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At a glance: Pressure loss, cold taps, noisy pipes, a boiler that won’t fire, patchy radiators, a worn-out pump, or a failing heat exchanger — these seven issues cover the vast majority of the callouts we attend across Essex and Hertfordshire. We’ll walk you through each one, what causes it, and what you can do right now.

There’s never a good time for your boiler to stop working. Whether it’s a cold morning in Dunmow or the middle of the school run in Harlow, a boiler fault tends to make itself known at the worst possible moment. The good news is that most problems follow a pattern, and once you know what to look for, you can often pinpoint the fault — or at least know what to tell your engineer — before anyone even picks up the phone.

We’re Gas Safe registered engineers, who fix combi boilers. We’re based in Great Dunmow, and we work across  Dunmow, Takeley, Sawbridgeworth, Harlow or Saffron Walden and the wider Essex area. These are the boiler faults we deal with day in, day out.

 

A quick look inside your boiler

A combi boiler pulls cold water directly from the mains and heats it on demand — no storage tank, no separate cylinder. That’s what makes them so popular in smaller homes. But because everything runs through a single unit, one failing part can knock out both your heating and your hot water at once.

The four components that come up most often in fault diagnosis are the heat exchanger (where heat transfers from the burner to the water), the circulation pump (which pushes heated water around your radiators), the diverter valve (which decides whether heat goes to the radiators or the taps), and the pressure sensor (which shuts the boiler down if the system pressure drops too far).

The seven faults we see most

1. The pressure keeps dropping

If there’s one thing we get called out for more than anything else, it’s low boiler pressure. Your system should sit between 1 and 1.5 bar when it’s cold and the heating is off. Drop below that and most boilers will shut themselves down — showing codes like F22 (Vaillant), F1 (Ideal) or E119 (Baxi).

Pressure loss almost always means water is finding its way out of the system. Sometimes it’s a slow drip at a radiator valve or a pipe joint that evaporates before it leaves a mark. Bleeding your radiators also lets water escape. Occasionally a faulty pressure relief valve is discharging water through the external overflow pipe — worth checking outside.

To top the pressure back up:

1. Switch the boiler off and give everything time to cool down fully.
2. Find the filling loop — it’s the braided silver hose with a valve at each end, typically underneath the boiler itself.
3. Open both valves gently. You’ll hear water flowing into the system.
4. Keep an eye on the gauge — once it reaches 1.5 bar, close both valves.
5. Restart the boiler and check it fires up normally.

If the pressure falls again within a few days, there’s a leak somewhere in the system. Keep topping it up without investigating and you risk causing further damage down the line.

 

2. Hot water has gone, but the radiators are fine

This is a classic sign of a diverter valve issue. The diverter valve is what tells your boiler to redirect its heat toward the hot water circuit when you open a tap. When it starts to stick or seize, the boiler just keeps heating the radiators and the taps run cold.

Before assuming it’s the valve, check a couple of things first. Is the hot water temperature on the boiler controls turned up — ideally between 50 and 60°C? Is the tap running at full flow? Some boilers need a decent flow rate before they’ll switch into hot water mode. A partial turn of the tap might not be enough. If neither of those is the issue, try holding the reset button for about four seconds and let the boiler go through its full startup.

If the problem is still there after a reset, the diverter valve almost certainly needs replacing. That’s not a job for a homeowner — it involves opening up the boiler and working with internal components, so it needs a Gas Safe engineer. Repair costs typically sit between £200 and £350 with parts and labour included.

 

3. The boiler is making strange noises

Your boiler shouldn’t be the loudest thing in the room. If it is, the noise itself is usually a clue about what’s going wrong.

A sound like a kettle coming to the boil — what engineers call kettling — points to restricted water flow through the heat exchanger. In hard water areas (and much of Essex qualifies), limescale builds up inside the exchanger over time and causes localised hotspots. Heating sludge — the dark, rusty debris that accumulates in older systems — can do the same thing. A powerflush can often clear sludge-related kettling; limescale sometimes needs chemical treatment or a new exchanger altogether.

Banging usually means air is trapped in the system or something has come loose and is vibrating when the pump runs. Gurgling tends to suggest a flow problem — air in the pipework, a struggling pump, or a condensate pipe that’s partially blocked. Bleeding your radiators from the top of the house downward is a good first step. If the noise carries on afterwards, it needs a proper look.

 

4. The boiler won’t come on at all

A boiler that’s completely dead — no lights, no display, no sounds — is often something simple. Work through this before calling anyone:

• Check the fused spur. There should be a switched spur on the wall near your boiler, usually labelled. If the fuse inside has blown, that’s a very cheap and easy fix — it takes a 3A fuse.
• Read the display. No screen at all means no power reaching the boiler. An error code or flashing light means it has power but has detected a problem — make a note of any code showing.
• Try a reset. Hold the reset button for five seconds and give the boiler time to complete its startup cycle. If it locks out again straight away, stop resetting. Every repeated lockout without a fix risks masking a more serious underlying fault.

 

5. Some radiators are cold, others are roasting

When your heating is uneven — upstairs freezing, downstairs tropical — it’s usually one of two things: trapped air or an unbalanced system.

Bleeding removes air that’s accumulated in the radiators. Turn the heating off and wait for things to cool. Using a radiator key, open the bleed valve at the top corner of each radiator — starting furthest from the boiler and working back toward it. Air will hiss out; close the valve as soon as water follows. Check your boiler pressure afterwards and repressurise if it’s dropped.

Balancing is the next step if bleeding doesn’t solve it. The radiators that heat up fastest are getting too much flow. Their lockshield valves — usually a capped valve that needs a spanner — can be turned clockwise to reduce their share. It’s a process of small adjustments and patience, but it makes a noticeable difference to comfort and efficiency.

 

6. The pump has stopped working

The pump is what keeps hot water moving around your home. When it fails, heat builds up inside the boiler with nowhere to go — and the boiler will typically lock out to protect itself.

The signs are fairly distinctive: the boiler fires and you can hear it working, but the radiators stay cold. You might also notice an unusual humming or grinding coming from inside the unit, or small leaks around the pump housing. In some cases a seized pump can be freed temporarily by tapping the casing — but that’s a sticking plaster, not a solution. Pump replacement is a Gas Safe job, and if your boiler is getting on in years, it’s worth having a conversation about whether repair or replacement makes more sense overall.

 

7. The heat exchanger is failing

The heat exchanger sits at the heart of the boiler. It’s where the energy from burning gas is transferred into the water that heats your home. When it starts to go, it tends to show up in a few different ways: kettling that won’t go away even after a powerflush, pressure that keeps dropping without any visible leak (a cracked exchanger leaks internally into the boiler), water temperature that swings unpredictably at the taps, or repeated overheating lockouts.

Replacing a heat exchanger is one of the more significant boiler repairs in terms of cost. For a boiler that’s ten years old or more, the repair cost can start to approach the price of a new boiler — so we’ll always give you an honest view of whether a fix or a replacement is the smarter investment.

 

Boiler error codes — what they mean

When to stop troubleshooting and call straight away

There are situations where the right move is to stop, leave the boiler alone, and pick up the phone. If you can smell gas anywhere near the appliance, if there’s sooty or black staining around the boiler casing, if the flame is burning yellow or orange rather than blue, or if your carbon monoxide detector has gone off — get everyone out and call the Gas Emergency line on 0800 111 999.

For non-emergency faults: internal components like the gas valve, PCB, heat exchanger, pump and diverter valve must always be handled by a Gas Safe registered engineer. It’s a legal requirement, and not one worth cutting corners on.

 

The easiest way to avoid all of this

An annual boiler service catches most of these problems before they become breakdowns. An engineer will clean the internal components, check gas pressures, test safety devices, and flag anything that’s starting to wear. It costs far less than an emergency callout — and it keeps your manufacturer’s warranty valid too. Checking your pressure once a month and bleeding your radiators each autumn takes less than ten minutes and makes a genuine difference to how efficiently your system runs.