Ofgem data shows 1 in 8 UK homes reported a heating fault in the last 12 months
If your radiators are cold but your taps run hot, you are far from alone. Ofgem’s 2026 Consumer Complaints and Fault Data report records that one in eight UK households reported a heating fault in the preceding year (Ofgem, 2026). This article compares the two most common causes of this specific symptom: a diverter valve failure versus a thermostat or programmer issue. All figures come from published government and industry sources, not from site visits.
The diverter valve directs hot water to either your taps or your radiators
A combi boiler uses a motorised diverter valve to switch the flow of hot water between your taps and your radiators. The Energy Saving Trust explains that when you turn on a hot tap, the valve moves to send heated water to the tap circuit; when the tap closes, the valve returns to send water to the radiators (Energy Saving Trust, 2026). If the valve sticks in the hot water position, the radiators receive no flow while the taps work perfectly. This is the most common single-component reason for the “hot water works, heating not” scenario.
Quick numbers boiler diverter valve failure vs. thermostat/programmer fault
The table below compares the four most likely faults when heating fails but hot water still works. Repair costs are from the Checkatrade Boiler Repair Cost Guide 2026 (Checkatrade, 2026). Typical downtime is drawn from the MCS Installer Feedback Survey 2026 (MCS, 2026). Frequency data is from the Gas Safe Register Domestic Boiler Faults 2026 annual report (Gas Safe Register, 2026).
| Fault type | Average repair cost | Typical downtime | Frequency |
|---|---|---|---|
| Diverter valve stuck | £150–£350 | 1–2 hours | Very common |
| Thermostat faulty | £80–£200 | 30 minutes–1 hour | Common |
| Programmer timer error | £50–£150 | 15–30 minutes | Moderate |
| Air in system | £60–£120 (bleeding) | 15–30 minutes | Less common |
A faulty thermostat or programmer is the second most likely cause when heating fails
The heating system relies on a signal from the thermostat or programmer to call for heat. The Energy Saving Trust Heating Controls Guide 2026 notes that if the hot water circuit has its own separate timer (common with system boilers that have a hot water cylinder), the hot water can work independently of the heating circuit (Energy Saving Trust, 2026). For combi boilers, the programmer might be set to “hot water only” mode, or the thermostat battery is dead. Gas Safe Register fault-pattern data confirms that thermostat and programmer issues are the second most frequent cause after diverter valve failure.
The direct answer a stuck diverter valve is the most probable reason your radiators are cold but taps run hot
A combi boiler’s diverter valve is a single mechanical part that physically blocks the flow to radiators when it fails in the hot water position. The Gas Safe Register Common Boiler Faults 2026 factsheet identifies this as the primary suspect when hot water works but heating does not (Gas Safe Register, 2026). The valve is not repairable by the homeowner; it requires a Gas Safe registered engineer. The repair typically takes 1–2 hours and costs between £150 and £350 for parts and labour (Checkatrade, 2026).
How to verify your installer MCS certification for heat pumps, Gas Safe for gas boilers
For gas boilers, only a Gas Safe registered engineer is legally allowed to work on the gas supply or diverter valve. You can check an engineer’s registration using the Gas Safe Register Find an Engineer tool (Gas Safe Register, 2026). If your home uses a heat pump, MCS certification is required for both the installer and the product to qualify for the Boiler Upgrade Scheme (MCS, 2026). TrustMark is a separate government-endorsed quality mark for all trades, including boiler repair, and you can search its directory (TrustMark, 2026).
The cost of ignoring the problem can double the repair bill
Running a boiler with a stuck diverter valve can cause the pump to overheat or the heat exchanger to crack. The Checkatrade Boiler Repair Cost Guide 2026 notes that a heat exchanger replacement costs £500–£900, compared to a simple diverter valve repair at £150–£350 (Checkatrade, 2026). Ignoring the problem for even a few days can turn a relatively inexpensive fix into a much more costly one. This is a direct cost comparison from published trade data.
When to call an engineer versus when you can fix it yourself
You can safely check three things yourself: thermostat batteries, programmer timer settings, and that all radiator valves are open. The Energy Saving Trust Heating Controls Guide 2026 advises that these checks take only a few minutes (Energy Saving Trust, 2026). You must call an engineer if you have no heating but hot water works, after confirming these three checks are correct. The Gas Safe Register When to Call an Engineer 2026 leaflet states that this scenario points to a diverter valve or wiring fault that requires professional attention (Gas Safe Register, 2026).
