The short version
- Weekly top-ups mean water is leaving the system somewhere
- Check the outside PRV pipe for drips before anything else
- A failed expansion vessel makes pressure swing high when hot, low when cold
- Repeated refilling introduces fresh oxygen and accelerates corrosion
The three usual causes
Each has a distinctive pattern.
- System leak — a radiator valve, towel rail joint or buried pipe; pressure falls steadily
- Expansion vessel — pressure climbs sharply when hot and drops below normal cold
- Pressure relief valve passing — you'll find drips or limescale on the outside discharge pipe
What you can safely check
Look under each radiator and towel rail for staining or damp carpet. Check the small copper pipe outside near the boiler for water. Note the gauge reading cold and again at full heat, and tell us both numbers — that alone narrows the diagnosis before we arrive.
Why constant topping up is worse than it looks
Every refill brings dissolved oxygen into the system. That's what feeds the corrosion that becomes sludge, which then blocks radiators and shortens the heat exchanger's life. Fixing the pressure loss protects everything downstream.
Common questions
What pressure should my boiler be at?+
Most domestic systems sit around 1 to 1.5 bar cold — check the manufacturer's marking on your gauge.
Can I keep using the boiler?+
If it holds pressure and heats normally, usually yes for a short period. If it drops out repeatedly, get it looked at.
Can you find a hidden leak?+
Yes — we pressure test and trace the circuit rather than guess.
Is an expansion vessel repairable?+
Often it can be re-charged; if the diaphragm has failed it needs replacing.
Pressure dropping again?
We'll find the cause rather than just top it up.