Central Heating Not Working But Hot Water Is Fine: Systematic Diagnosis

When your central heating stops working but hot water remains fine, the boiler itself is almost certainly not the problem. The fault most likely lies in a stuck diverter valve, a faulty zone valve, a thermostat issue, or an incorrect programmer setting - each of which affects only the heating circuit, leaving your hot water supply untouched.

Why Can Heating and Hot Water Fail Independently?

Your boiler produces heat from a single burner or heat exchanger, but two separate circuits carry that heat to its destination. One loop runs through your radiators and underfloor heating pipes - the central heating circuit. The other heats the water at your taps and in your shower - the domestic hot water, or DHW, circuit. These circuits are kept separate by valves, pumps, and electronic controls, which is why one can fail while the other continues running perfectly.

The type of boiler you have determines how that separation is managed. In a combi (combination) boiler - the most common type installed in UK homes - an internal component called a diverter valve physically switches water flow between the two circuits on demand. When you open a hot tap, the boiler detects the flow and the diverter valve moves to prioritise domestic hot water. When the tap is closed, the valve should move back so the boiler can serve your radiators instead. In a system boiler with a hot water cylinder, the separation is handled by motorised zone valves and a separate circulation pump, usually located in the airing cupboard. If you are unsure which type of system you have, the Go Assist boiler questions and answers guide explains the differences in plain English.

What Are the Most Common Causes?

When hot water works but central heating does not, experienced engineers encounter the same handful of faults time and again. Here are the most likely culprits, roughly in order of how frequently they are seen:

• Stuck or faulty diverter valve (combi boilers): The valve gets stuck in the hot water position, permanently blocking the heating circuit from receiving any flow.
• Failed motorised zone valve (system boilers): The electrically operated valve for the heating circuit fails to open, so radiators never receive hot water regardless of what the thermostat demands.
• Thermostat set too low or malfunctioning: A thermostat that reads the room as warmer than it actually is will never send the signal to start the heating.
• Programmer or timer set incorrectly: After a power cut or clock change, the heating schedule may have been wiped or reset, effectively switching the heating off permanently until it is reprogrammed.
• Circulation pump failure: A seized or burned-out pump means hot water from the boiler simply cannot circulate around the radiator circuit.
• Air locks in the system: Air trapped in the radiator pipes stops water circulating properly, leaving some or all radiators cold even though the boiler is firing.
• Low boiler pressure: If system pressure drops below around 1 bar, most modern boilers automatically shut down the heating circuit as a protective measure.
• Wiring or PCB fault: Less common but possible - a wiring fault or printed circuit board failure can prevent the heating circuit from ever receiving an activation signal.

Is a Stuck Diverter Valve to Blame?

If you have a combi boiler, a stuck diverter valve is the first thing most heating engineers will suspect. The diverter valve switches water flow between the heating and hot water circuits, and after years of constant switching - sometimes thousands of operations per week in an active household - it can become sluggish, corroded, or completely seized. When it sticks in the domestic hot water position, your taps and shower continue to work normally but the heating circuit never opens, so every radiator stays cold.

There is a straightforward field test you can carry out. Set the heating to full and wait for the boiler to fire. Then carefully place your hand on the large flow pipe coming out of the boiler. If that pipe remains cool while your hot water works normally, the diverter valve is almost certainly the culprit. An experienced engineer can typically confirm the diagnosis within minutes of arriving on site.

Diverter valve replacement involves draining the system, partially dismantling the boiler casing, and working with gas-connected components - so it must be carried out by a Gas Safe registered engineer. The valve itself typically costs between £30 and £80, but with labour and call-out, the full repair usually runs to between £150 and £350, depending on the boiler make and model and your location in the UK. Parts for popular brands are generally readily available, meaning most repairs can be completed in a single visit.

On older boilers - particularly those more than 12 to 15 years old - a worn diverter valve can be a sign that the boiler is approaching the end of its serviceable life. If the repair cost is approaching a significant proportion of a new unit, it is worth exploring boiler installation options and comparing the long-term figures before committing to the repair. A modern, A-rated boiler will also reduce your energy bills considerably compared with an ageing unit running at lower efficiency.

Could Your Thermostat or Programmer Be the Problem?

Before assuming a mechanical fault, always rule out a settings issue first - it is surprisingly common and costs nothing to check. The room thermostat and programmer are the brain of your heating system, telling the boiler when to switch on and where to send heat. If either is set incorrectly or has developed a fault, the boiler will fire for hot water on demand but simply never receive the signal to run the central heating circuit.

Start with the room thermostat. If it is a traditional dial type, turn it up well above the current room temperature - setting it to around 20 or 21 degrees Celsius is a good test point. If it has a digital display, check that the batteries are fresh and that the target temperature shown is higher than the current room reading. Smart thermostats can sometimes lose their schedules after a software update or a loss of Wi-Fi connection, so check the app settings carefully.

Next, inspect the programmer or timer unit, usually mounted near the boiler or on a hallway wall. After a power cut or the end of British Summer Time, these units frequently revert to a factory default schedule - often with central heating switched off entirely. Try manually overriding the timer to continuous heating mode and wait 20 minutes. If the radiators begin to warm up, the problem is simply the programme settings and you just need to enter the correct schedule again.

It is also worth noting that some system boiler controllers give priority to heating the hot water cylinder over the central heating circuit. If the cylinder thermostat is set very high, the boiler may spend a disproportionate amount of time re-heating the cylinder, leaving little time for the heating circuit to run. Adjusting the cylinder thermostat to around 60 degrees Celsius is the correct setting for both comfort and energy efficiency.

If you have adjusted all the settings and the heating still fails to respond, the thermostat itself may have developed a fault and is no longer sending the correct signal. A basic replacement room thermostat costs around £20 to £50, while a smart unit runs from around £100 to £200, with installation adding roughly £50 to £100 in most parts of the UK.

What About Zone Valves and the Circulation Pump?

If you have a system boiler with a separate hot water cylinder, your heating and hot water are controlled by motorised zone valves - typically a two-port valve for each circuit, or a single mid-position three-port valve that handles both. When the zone valve for the heating circuit fails - either because the actuator motor burns out or the mechanical head seizes - the hot water cylinder circuit continues to work normally through its own separate valve. The result is exactly the symptom you are experiencing: reliable hot water, no central heating. The actuator head can often be replaced without draining the system, keeping labour time and cost down. Total costs typically fall between £100 and £280, depending on the valve type and the engineer's location.

The circulation pump is responsible for pushing hot water from the boiler around the entire radiator circuit. If it seizes - which often happens after a long period without use, such as at the very start of winter - water stops moving through the radiators even though the boiler fires normally. A useful diagnostic clue: radiators close to the boiler may feel faintly warm while those further along the circuit are completely cold, suggesting water is not being pushed far enough around the system. In some cases an engineer can free a seized pump by carefully turning the impeller shaft by hand, but a pump that cannot be freed will need replacing, typically at a total cost of between £150 and £400.

If radiators are consistently warm near the boiler but cold further around the circuit, this pattern can also indicate an air lock rather than pump failure. Bleeding the radiators - starting with the one furthest from the boiler - is a task any homeowner can tackle, and the full method is covered in the next section.

What Checks Can You Safely Do Yourself?

Before picking up the phone to book an engineer, run through this checklist. Working through it methodically could resolve the problem for free - and if it does not, you will be able to give the engineer a far clearer picture of what has already been ruled out, making the diagnosis faster and potentially cheaper.

1. Check the boiler pressure gauge. The gauge on your boiler - usually a dial or digital readout on the front panel - should read between 1 and 1.5 bar when the system is cold. Below 1 bar, most modern boilers will automatically shut down the heating circuit as a safety measure. Repressurise via the filling loop (a small silver or grey valve, usually located beneath the boiler), following the instructions in your boiler manual. Top the pressure up to around 1.2 bar, then switch the boiler back on and check whether heating resumes.
2. Review the thermostat and programmer settings. Turn the thermostat setpoint well above the current room temperature and manually override the programmer to continuous heating. Wait 20 minutes and observe whether the radiators begin to warm up.
3. Bleed the radiators. Using a radiator bleed key, open the bleed valve at the top end of each radiator, starting with the one furthest from the boiler. Hold a cloth underneath. If air hisses out, keep the valve open until water runs steadily from the opening, then close it. Repeat for every radiator in the house. Re-check the boiler pressure gauge after bleeding and repressurise if it has dropped below 1 bar.
4. Note any error or fault codes. Most modern boilers display a fault code on a small screen when something goes wrong. Write the code down and look it up in your boiler manual or on the manufacturer's website. Codes relating to flow problems, pump faults, or valve issues are particularly relevant when only the heating circuit is affected.
5. Reset the boiler. Many boilers have a reset button that clears a lockout condition. Press and hold it for a few seconds, allow the boiler to restart fully, and check whether the heating now responds. Only attempt this once or twice - repeatedly resetting without finding the underlying cause can mask a more serious fault developing inside the unit.
6. Check the programmer fuse. If the programmer display is completely blank, check the fused spur socket it is connected to. A blown fuse will take out the programmer entirely, leaving the boiler unable to receive any heating signal even though the manual hot water override continues to function normally.

How Much Will the Repair Typically Cost?

Repair costs vary based on the specific fault, your boiler make and model, and your location in the UK, but the following ranges give a realistic picture of what most homeowners can expect to pay.

• Diverter valve replacement: £150 to £350 in total, including parts and labour. Popular boiler brands generally have parts in stock, keeping waiting times short.
• Zone valve replacement: £100 to £280, depending on the valve type and whether the system needs draining down.
• Circulation pump replacement: £150 to £400, including any drain-down work required.
• Thermostat or programmer replacement: £70 to £300, depending on whether you choose a basic model or a smart unit.
• PCB (printed circuit board) repair or replacement: £200 to £600. On an older boiler this can approach the cost of a new installation and is worth comparing carefully before proceeding.
• Engineer call-out and diagnosis: Typically £60 to £120 as a standalone charge, though most engineers absorb this into the total if they carry out the repair during the same visit.

Most heating faults in the common categories above are resolved for between £150 and £400 when caught and addressed promptly. Leaving the problem unaddressed carries a real risk of secondary damage - a seized pump left running without adequate water flow, for example, can eventually harm the heat exchanger. An annual boiler service - typically £80 to £120 - is the most cost-effective way to catch developing faults before they escalate into an expensive breakdown at the worst possible time of year.

When Should You Call a Gas Safe Engineer?

Gas boilers and the components directly connected to them must be worked on only by Gas Safe registered engineers. This is a legal requirement, not bureaucratic red tape - working on internal boiler components, gas connections, or gas-carrying pipework without the correct registration is both illegal and genuinely dangerous.

Call a Gas Safe engineer immediately if any of the following apply:

• You smell gas anywhere near the boiler or pipework. Do not touch any switches - leave the property immediately and call the National Gas Emergency Service on 0800 111 999.
• The boiler is making unfamiliar banging, kettling, or persistent clicking noises alongside the loss of heating.
• A fault code points to a combustion, gas pressure, or heat exchanger problem.
• You have worked through the DIY checklist above and the heating has still not been restored.
• The boiler is going into lockout repeatedly, even after resetting.

For faults that are clearly not gas-related - such as a thermostat wiring issue or a seized zone valve actuator - a qualified heating engineer can carry out the repair. However, since most call-outs involve at least a visual inspection of the boiler, a Gas Safe registered engineer is almost always the safest and most practical choice.

Go Assist connects homeowners with Gas Safe registered heating engineers across the UK, with experience covering a wide range of boiler makes and models. Whether the fault turns out to be a stuck diverter valve, a failed zone valve actuator, or something less common, the aim is always to diagnose accurately on the first visit and complete the repair as efficiently as possible.

If your boiler is reaching the end of its serviceable life and repair costs are beginning to mount, Go Assist can help you weigh up the alternatives. Our boiler installation service covers a full range of modern, energy-efficient units, and our engineers can walk you through the most cost-effective route for your home and budget.