Why It Happens and How to Fix It
If you’ve noticed your boiler losing pressure when the heating is off, you’re not alone. Many homeowners spot the pressure dropping on cold days or even when their radiators haven’t been on for hours. While it might seem minor at first, ignoring it can lead to bigger problems with your heating system or even a complete breakdown. In this guide, we’ll explain why it happens, how to troubleshoot it, and when it’s time to call a professional.
Why Is My Boiler Losing Pressure When Heating Is Off?
Boiler pressure is crucial for keeping your central heating running efficiently. Your system usually operates at around 1–1.5 bar when cold and 1.5–2.5 bar when hot. If the pressure drops below this, your boiler may stop working or display warning lights.
When your boiler loses pressure with the heating off, the most common causes are:
Leaks in the heating system – Even tiny leaks in radiators, pipes, or valves can cause pressure to slowly drop when the system is not in use.
Faulty pressure relief valve (PRV) – The PRV releases water if the system is over-pressurised. If it’s worn or stuck, it may leak slowly even when the boiler is off.
Expansion vessel issues – The expansion vessel is a small tank inside your boiler that absorbs pressure changes. If it loses its air charge, your boiler can bleed pressure even when not heating.
Air in the system – Air pockets in radiators or pipes can cause pressure fluctuations. The pressure may drop once the system cools down and the air shifts.
How to Tell If It’s a Serious Problem
A small drop in pressure over a few days is normal, but if you notice:
Pressure dropping below 1 bar
Frequent need to top up the system
Leaks around radiators or boiler joints
Boiler locking out or showing error codes
…then it’s likely more than a minor issue. Ignoring it could damage your boiler or central heating system.
Simple Things You Can Try at Home
Before calling a professional, there are a few safe steps you can take:
Check for visible leaks – Look at radiators, pipework, and your boiler for any drips or damp patches.
Top up the boiler pressure – Most modern boilers have a filling loop. Open it slowly to increase pressure to the recommended level (usually 1.5 bar when cold). Don’t overfill.
Bleed your radiators – Releasing trapped air can stabilise pressure. Use a radiator key, and have a cloth handy.
Monitor pressure over a few days – Note how quickly it drops. Slow, gradual drops may indicate minor issues; rapid drops usually need professional attention.
⚠️ Important: Do not try to tamper with the expansion vessel, PRV, or internal components yourself. These parts are under pressure and can be dangerous if handled incorrectly.
Why You Should Call a Gas Engineer
If topping up the system or bleeding radiators doesn’t fix the problem, or if you notice leaks or repeated pressure drops, it’s time to call a Gas Safe engineer. They can:
Diagnose leaks or faulty valves
Test and recharge the expansion vessel
Repair or replace the PRV safely
Ensure your boiler is operating efficiently
Calling a professional is safer, faster, and can prevent expensive boiler breakdowns later. Plus, regular maintenance can prolong the life of your system and keep your energy bills down.
A boiler losing pressure when heating off is a common issue, but it shouldn’t be ignored. While minor pressure drops can be topped up at home, persistent or rapid pressure loss usually points to leaks, faulty valves, or expansion vessel problems. The safest and most effective solution is to call a Gas Safe engineer, who can diagnose and fix the problem properly.
If your boiler is struggling, don’t wait until it completely fails contact a professional today to keep your heating running smoothly all year round.
Book a Gas Safe Engineer Today
Don’t let low boiler pressure disrupt your heating. Our qualified Gas Safe engineers can quickly diagnose and fix leaks, faulty valves, or expansion vessel issues keeping your home warm and worry-free.