Boiler Breakdowns and Their Cause
I have been to thousands of boiler breakdowns over my working career. They range from a simple power supply switch accidentally being knocked by a spaghetti jar, to a panicked call from a customer who’d tried to fix his boiler himself, made things worse, and was dreading his mum getting home to find out.
I’ll get to that story shortly. First, a bit of background on how boilers used to work — and how they work now.
The Thermocouple Days
In the early days, before all these electronically controlled boilers emerged, boilers were very simple to fix. One of the most common faults was the thermocouple.
The thermocouple is a small metal probe that sits directly in the pilot flame. Its main job is to prove the pilot is lit — it sends a signal to the gas valve, through a tiny electrical voltage, to the electromagnet inside the valve, essentially saying “I’m on.”
Over time, these thermocouples would burn out. Some would last for years, others less than one. What I found was that if the pilot flame was set too high, it acted like a burning torch and eventually burned the thermocouple out. If the pilot was set to a lower flame — but still within the manufacturer’s settings — it would last for years.
The recommendation was always to change the thermocouple at every service, but that got missed more often than not.
They were an easy repair, straightforward enough to fix. Believe it or not, there are still plenty of boilers out there today that use this exact method of lighting. Before I retired, I had around 12 annual boiler services a year on boilers averaging 40 years old — and still going.
Very rarely would you need to replace a gas valve on these older boilers.
The Panicking Son
Now, back to that call.
When I picked up the phone, I could sense from his voice that he was panicking. I had two more small jobs to finish first and told him I’d be there in about two hours.
I’ll never forget his face when I arrived. He opened the door and said, “Quick, my mum will be home in about an hour — please, please fix it before she gets back.”
I asked him to show me the boiler. What I found was a boiler in bits, scattered across the floor. I couldn’t believe it. “You told me it wasn’t working,” I said, “not that it was in bits. What happened?”
His story came out: that morning he’d noticed the hot water was cold, told his mum not to worry, that he’d get someone in to fix it — and then, once she’d left for work, decided to have a go at fixing it himself.
It wasn’t actually a boiler that ran heating and hot water — it was an old Chaffoteaux water heater. He’d isolated the water supply and stripped it down piece by piece, damaging parts along the way, and by the time he called me he couldn’t even remember where half the parts went.
I had a look at everything laid out on the floor and told him it couldn’t be repaired — it needed a new water heater entirely. Inside, I was quietly laughing; this guy had genuinely thought he could fix a water heater himself, and now he was panicking about what his mum would say.
Right as I was explaining all this, his mum walked through the door. His face went bright red. “What’s going on here?” she asked. I introduced myself and explained the situation — that they’d need a replacement. Her son quietly disappeared into another room.
She agreed to the price, and I went back two days later to install a new, modern fan-flue water heater. She later told me her son had been officially barred from doing any work in the house ever again. A strange one to be in the middle of.
How Modern Boilers Actually Work
Today’s boilers are all electronic ignition — no thermocouple, and a completely different way of working.
Most modern boilers go through an ignition sequence to fire up:
- A call for heat comes from something like a thermostat.
- The fan starts running, which opens an air pressure switch.
- That switch sends a signal to the printed circuit board (PCB) to start the electronic spark.
- The spark opens the gas valve, letting gas through to the burner.
- Once the flame is lit, a flame-sensing detector tells the PCB to keep the gas valve open.
Nearly every modern boiler works in a very similar way. Once you understand this startup sequence, diagnosing a breakdown becomes a process of testing each section one at a time — proving whether it’s working or not — until you find the fault.
Believe it or not, every make of boiler has its own common fault. It doesn’t matter whether it’s an expensive brand or a cheap one — so when I took a call about a breakdown, I always asked for the make and model first, just to make sure I had the right parts on board.
There was one boiler brand (no names) that, as soon as it reached a certain age, would reliably throw up the same fault codes. As soon as you got the call, you knew it needed a new PCB and gas valve.
Here’s the bit I could never quite get my head around: both of those parts, at the time, cost more than a brand new boiler. No guarantee on the PCB — that was their policy — and only a year’s warranty on the gas valve. Meanwhile, a new boiler came with two years’ warranty, and the price difference was give or take twenty pounds. Labour was higher for a full replacement, of course, but it still didn’t add up in the customer’s favour. That brand became well known for the problem — almost like they’d built in a timer for the boiler to fail, always just after the warranty ran out.
You start to learn what to look for the longer you’re in the trade.
The Spaghetti Jar
There was one breakdown I got called to in central London. I always tried to avoid London — the traffic and parking were brutal — but this lady offered to cover all my expenses and more. I’d never dealt with her before; a close friend of hers had recommended me.
I arrived at 8am sharp. She lived on the ninth floor of a tower block in Notting Hill. She opened the door and said, “My friend told me you’d arrive on time, without fail.” I asked where the boiler was, and she led me to a cupboard in the kitchen.
It was a combination boiler, so the first thing I checked was water pressure — all fine. She offered me a tea or coffee and said she’d have her housekeeper make it. This was one of those enormous flats in central London, worth an absolute fortune — the family were from another country’s high society, and this was their London base for three months of the year.
Back to the boiler: no lights showing at all. I traced the power supply to a switch just outside the boiler cupboard, mounted on the wall above the worktop. In front of it stood four tall storage jars — one of them for spaghetti.
I moved the jar and found the switch was off. Flicked it back on, and the boiler sprang to life. I called the lady over and explained — the housekeeper had probably knocked it without realising. I asked when the boiler was last serviced; she wasn’t sure, but told me to go ahead and do it if I thought it needed it, so I did.
Before I left, I checked the radiators and hot water — all good. As I was writing up the invoice, she insisted I charge for a full day plus parking. I said that wasn’t necessary; she insisted anyway. On my way out, I gave her one last piece of advice: “Remember the jars. If it stops again, check the jars first.”
Two days later, she called: “Steve, the boiler’s not working again.” I asked if she’d checked the switch behind the jars. She had — but it still wasn’t working.
I went back the next day, checked the power supply first, and sure enough — off again. She explained she just wanted to be sure everything was safe. This happened three more times. I even moved the jars out of the way entirely, but the housekeeper kept putting them back. Each time, I was paid for a full day regardless. I carried on working for that lady for seven years, right up until I retired.
Sometimes that’s genuinely all it is — a power supply switched off by mistake.
Before You Call an Engineer
A good number of the breakdowns I attended over the years turned out to be something simple — a switched-off isolator, low water pressure, a tripped breaker. Before you pick up the phone, it’s always worth running through the basics yourself first.
I’ve put together a free checklist covering exactly what to check, and the warning signs that mean you should call an engineer straight away rather than investigating further.
