THE SHORT ANSWER
NORMAL OR NOT
The rate of loss is the diagnosis. Make a note of the date every time you top up and you will know within a month.
Nothing to worry about
1 to 1.5 bar
Cold, on the gauge at the front
Rises when hot
Up to around 2 bar is expected
Top-up frequency
Once or twice a year
After bleeding
A small drop is normal, just top up
After a repair
One top-up as the system settles
Small seasonal drift
Over months, not weeks
Get it looked at
Weekly top-ups
Water is going somewhere
Daily top-ups
Find it now, not next month
Drops to zero
Boiler will lock out and stay out
Over 2.5 bar hot
Expansion vessel has likely failed
Dripping outside pipe
The relief valve is passing
Damp under a radiator
A valve or the radiator itself
WORKING OUT WHICH
1
Months between top-ups is normal. Weeks means something is wrong. Days means find it now. The rate is the single most useful thing you can tell an engineer on the phone, so make a note of the date each time you top up.
2
Check the outside wall near the boiler for the small copper pressure relief pipe. If it is dripping or you see staining below it, the relief valve is passing and that is your loss. Then check under radiators and around valves for staining, which is the other common source.
3
Pressure that climbs high when hot and then sits low when cold usually points at the expansion vessel having lost its air charge. That is a recharge or a replacement, not something a top-up will ever fix.
WORKING OUT WHICH
Once or twice a year
Every few weeks
Every few days
High when hot, low when cold
COMMON QUESTIONS
What pressure should my boiler be?
How do I top up boiler pressure?
How often is it normal to top up?
Why does my boiler keep losing pressure?
Is it dangerous to keep topping it up?
Can you find the leak?
BEFORE YOU GO
More of the same, written by the engineers who do the work.