If your feed and expansion tank in the loft keeps topping up more often than expected, it’s usually not just a minor quirk. Many homeowners only notice it as a quiet refill sound or a ball valve activating more frequently.
But this behaviour is often one of the clearest signs that your heating system is losing water somewhere.
This is exactly the type of issue our Central Heating Leak Detection service is designed to diagnose properly.
What This Situation Usually Means
In an open-vented central heating system, the feed and expansion tank maintains the correct water level. It should only top up occasionally to compensate for normal changes in temperature and expansion.
If it’s topping up regularly, it usually means water is leaving the system and being replaced continuously.
This is a strong indication that the system is no longer stable and is trying to compensate for a loss.
Why This Happens
The most common cause is water loss within the heating system, even if it is not visible.
What typically happens is:
Water escapes slowly from pipework or connections
The tank replaces the lost water automatically
The cycle continues without obvious signs inside the home
Because many heating pipes run under floors or behind walls, the leak can remain hidden for a long time.
What People Often Misunderstand
It’s common to assume the issue lies with the tank itself, such as a faulty float valve. While this can happen, it usually results in continuous overflow rather than repeated topping up linked to heating use.
Another misunderstanding is that small top-ups are normal. Occasional adjustment is expected, but regular or frequent refilling is not.
If the tank keeps activating, it is usually responding to something happening elsewhere in the system.
What to Do Next
The key is to identify whether water is being lost and why.
Look for patterns such as:
The tank refilling more when the heating is on
Radiators needing bleeding more often
Subtle changes in how the system performs over time
Even without visible leaks, these signs often point to water loss somewhere within the system.
The next step is proper investigation to locate the source. You can learn more about the process on our how we find central heating leaks page.
FAQ
Is it normal for a feed and expansion tank to top up?
Occasionally, yes. However, frequent topping up usually indicates that water is being lost somewhere in the system.
Could the ball valve be the issue?
It’s possible, but a faulty valve typically causes continuous filling rather than repeated topping up linked to system use.
Can there be a leak without visible water?
Yes. Many heating leaks occur in hidden areas such as under floors or behind walls.
Will this problem get worse over time?
In most cases, yes. Ongoing water loss can lead to air entering the system and further instability.
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 feed and expansion tank keeps topping up, it’s usually not something to ignore. It’s a sign the system is compensating for a loss somewhere.
Understanding that early helps you move from reacting to symptoms to identifying the real cause — and resolving it properly.