If your combi boiler keeps dropping below 1 bar every few days — or even every few hours — you're not alone. It's the single most common boiler complaint we hear in Lancashire, across Burnley, Blackburn, Preston, Bury and everywhere in between.
The good news: 5 out of 6 times it's fixable, often without replacing the boiler. Here are the six actual causes — in order of how often we see them — what you can safely check yourself, and roughly what each fix costs in 2025.
1. A small leak somewhere in the system (most common)
Even a pinhole leak you can't see will gradually drop system pressure over days or weeks. Common suspects:
- Radiator valves under floorboards or behind furniture (we find these regularly in Burnley's Victorian terraces)
- Towel-rail T-fittings in bathrooms
- Compression joints under sinks where someone has bumped a fitting
- The boiler itself (heat exchanger pinhole — usually only on boilers 12+ years old)
DIY check: wipe every accessible joint with kitchen roll for a day. Look for damp patches, salt stains, green corrosion. If the boiler is in a cupboard, check the floor underneath it for water marks.
Fix cost: £85 callout + the part. Most small leaks: £100–£180 total.
2. Failed expansion vessel (second most common)
The expansion vessel is a sealed metal vessel inside the boiler with a rubber diaphragm and pressurised air on one side. When the system heats up, it absorbs the expansion. When it fails (rubber perishes, air pressure drops), the system pressure rises sharply when hot, then drops dramatically when cool.
Diagnostic signs:
- Pressure jumps high (2.5–3 bar) when heating is on
- Then drops below 1 bar when system is cold
- PRV (pressure relief valve) outside the house drips during heating cycles
Fix cost: £160–£260 depending on boiler. Sometimes a re-pressure of the existing vessel works (£85 callout) but on Worcester / Baxi / Vaillant boilers 5+ years old, a replacement vessel is the proper fix.
3. Stuck-open PRV (pressure relief valve)
The PRV is a safety valve that opens if system pressure exceeds about 3 bar. If a tiny bit of debris (sludge) gets stuck in the seat, it can leak permanently — sending water out of the discharge pipe (usually a copper pipe sticking through an external wall, often dripping onto your patio).
How to check: go outside, find the copper pipe coming out of the wall near the boiler. If water is dripping or trickling out — that's a stuck PRV.
Fix cost: £120–£180 for a new PRV cartridge.
4. Bleeding radiators too often
Bleeding a radiator removes a tiny bit of water as well as air, dropping system pressure. If you bled them last weekend and now your pressure is at 0.8 bar — you just need to re-pressurise (see below).
How to re-pressurise: Find the “filling loop” — a short braided silver hose with two black handles, usually underneath the boiler. Slowly open one handle, watch the pressure gauge, close at 1.2 bar. Don't exceed 1.5 bar cold.
If you have to do this more than once a month — there's a real fault, get a Gas Safe engineer out.
5. Auto Air Vent (AAV) leaking
Inside the boiler is an automatic air vent that releases trapped air. When it fails it can drip water continuously into the boiler chassis. You'll often see this as condensation or water in the boiler drip tray.
Fix cost: £100–£150.
6. Heat exchanger pinhole (worst case)
On boilers older than 10–12 years (especially older Ideal, older Vaillant), the heat exchanger can develop tiny pinholes. Water then drips into the boiler casing and you'll find rust marks inside the chassis or condensate trap area. If this is your problem, you're usually looking at a replacement boiler — a new heat exchanger costs nearly as much as a new combi.
What to do right now
- Re-pressurise to 1.2 bar using the filling loop (see Section 4)
- Note how quickly it drops. Hours? Days? Weeks?
- Check the PRV pipe outside — is water dripping out?
- Walk every room and look behind/under radiators for damp marks
- If it's dropping daily — call us. Don't keep topping up: too many top-ups dilute the inhibitor and accelerate sludging.
How much does it cost to fix?
Most pressure-loss faults in Lancashire homes cost between £85 and £260 all-in. Our boiler repair callout is a fixed £85 with a transparent quote before we do any work — no nasty surprises.
FAQ
Is it safe to keep using the boiler if pressure is low?
Below 0.5 bar, modern boilers lock out for safety. Between 0.5–1 bar you can use it short-term but the inhibitor is being diluted every time you refill.
Can I refill more than once a week?
Yes if needed, but if you're refilling more than once a week, there's a real fault — get it diagnosed.
Same-day callouts in Burnley, Blackburn, Preston?
Yes — we cover all of Lancashire same-day for pressure-loss faults. Book a free diagnostic or call 01282 914 044.