Read Time: 9 minutes
If you’ve topped up your boiler pressure two, three, even five times this week and it keeps sliding back down, you’re not doing anything wrong. Your boiler is telling you something is actually broken. Repressurising a boiler is meant to be a once-in-a-while job, not a daily ritual. When it becomes a daily ritual, the pressure loss has a physical cause somewhere in the system, and no amount of topping up the filling loop will fix it.
This is one of the most common calls our engineers get across North and East London, and it’s almost always one of five things. Below, we’ll walk through exactly why this happens, how to tell which one applies to you, and when it’s time to stop repressurising and call a boiler repair service London homeowner’s trust.
Quick Answer: Why Your Boiler Pressure Keeps Dropping
If repressurising isn’t holding, the most likely causes are a pinhole leak somewhere in the pipework, a faulty pressure relief valve, a failed expansion vessel, a leaking heat exchanger, or a stuck filling loop valve letting pressure creep in and out unevenly. A one-off pressure drop is normal. A pressure drop that returns within hours or days is a fault that needs diagnosing a Gas Safe engineer, not another top-up.
Why Repressurising Alone Isn’t Fixing It
Repressurising simply adds more water back into a sealed heating system. It doesn’t fix whatever is letting water or pressure escape in the first place. Think of it like reinflating a tyre with a slow puncture: the tyre holds air for a while, then goes flat again, because the actual hole was never patched.
The same logic applies to your boiler. If you’re repressurising and it holds for weeks, that’s normal wear and tear. If it drops again within a day or two, or drops faster each time, there’s an active fault that needs a proper diagnosis, not a home fix.
The Real Reasons Your Boiler Won’t Hold Pressure
1. A Pinhole Leak in the Pipework or Radiators
Small leaks are often invisible to the eye but easy for an engineer to trace with pressure testing and dye checks. Look for:
- Damp patches near pipe joints, under radiators, or around the boiler casing
- Faint white or green limescale marks near a fitting (a sign of a slow, dried leak)
- A drop in pressure that’s gradual rather than sudden
Even a leak the size of a pinhead can drain your system of pressure over 24-48 hours.
2. A Faulty Pressure Relief Valve (PRV)
The PRV is a safety device designed to release water if pressure gets too high. When it fails, it can release small amounts of water even at normal operating pressure, causing a slow but steady drop. If you notice water dripping from a pipe outside your property (the external overflow pipe), a failing PRV is very likely the cause.
3. A Failed Expansion Vessel
Every sealed heating system has an expansion vessel that absorbs the natural expansion of water as it heats up. If the rubber diaphragm inside fails or the vessel loses its charge of air, your system pressure will rise and fall unpredictably, and topping it up won’t hold for long. This is one of the most missed causes because it isn’t visible from the outside; it requires a pressure gauge test by an engineer.
4. A Leaking Heat Exchanger
This is the more serious end of the scale. A cracked or corroded heat exchanger allows water to escape internally, sometimes without any visible sign at all. It’s more common in boilers over 10-12 years old that haven’t had regular servicing. If your boiler is older and losing pressure repeatedly, this should be ruled out early rather than last.
5. A Stuck or Leaking Filling Loop
The filling loop is the flexible braided hose you use to repressurise the system. If the valves on either end don’t seal fully shut after use, mains water can continue seeping into the system, sometimes actually pushing pressure too high before it drops again through the PRV. A loose or worn filling loop is a simple fix, but one many homeowners never think to check.
Repressurising Too Often Can Make Things Worse
Here’s what a lot of people don’t realise: repeatedly repressurising a boiler that has an underlying leak can actually accelerate corrosion inside the system, push sludge and debris around the pipework, and put extra strain on the pump and heat exchanger. What feels like a harmless quick fix can quietly shorten the life of your boiler if the real fault is never addressed.
This is the same pattern we see with Vaillant boilers throwing an F22 fault code repeatedly. You can read our breakdown of that specific issue in our guide to emergency Vaillant boiler pressure repairs and the F22 fault code, which covers exactly why that model behaves this way. If you own a Vaillant system specifically and pressure keeps dropping, our guide on Vaillant boilers losing pressure in London homes goes into brand-specific causes in more detail.
If you drive a Worcester Bosch boiler, the diagnostic steps differ slightly. Our engineers have documented the most common fault codes in our guide to Worcester Bosch boiler repair, fault codes, and low-pressure emergency fixes.
How to Check Before You Call an Engineer
Before booking a repair, a quick visual check can help narrow down the cause and save time on the visit:
- Check the pressure gauge reading: anything consistently below 1 bar or rapidly climbing above 2.5 bar needs attention.
- Look under the boiler and along visible pipework for damp patches, drip marks, or limescale staining.
- Check the external overflow pipe on an outside wall for signs of dripping; this points to a PRV issue.
- Note how quickly pressure drops: hours suggest a leak, while days-to-weeks suggests a smaller fault like the expansion vessel.
- Check your filling loop is fully closed after topping up (both valves should sit in a straight, closed position).
If you’ve done all of this and pressure is still dropping, it’s time to stop repressurising and get a proper diagnostic. For step-by-step guidance on the repressurising process itself, see our detailed guide on how to restore boiler pressure safely.
What Happens During a Professional Pressure Diagnostic
When one of our Gas Safe engineers attends a repeated pressure-loss call, we don’t just top up the system and leave. A proper visit includes:
- A full visual inspection of pipework, radiator valves, and joints for leaks
- Pressure testing the expansion vessel’s air charge
- Checking the PRV for correct operation and signs of weeping
- Inspecting the filling loop and isolation valves
- Assessing the heat exchanger and pump seals where age or corrosion is suspected
- A clear explanation of the fault and an honest, upfront repair quote before any work begins
This is the same standard of diagnostic work behind every Worcester Boiler Repair London and Worcester Boiler Service London visit we carry out, whether it’s a routine annual service or an urgent pressure fault.
When to Treat This as an Emergency
Most pressure faults aren’t dangerous on their own, but you should treat it as urgent and book emergency boiler repair in London if you notice any of the following alongside the pressure drop:
- A smell of gas near the boiler
- Visible water pooling near electrical components
- The boiler locking out repeatedly and refusing to reset
- No heating or hot water during cold weather, especially with vulnerable household members
Our engineers cover Hackney, Stoke Newington, Clapton, Walthamstow, Islington, and the surrounding areas with same-day emergency callouts where possible.
Get It Fixed Properly, Not Just Topped Up
Constant repressurising is a symptom, not a solution. Whether the cause turns out to be a small leak, a tired expansion vessel, or a failing PRV, a qualified engineer can usually diagnose it in one visit and give you an honest repair quote on the spot. Our team at Boiler Services London are Gas Safe registered, accredited by Worcester Bosch and Vaillant, and available for Worcester Bosch boiler servicing in London as well as full diagnostic repairs across all major boiler brands.
Don’t keep guessing with the filling loop. Book a diagnostic visit today and get it fixed properly, the first time.
📞 Call us now on 07877767776 🌐 Or book online here
Frequently Asked Questions
This usually means there’s an active leak or fault somewhere in the system, commonly a pinhole leak in the pipework, a worn pressure relief valve, or a failed expansion vessel. Repressurising only refills the system; it doesn’t fix whatever is letting pressure escape.
Low pressure alone usually isn’t dangerous, but it can cause the boiler to lock out and leave you without heating or hot water. If you also smell gas or see water near electrical parts, treat it as an emergency and turn off the boiler immediately.
A well-functioning sealed system might need topping up once every few months, if at all. Needing to repressurise weekly or more often is not normal and indicates a fault that needs diagnosing.
Yes. If the rubber diaphragm inside the expansion vessel fails or loses its air charge, the system can’t absorb the normal expansion of heated water, causing pressure to fluctuate and drop unpredictably even without a visible leak.
Costs vary depending on the cause: a filling loop or PRV replacement is typically a lower-cost fix, while an expansion vessel replacement or heat exchanger repair costs more. Our engineers always provide a clear, upfront quote after diagnosis before starting any work.