What it means
What Low pressure is telling you
A sealed heating system should sit around 1 to 1.5 bar cold. Below 1 bar most boilers will either run badly or lock out entirely.
Topping it up once after bleeding radiators is completely normal. Topping it up every week is not — that water is going somewhere.
The three real causes are a leak on the pipework or a radiator, a failed pressure relief valve discharging outside, or a knackered expansion vessel that can no longer absorb expansion.
Before you call
Safe checks you can do yourself
- Repressurise using the filling loop to 1.2 bar with the boiler cold
- Check under radiators and around valves for damp patches or rust staining
- Look for a small copper pipe outside near the boiler — if it's dripping, the pressure relief valve is passing
- Note how quickly it drops: days means a small leak, hours means a bigger one
Never remove the boiler casing or work on the sealed combustion chamber yourself — by law that’s Gas Safe registered engineers only. If you smell gas, call 0800 111 999 immediately.
How we fix it
What we do on the visit
- Leak detection across pipework, radiators and the boiler itself
- Expansion vessel pressure tested and recharged, or replaced if the diaphragm has gone
- Pressure relief valve replaced where it's passing
- Radiator valve and joint repairs, and inhibitor restored after any drain-down
How urgent is it?
Not an emergency, but a system losing pressure weekly is leaking somewhere. Get it found before it damages floors or ceilings.
Costs
What a repair costs
Our diagnostic visit is £99 inc VAT and it comes off the bill if we carry out the repair. We give you a fixed price before touching anything — no hourly rates, no surprises on the invoice.
We’re Gas Safe registered, Which? Trusted Trader endorsed and rated 4.9 across 206+ reviews. We carry the common Worcester Bosch and Viessmann spares on the van, so most faults are fixed on the first visit rather than booked back in.
Common questions
How often should I have to top up my boiler?
Once or twice a year at most, usually after bleeding radiators. Weekly or monthly top-ups mean there's a leak or a failed expansion vessel.
Is it safe to keep topping up the pressure?
Short term, yes. But every top-up introduces fresh oxygenated water, which accelerates corrosion and sludge inside the system — so you're trading a cheap fix now for an expensive one later.
What does leak detection cost?
Our standard diagnostic is £99 inc VAT and comes off the repair. Most pressure loss is traced and fixed in a single visit.
Book it in
Get an engineer out
Step 1 / 5
60 second boiler quote
No callbacks unless you want one
What can we help with?
Pick the option that fits — we'll tailor the next questions.
More help
Other fault codes
Worcester Bosch
EA
No flame detected — ignition lockout
Worcester Bosch
E9
Overheat — the boiler has got too hot
Worcester Bosch
A1
Pump fault or poor water circulation
Worcester Bosch
C6
Fan fault — fan speed out of range
Worcester Bosch
D5
Control board or communication fault
Viessmann
F2
Temperature limiter — overheat lockout