If you can hear water running or trickling in your loft tank more often than usual, it’s usually not something to ignore. Many homeowners first notice this as a quiet but constant sound — especially when the heating is on or shortly after it has been running.
In most cases, this points to one key issue: your central heating system may be losing water somewhere.
What This Situation Usually Means
In open-vented heating systems, the header tank (also known as the feed and expansion tank) maintains the correct water level in the system. When everything is working properly, it should only refill occasionally.
If it keeps topping up regularly, it usually means water is leaving the system faster than it should. The tank is simply replacing what’s being lost.
This is why the sound of constant refilling is such an important early signal. The system is no longer behaving like a closed, stable system.
Why This Happens
The most common reason is a leak somewhere within the heating system. This could be:
Pipework under floors or behind walls
Radiators or valves slowly losing water
Connections that only leak when the system is hot and under pressure
Because these leaks are often hidden, there may be no visible signs inside the property. Instead, the header tank quietly compensates by refilling again and again.
You can learn more about typical locations on our where central heating leaks occur page.
What People Often Misunderstand
It’s easy to assume the issue is with the tank itself — for example, a faulty ball valve. While that can happen, it’s far less common than ongoing water loss.
Another misconception is that small refills are normal. Occasional topping up is expected, but regular or continuous refilling is not.
If the tank is constantly active, it’s usually reacting to something happening elsewhere in the system.
What to Do Next
The key is to confirm whether water is actually being lost and where it is going.
Look for patterns such as:
The tank refilling more when heating is on
Radiators needing bleeding more often
Subtle drops in system performance over time
Even if you cannot see a leak, that does not mean one isn’t there. Many central heating leaks occur in hidden areas and only show indirect signs like this.
If you suspect your system is losing water, the next step is proper diagnosis. You can read about the process on our how we find central heating leaks page.
For a broader overview of how these issues develop, visit Central Heating Leak Detection.
FAQ
Is it normal for a header tank to refill?
Occasional refilling is normal. However, frequent or constant refilling usually indicates that water is being lost somewhere in the system.
Could it just be the ball valve?
It’s possible, but less common. A faulty valve usually causes continuous flow regardless of heating use, whereas leak-related refilling often follows system activity.
Can a leak exist without visible water?
Yes. Many heating leaks occur under floors, behind walls, or in concealed pipework where water is not immediately visible.
Will the problem get worse if I leave it?
In most cases, yes. Ongoing water loss can lead to air entering the system, reduced performance, and further damage over time.
Do you only cover my area?
If you’re outside these areas, don’t worry — our team covers most of the South East, providing trusted leak detection services throughout London, Kent, Sussex, and Surrey.
Final Thoughts
If your header tank keeps refilling, it’s usually not just a minor quirk. It’s a signal that the system is trying to compensate for something.
Understanding that early can help you avoid bigger issues later — and ensure the problem is properly identified rather than temporarily masked.