Quick Answer
A new boiler that keeps breaking down is usually not a faulty boiler. In most early failures we see, the cause sits in the installation or the system around it. The boiler is often fine, and the fix lies in the pipework, the water or the settings.
Four faults come up again and again. A dirty heating circuit that was never flushed. Gas pipework too small for the new boiler. A missing magnetic filter or unregistered logbook. A bypass that was never set up. Each one needs a different fix, so testing comes first.
A diagnostic visit is £120 on weekdays, and you get a fixed written quote before any repair. Nothing is replaced until you agree the price. We are Gas Safe registered (657972) and cover West and North West London.
Most people blame the manufacturer first. Our lead engineer Adam Said sees it differently. Before you write the boiler off, check how it was fitted.
Key Takeaways
- Check the install first: Most early breakdowns trace back to a dirty system, undersized gas pipe, missing filter or bad commissioning, not a faulty boiler.
- Warranty depends on paperwork: No Benchmark logbook, filter or registration can void a manufacturer guarantee, so find those records before you call anyone.
- Resetting repeatedly is a warning: One reset is fine. A second lockout within days means a fault is present and needs an engineer.
- Fixed prices from £120: Weekday diagnostic visit £120, water test and inhibitor £50 with a service, powerflush from £499, across West and North West London.
Four faults behind most early boiler breakdowns
A new boiler is only as good as the fitting job and the system it joins. Here is what we find most often, and what fixes it.
Unflushed Magnetite Sludge
Plate Heat Exchanger BlockageOld radiator circuit connected to the new boiler without a machine powerflush, so black iron oxide circulates into the new appliance
Shower cycles hot then cold, radiators cold at the bottom, pump noise, black water at the bleed valve
Bleed a radiator into a white cloth. Black or brown water is a warning sign. Stop there
System water test, magnetic filter check, then a chemical clean or full powerflush and fresh inhibitor
Undersized Gas Supply Pipe
Flame Lockouts Under LoadA bigger combi fitted onto the old 15mm supply pipe, so working gas pressure drops when the boiler runs flat out
Fires and runs fine in mild weather, then locks out with an ignition or flame fault when heating and hot water run together
Note when it locks out and what else was running. Never touch gas pipework yourself
Working gas pressure measured at full rate, then the supply run upsized (22mm where the load needs it)
Missing Magnetic Filter and Void Warranty
Unregistered Benchmark LogbookCorner-cut fit with no magnetic filter, no recorded flue gas readings and no registration, so the guarantee can be refused
Benchmark logbook blank or missing, no filter on the return pipe, no guarantee paperwork from the installer
Look for the logbook and the filter (a small cylinder on the pipe near the boiler). Photograph both
Fit a magnetic filter, complete missing records, check registration and advise on the manufacturer’s terms
Hydraulic Bypass and Commissioning Calibration
Circulation Restriction and OverheatModulating pump pushing against closed radiator valves because the bypass was omitted or set wrongly
Boiler overheats and locks out when most radiator valves have closed, kettling or whistling from the pipework
Check that at least one radiator or the bathroom towel rail stays open. Do not adjust any bypass valve
Set the automatic bypass valve to the boiler maker’s figures, balance the radiators and recheck flow and return temperatures
A dirty system kills a clean boiler
Modern condensing boilers have very narrow waterways in the plate heat exchanger. Old radiators corrode inside and shed black iron oxide, called magnetite. Baxi explains that these particles travel round the circuit and can clog boiler components.
Connect a new boiler to that water and the debris goes straight in. It settles in the heat exchanger. It wears the pump. Within months the shower runs hot, then cold, and fault codes return.
The standard that covers this is BS 7593:2019. It expects the system to be cleaned, treated with inhibitor and fitted with a filter before a new boiler goes on. A quick drain and refill does not meet it. Compacted sludge needs a machine powerflush.
This is the fault we see most on repeat call-outs to homes in Kensal Rise and Queen’s Park. Replacing a part will not cure it. The water has to be cleaned first.
A bigger boiler on the old gas pipe
Old heat-only boilers were often around 15 to 18kW. A modern combi is typically 30 to 36kW. That is nearly twice the gas demand.
If the installer kept the old 15mm supply pipe, the boiler can be starved when it runs flat out. Working gas pressure at the boiler drops. The flame goes unstable, and the boiler locks out with an ignition or flame fault code.
The giveaway is timing. It runs fine in mild weather, then fails on the first cold morning when heating and hot water run together. The boiler maker’s instructions give the minimum working inlet pressure, normally around 20 mbar. Only a Gas Safe engineer can measure it and size the pipe. Do not touch gas pipework yourself.
No filter, no logbook, no guarantee
A boiler guarantee is a contract with conditions. Vaillant’s extended-guarantee terms ask for flushing, a filter and inhibitor. Worcester Bosch lists filters as a condition for particular guarantees. Terms differ by model, so check yours.
The Benchmark logbook is the proof. It records the flue gas readings, the cleaning, the filter and the inhibitor. A blank logbook is a red flag. So is a boiler with no magnetic filter near it.
If the paperwork is missing, a manufacturer can decline a claim. That is why we fit a magnetic filter, complete any missing records and tell you plainly where you stand.
Pump pushing against closed valves
Modern pumps modulate their speed. When the thermostatic radiator valves close, the pump can push against a dead end. An automatic bypass valve gives the water somewhere to go.
If the installer left it out, or set it wrongly, the boiler overheats and locks out. You may hear whistling or kettling from the pipes. The fix is calibration. We set the bypass to the manufacturer’s figures and balance the system.
What you can safely check yourself
Start with the basics.
- Look at the pressure gauge. A cold system should read around 1 to 1.5 bar.
- Check the programmer and thermostat batteries.
- Bleed one radiator into a cloth and look at the water colour.
- Find your Benchmark logbook and photograph the commissioning pages.
- Note the fault code, the time it happened, and what else was running.
Reset the boiler once if the manufacturer’s instructions allow it. If it locks out again within hours or days, stop. Resetting over and over forces the boiler to retry against a real fault.
Stop and call a Gas Safe engineer if
Call us straight away if the boiler locks out repeatedly, the pressure keeps dropping, the water from a bleed valve is black, or you hear banging or whistling from the pipes.
If you smell gas, or a carbon monoxide alarm sounds, call the National Gas Emergency Service on 0800 111 999 first. Open doors and windows and leave the property. Do not use switches or flames.
Removing the boiler casing, working on gas pipework and adjusting the bypass are Gas Safe jobs. They are not safe DIY, and the law requires registered competence for gas work.
What our engineer does on the visit
- We ask what the boiler is doing, the fault codes and when it began.
- The commissioning record, filter and installation get checked against the manufacturer’s instructions.
- System water is tested, and the pump and heat exchanger are observed under load.
- Working gas pressure is measured at the boiler while it runs at full output.
- Next comes the bypass and circulation check.
- You get the findings, the evidence and a fixed quote in writing.
You decide whether to go ahead. Nothing starts until you have agreed the price.
What it costs to diagnose and fix
Everything below is a fixed price. We quote before any work begins.
Is Your New Boiler Constantly Locking Out?
Our Gas Safe registered engineers (657972) find the real cause for a fixed £120 diagnostic fee on weekday daytime visits. You get a written quote before any repair.
Problems that look like a faulty new boiler
Not every early fault is one of the four above. A leak can drop the pressure. Flat thermostat batteries can stop the boiler firing. In frost, a frozen condensate pipe can lock it out. Our guide to boiler losing pressure covers the most common of these.
An engineer rules each one out before blaming the installation. That is why a diagnostic visit comes first. If the boiler is old enough that repairs keep stacking up, our guide on whether to repair or replace your boiler helps you decide.
Protect the boiler once it is fixed
Once the cause is fixed, keep it fixed. A yearly service checks the filter, the water and the combustion readings. It also keeps your guarantee valid. A boiler servicing visit is £120, or you can spread it with the Homeowner Plan, which includes priority booking if the boiler breaks down, at £9.99 a month.
If the installation was so poor that the work cannot be put right, a proper boiler installation may be the better route. Do not decide that before you have a written diagnosis.
Frequently asked questions
Is a new boiler that keeps breaking down covered by warranty?
Usually yes, if the fault is a manufacturing defect and the guarantee conditions were met. Manufacturers typically ask for a registered installation, a clean system, a magnetic filter and annual servicing. If an installation fault or a dirty system caused the breakdown, the claim can be declined. Check the terms for your exact model and keep your Benchmark logbook safe.
Can sludge break a new boiler?
Yes. If a new boiler is connected to radiators that were never cleaned, black magnetite circulates into it. It blocks the narrow waterways in the heat exchanger and wears the pump. Hot water turns lukewarm, the boiler cycles or overheats, and fault codes return. A water test and a clean usually confirm it. Replacing parts alone rarely fixes it.
Why does my new boiler lock out in winter?
Winter is when the boiler works hardest, so weak spots show. Common causes are undersized gas pipework starving the burner at full output, a blocked heat exchanger, a frozen condensate pipe, or a pump struggling against debris. If it runs fine in spring but locks out on cold mornings, tell the engineer. That pattern points to supply or flow, not a faulty board.
What is the Benchmark logbook?
It is the commissioning record the installer completes when your boiler is fitted. It should show the flue gas readings, whether the system was cleaned, the inhibitor and magnetic filter fitted, and the installer’s details. Manufacturers often ask for it before they honour a guarantee. Service visits are then stamped in the same book. If yours is blank, ask your installer for it.
How do I know if my gas pipe is undersized?
You cannot tell by looking, and you must not work on gas pipework yourself. The giveaway is a boiler that fires fine at low demand but locks out when the shower and radiators call for heat together. A Gas Safe engineer measures the working gas pressure while the boiler runs at full output. A big drop means the pipe is too small.
Can I claim against my original installer?
You can ask them to return and put it right. Most installers offer a workmanship guarantee, and a missing flush, filter or logbook is an installation matter. Keep your invoice, commissioning record and any messages. Get a written engineer’s report on what is wrong before you contact them. If they refuse, Gas Safe Register can look into unsafe or non-compliant gas work.
What does a diagnostic visit cost?
A weekday daytime diagnostic visit is £120. Saturday daytime is £160 and weekday evenings are £180. Overnight, Sunday and bank holiday emergencies are £220. The engineer tests, finds the cause and gives you a fixed price in writing before any repair starts. System water testing and cleaning are quoted separately once we know what your system needs.
Is it safe to keep resetting my boiler?
One or two resets are fine. If the boiler locks out again within hours or days, stop resetting it. Repeat lockouts mean something is wrong, and each reset forces the boiler to try again against that fault. If you smell gas or a CO alarm sounds, leave the property and call the National Gas Emergency Service on 0800 111 999 straight away.
Need an engineer for a new boiler that keeps locking out?
Tell us the fault code, how old the boiler is, and what you have already tried. We will get an engineer to you with a clear fixed price agreed before any work starts. For urgent faults, see our emergency boiler repair page.
- WhatsApp us and book in 60 seconds
- Call 020 8051 2124. The line is answered 24/7 and an engineer gets your details.
- Book online: boiler repairs
We cover Kensal Rise, Queen’s Park, Acton, Ealing and the wider West and North West London area. We’re Gas Safe registered (657972). Heating that just works, no drama.
