiler. It's pressurized (1.0-1.5 bar when cold) so it can circulate efficiently and handle expansion when hot.
If pressure drops: There's less water in the system. It's escaping somewhere. Boiler detects low pressure and locks out (safety feature—running with low pressure damages pump and heat exchanger).
Where The Water Is Going (Common Causes)
1. Pressure Relief Valve (PRV) Leaking
Most common cause. PRV is a safety valve—if pressure gets too high (over 3 bar), it opens and dumps water outside via a copper pipe. But they can fail and leak constantly.
How to check: Follow the copper overflow pipe from boiler to outside. Is it dripping? Is ground wet underneath?
Fix cost: £120-220 (new PRV + labour)
2. Expansion Vessel Failed
Expansion vessel absorbs pressure increase when water heats up. When it fails, pressure shoots up, PRV opens, water escapes. Pressure drops. You top it up. Heats up again. Cycle repeats.
How to check: If pressure is fine when cold but shoots to 2.5+ bar when heating on, expansion vessel is suspect.
Fix cost: £180-320 (re-pressurize or replace expansion vessel)
3. Radiator Valve Leaking
Compression fittings on radiator valves can weep, especially if disturbed (e.g., after bleeding radiators).
How to check: Feel around each radiator valve. Wet? Put tissue around base of valve—if it gets damp over 24 hours, that's your leak.
Fix cost: £80-150 (tighten or replace valve)
4. Boiler Internal Leak
Heat exchanger, pump seals, or diverter valve leaking inside boiler. Water drips into drip tray (if it has one) or onto floor.
How to check: Open boiler casing (if safe/competent—or get engineer to do it). Look for water stains, corrosion, drips.
Fix cost: £150-450 (depending on part)
5. Hidden Pipe Leak
Pipe under floorboards, behind walls, or in loft. Hardest to find.
How to check: Look for damp patches on ceilings/floors, wet carpet, unexplained water damage.
Fix cost: £200-600 (find leak + repair + make good)
How To Find The Leak (DIY Checks)
Step 1: Visual inspection
- Check PRV overflow outside—is it dripping?
- Check all radiator valves for wetness
- Check boiler drip tray (if accessible)
- Check visible pipework for corrosion/wetness
- Check ceilings/floors for damp patches
Step 2: Pressure test method
- Top up boiler to 1.2 bar when cold
- Mark current pressure with phone photo
- Turn heating OFF (don't run it)
- Wait 24 hours
- Check pressure—has it dropped?
If YES (pressure dropped when cold): External leak (radiator valve, PRV, pipe leak)
If NO (pressure same when cold): Expansion vessel issue (pressure only drops after heating cycles)
💡 Pro Tip: Tiny leaks evaporate quickly on hot pipes. You might never see the water. But the pressure drop is real. That's why engineers use pressure testing over 24 hours—it doesn't lie.
Can You Just Keep Topping It Up?
Short answer: No.
Why not:
- Damages boiler: Constant fresh water introduces oxygen, causes corrosion inside heat exchanger
- Scale buildup: Fresh tap water has dissolved minerals. Over-filling repeatedly causes scale deposits that block heat exchanger (expensive repair)
- Boiler warranty: Most manufacturers void warranty if system not maintained properly
- Leak could get worse: Small leak today = flood tomorrow when pipe finally splits
Topping up is fine ONCE while you arrange an engineer. Doing it weekly/monthly? You're causing more damage.
When To Call Gas Safe Engineer
Call if:
- Pressure drops more than 0.5 bar per week
- You can't find the leak visually
- PRV needs replacing (gas work—legally requires Gas Safe)
- Expansion vessel needs work
- Leak is inside boiler casing
- Leak is under floor/behind walls
What engineer does:
- Pressure test system properly
- Thermal imaging to find hidden leaks
- Check expansion vessel pre-charge pressure
- Test PRV function
- Inspect all radiator valves
- Quote repair before starting work
💷 What Repairs Cost
- PRV replacement: £120-220
- Expansion vessel re-pressurize: £80-140
- Expansion vessel replacement: £180-320
- Radiator valve replacement: £80-150
- Leak detection + repair (hidden): £200-600
- Boiler internal seal/gasket: £150-350
- Heat exchanger (if corroded from over-filling): £400-800
How To Top Up Boiler Pressure (Safely)
Only do this if you need emergency heating while arranging engineer:
- Find filling loop (flexible braided hose under boiler, or built-in on Worcester models)
- Turn both ends (usually two valves/taps)
- Watch pressure gauge while water flows
- Stop at 1.0-1.2 bar (don't overfill to 2+ bar)
- Close both valves tightly
- Check for leaks from filling loop itself
⚠️ Don't Overfill: If you overfill to 2.5+ bar, PRV will open and dump water. Now you've made the problem worse. Fill slowly, watch the gauge.
Preventing Pressure Loss
- Annual service: Engineer checks PRV, expansion vessel, system pressure before faults develop
- Don't disturb radiator valves: Tightening/loosening can damage seals
- Inhibitor in system: Protects against corrosion (should be topped up every 5 years)
- Fix small leaks immediately: Don't wait till it gets worse
Need Pressure Issue Fixed?
UK Boiler Response: 0333 600 0990
Gas Safe engineers. Pressure testing equipment. Thermal imaging for hidden leaks. PRV/expansion vessel specialists. Fixed pricing before we start. Same-day appointments available.