Ever feel like your boiler is toying with you?
One day it’s working perfectly. The next day it refuses to heat the house because the pressure has dropped — again.
Low boiler pressure is one of the most common heating issues in UK homes, especially during autumn and winter. The good news? It’s usually something that can be fixed without calling an engineer.
This guide explains:
✅ What boiler pressure is
✅ The real reasons it keeps dropping
✅ Step-by-step repressurising instructions
✅ When a low pressure problem signals something more serious
✅ How to get a free boiler replacement if yours qualifies
🔧 What Is Boiler Pressure?
Boiler pressure refers to the water pressure within your heating system — not the gas pressure.
It keeps hot water flowing from the boiler, through your pipes, into your radiators.
Your boiler gauge should look like this:
- 1.0–1.5 bar when the system is cold (ideal)
- Up to 2.0 bar when the system is heating (normal)
- Below 1.0 bar — your boiler may stop working
If your boiler cuts out when pressure drops, it’s doing its job — protecting the system from damage.
🚨 Main Reasons Boiler Pressure Keeps Dropping (Ranked From Most to Least Common)
✅ 1. Small Leak Somewhere in the Heating System
This is the #1 cause of persistent pressure loss.
Leaks can occur in:
- Radiator valves
- Towel rails
- Copper pipe joints under floors
- Boiler components (PRV, heat exchanger)
➡️ Even tiny, slow leaks will cause pressure to drop every 24–48 hours.
✅ 2. Faulty Pressure Relief Valve (PRV)
Your PRV is a safety feature that opens to release pressure if the system gets too high.
If it becomes faulty, it may release water constantly, even when pressure is normal.
Check outside — is water dripping from the copper pipe?
If yes, your PRV may need replacing (£90–£200).
✅ 3. Expansion Vessel Failure
Your expansion vessel absorbs excess pressure. If it loses air or fails:
- Pressure rises too high when heating
- Safety valve kicks in and dumps water
- Pressure then drops too low
➡️ You’ll see pressure spike when heating starts, then crash to 0.
✅ 4. Radiators Recently Bled
Bleeding radiators releases trapped air — but it also releases pressure from the boiler.
If you’ve bled radiators recently, you must repressurise the boiler afterwards.
✅ 5. Filling Loop Left Slightly Open
Sometimes the small valves on the silver filling loop don’t close fully.
If that happens, your system may overfill, pressure rises, safety valve opens, water is dumped, pressure drops — repeat.
🛠️ DIY: How to Repressurise a Boiler (the Right Way)
Works for Worcester, Vaillant, Ideal, Glow-worm & more.
✅ Step-by-step:
- Turn the boiler off and allow system to cool.
- Locate the filling loop underneath — a silver flexible hose or keyless lever.
- Slowly turn both filling valves until you hear water flow.
- Watch pressure gauge rise to 1.4–1.6 bar.
- Close the valves firmly.
Do NOT overfill past 2.0 bar.
Too much pressure can damage internal components.
🧪 Test to Identify a Leak (Simple Homeowner Test)
- Turn off boiler
- Repressurise to 1.5 bar
- Turn off heating completely overnight
➡️ If pressure drops while everything is off, you likely have a leak.
❓ When Should You Call an Engineer?
Call a professional when:
- Pressure drops more than once per week
- There’s water dripping from the boiler
- Pressure only drops when heating is turned OFF
- You suspect a leak behind walls or flooring
💷 UK Government FREE Boiler Grants (ECO Scheme)
If your boiler is:
- Over 8 years old
- Inefficient
- Losing pressure repeatedly
…you may qualify for a FREE replacement boiler through ECO grants.
✅ Free upgrade
✅ Installed by an accredited heating installer
✅ No repayment required (grant, not a loan)
Many households qualify due to income, EPC rating, insulation, or benefits.
👉 Check eligibility (60 seconds — no calls required).