A boiler that loses pressure once is normal. A boiler that loses pressure repeatedly, every few weeks, every few days, or daily, has an underlying fault that won't fix itself by topping up. Here's what's actually going on, ranked by likelihood, and what each fix costs in London.
The four real causes of repeated pressure loss
1. Failing expansion vessel (most common)
Inside every modern combi boiler is a small steel vessel split internally by a rubber bladder. One side holds water from your central heating system. The other side holds compressed nitrogen at the factory-set pressure. As the system heats up and water expands, the bladder absorbs the extra volume, keeping system pressure within the safe band (typically 1,2.5 bar).
Over time, usually around 5,10 years, the rubber bladder perforates. The nitrogen leaks across to the water side, the cushion is lost, and now there's nothing to absorb expansion. The boiler's pressure-relief valve (PRV) opens at 3 bar to discharge the excess water down the drain pipe. You see the pressure gauge drop afterwards and assume there's a leak, but the water actually went out via the PRV, not into the property.
How to diagnose: A Gas Safe engineer can re-pressurise the air side of the expansion vessel via the Schrader valve (it looks like a car-tyre valve). If pressure won't hold, the bladder is gone. Replacement vessel: typically £180,280 inc VAT for an internal replacement. Some boilers have an external option.
2. Failing pressure-relief valve (silent leak)
The PRV is the safety valve at the top of your boiler that opens at 3 bar to dump excess pressure. Over time, debris or limescale can hold the valve slightly open, meaning every time the boiler heats up and pressure rises (even within normal range), water trickles out through the discharge pipe.
You won't see this leak inside the property, the discharge pipe usually runs to outside the building, often hidden behind kitchen units or in a cupboard. Telltale signs: copper-colour staining around the discharge outlet, dripping after the boiler has been running, or a barely-audible hiss from the boiler casing.
How to diagnose: Engineer inspects the discharge pipe outside. In some cases isolates the boiler and measures pressure decay. PRV replacement: typically £140,220 inc VAT including the valve.
3. Hidden system leak (radiator, pipe, joint)
Slow-rate leaks somewhere in the central heating circuit, radiator valve weep, hairline pipe crack, pump seal failure, soldered joint corrosion. These are often invisible because the leak is so slow that the water evaporates before it makes a visible puddle. Common locations:
- Behind radiator valves (turn off heating, watch for 15 minutes, any droplet?)
- Under floors where pipework runs (warm patches, lifted boards, faint damp smell)
- Pump body and circulator seals (drips behind the boiler casing)
- TRV (thermostatic radiator valve) bonnets, particularly the older Drayton/Honeywell models
- Underfloor heating manifolds, often overlooked because the manifold is in a cupboard
How to diagnose: Visual inspection plus pressure-test isolation. For deeply hidden leaks, our team uses thermal imaging cameras (FLIR) and acoustic detection, see leak detection. Repair costs vary widely. Typical radiator-valve repair £150,240, hairline pipe repair £200,400 plus access cuts.
4. Air vents auto-bleeding
Less common but worth ruling out. Some boilers and unvented cylinders have automatic air-release valves that vent trapped air. If the AAV (automatic air vent) sticks open with limescale, the system slowly bleeds water vapour to atmosphere. Easy fix, replacement valve £40,80, plus labour.
What you should NOT do
- Don't keep topping up indefinitely. Each time you re-pressurise, you're diluting the corrosion inhibitor (Sentinel X100, Fernox F1) that protects your boiler. After 3-4 top-ups the inhibitor is depleted and corrosion accelerates, meaning a £200 expansion vessel becomes a £600 heat-exchanger replacement within 2 years.
- Don't ignore the cause. The expansion vessel/PRV/leak isn't going to heal itself. Fix it once, properly.
- Don't add antifreeze or random additives. The system needs a balanced inhibitor, not unrelated chemistry. Adding the wrong thing can void your manufacturer warranty.
What it costs to fix in London (inc VAT)
| Fix | Typical Cost | Time on site |
|---|---|---|
| Diagnostic visit | £110,180 | 30,60 min |
| Re-pressurise expansion vessel | £140,180 | 45,60 min |
| Replace expansion vessel | £180,280 | 60,90 min |
| Replace pressure-relief valve (PRV) | £140,220 | 45,60 min |
| Radiator valve weep repair | £150,240 | 45,60 min |
| Hidden leak detection (thermal + acoustic) | £180,360 | 60,120 min |
| Power flush (if recommended for system rejuvenation) | £450,650 | 4,6 hours |
Quoted at our day rate (Mon-Fri 7am-6pm). Out-of-hours rates apply outside these times, see pricing.
When to consider boiler replacement instead
If your boiler is over 12 years old, the expansion vessel or PRV repair quote exceeds £400, AND you've had multiple repairs in the last 24 months, replacement usually pays back within 3-5 years on energy savings alone. New A-rated combis from Worcester Bosch, Vaillant, Ideal and Viessmann run at 92%+ efficiency vs the 70-80% of older units. See boiler installation →
FAQ
Is it dangerous to leave a boiler that keeps losing pressure?
How often should I need to top up my boiler?
Can I replace the expansion vessel myself?
Will my landlord pay for this if I'm renting?
Need a London engineer to diagnose your boiler pressure issue? Call 020 8050 5306, Gas Safe engineers on-site within 30 to 60 minutes, day or night. More on heating & boiler repair →