Why Your Boiler Keeps Losing Pressure in Winter: You top up your boiler pressure, the heating comes back on, and a few days later the gauge has dropped again. It’s one of the most common winter call-outs we deal with, and it usually points to a specific fault in your heating system.
The good news is that most causes are easy to identify. In this guide, we explain why your boiler keeps losing pressure in winter, what you can safely check yourself, and when it’s time to call in a professional.
What Should Your Boiler Pressure Be?
Most combi and system boilers should sit between 1 and 1.5 bar when the heating is off. When the heating is running, it’s normal for the pressure to rise slightly, often up to around 2 bar.
If your gauge drops below 1 bar, your boiler may struggle to heat your home or switch itself off completely to protect its components. Always check your boiler manual, as recommended levels vary slightly between manufacturers.
Why Boiler Pressure Drops More in Winter
Your heating system works harder in winter than at any other time of year. It runs for longer, heats up and cools down more often, and puts extra strain on every joint, valve and seal. Small weaknesses that go unnoticed in summer tend to show themselves as soon as the weather turns cold.
The 6 Most Common Causes of Boiler Pressure Loss
1. A Hidden Leak in the System
A leak is the most likely cause of a boiler that keeps losing pressure. Even a slow drip from a radiator valve or pipe joint can drain enough water over a few days to drop the pressure noticeably.
Check for:
- Damp patches on floors, walls or ceilings
- Rust or white limescale marks around radiator valves
- Drips beneath the boiler itself
- Wet carpets or stained skirting boards
Leaks under floorboards or behind walls can be harder to find and usually need a professional.
2. Bleeding Your Radiators
Bleeding radiators releases trapped air, but it also releases a little pressure. Many people bleed their radiators at the start of winter and forget to top up the boiler afterwards. If your pressure dropped straight after bleeding, this is likely the only cause.
3. A Faulty Pressure Relief Valve
Your boiler has a pressure relief valve (PRV) that releases water if the pressure gets too high. If it’s worn or faulty, it can let water escape even at normal pressure.
A quick check: look for a small copper pipe running from your boiler through an outside wall. If it’s dripping or there’s a wet patch beneath it, the PRV may be the problem.
4. A Failing Expansion Vessel
The expansion vessel absorbs the extra volume of water as it heats up. Over time, it can lose its air charge. When this happens, the pressure shoots up when the heating is on, the PRV releases water, and then the pressure drops too low once the system cools.
If your gauge swings dramatically between hot and cold, a faulty expansion vessel is a strong suspect. This needs a Gas Safe registered engineer to recharge or replace it.
5. Recently Repaired or Installed Components
New radiators, valves or pipework can release trapped air over the following weeks. As that air escapes, the pressure naturally drops. A few top-ups after recent work is normal, but if it continues beyond a couple of weeks, something may not be sealed properly.
6. An Internal Boiler Leak
Sometimes the leak is inside the boiler, from a worn seal, a cracked heat exchanger or a failing pump. You should never remove the boiler casing yourself. If you suspect an internal fault, turn the boiler off and call a qualified engineer.
How to Safely Repressurise Your Boiler
Most modern boilers have a filling loop, usually a flexible hose or built-in lever beneath the boiler. The general process is:
- Turn off your boiler and let it cool down
- Locate the filling loop and check both valves are connected
- Slowly open the valves and watch the pressure gauge
- Close the valves once the gauge reaches around 1.2 to 1.5 bar
- Turn the boiler back on and reset it if needed
Every boiler is slightly different, so always follow your manufacturer’s instructions. If you’re unsure, it’s better to ask a professional than risk overfilling the system.
When to Call an Emergency Plumber
Topping up your boiler once or twice a year is normal. Having to do it every week is not. Call a professional if:
- The pressure drops again within a few days of topping up
- You can see water leaking from the boiler or pipework
- The pressure rises above 2.5 to 3 bar when the heating is on
- Water is dripping from the outside pressure relief pipe
- Your boiler keeps locking out or showing fault codes
Ignoring pressure loss can lead to water damage, a broken boiler and no heating on the coldest nights of the year.
