The short version
- Most systems want roughly 1 to 1.5 bar when cold
- Fill slowly, in short bursts, watching the gauge the whole time
- Close both filling loop valves fully when you finish
- Needing to do this every few weeks is a fault, not maintenance
Step by step, cold system
Give yourself ten minutes and do it with the heating off and the system cold, because hot water reads higher and you will overfill.
- Turn the heating off at the programmer and let the system cool
- Find the filling loop, usually a braided silver hose or an internal key under the boiler
- Open the first valve, then open the second slowly until you hear water moving
- Watch the gauge and stop at about 1.2 bar, or the mark shown on your gauge
- Close both valves fully, in reverse order
- Reset the boiler if it was locked out, then run the heating and check for leaks
If you overfill it
Above about 2.5 bar cold, the pressure relief valve will start discharging outside. Bleed a small amount out of a radiator bleed valve into a cloth and cup until the gauge settles back down. Do not leave a system sitting high, because it stresses seals and the relief valve.
Why the pressure keeps dropping
A sealed system does not consume water, so if the pressure falls the water has gone somewhere. There are three realistic explanations and they behave differently.
- A leak on a radiator valve, towel rail joint or buried pipe, giving a steady slow fall
- A failed or discharged expansion vessel, giving pressure that climbs high when hot and drops below normal when cold
- A pressure relief valve passing, shown by drips or limescale on the small pipe outside
Leak or expansion vessel: how to tell
Note the gauge cold, then again at full heat. A swing of more than about a bar between the two points at the expansion vessel. A gauge that simply creeps down over days, with no dramatic swing, points at a leak or a passing relief valve. Tell us both readings when you call and you save us a diagnostic step.
Why constant topping up causes damage
Every refill brings fresh oxygenated water into the system. That feeds corrosion, which becomes the black sludge that blocks radiators and shortens the life of the heat exchanger and pump. Fixing the cause is cheaper than the parts it eventually costs you.
Common questions
What pressure should a combi boiler be?+
Around 1 to 1.5 bar cold on most domestic systems. Check the marking on your own gauge, because manufacturers vary.
Is it safe to repressurise the boiler myself?+
Yes. The filling loop is outside the sealed appliance and is designed to be used by the householder. Anything behind the casing is not.
How often is too often?+
Once or twice a year after bleeding radiators is normal. Weekly or monthly means there is a fault to find.
My boiler has no visible filling loop, what now?+
Some models use an internal key or lever built into the underside. Check the manual, and if you cannot identify it, do not force anything, call us.
Can you find a hidden leak under the floor?+
Yes. We pressure test and trace the circuit rather than lifting floors at random.
Pressure dropped again after topping up?
We find the cause, whether it is a leak, the expansion vessel or the relief valve.