Establish whether the heating still works, whether you have a cylinder or a combi, and whether there is no hot water at all or it runs out quickly. Those three answers narrow the fault to a handful of causes.
The three questions
- Does the heating still work? If radiators heat but there is no hot water, the boiler is firing and the fault is downstream of it. If neither works, look at the boiler or the controls.
- Cylinder or combi? A combi heats on demand with no store. A cylinder system stores it. The fault lists barely overlap.
- No hot water at all, or hot water that runs out quickly? Nothing at all is a heat source or control failure. Running out quickly is capacity, coil or thermostat – a different problem.
Having those three answers ready when you call saves time at both ends and frequently identifies the fault before anybody arrives.
Cylinder systems
| Symptom | Likely cause | Notes |
|---|---|---|
| Heating works, no hot water at all | Motorised valve stuck, cylinder thermostat failed, or wiring centre fault | The most common combination by some distance |
| Lukewarm, never properly hot | Thermostat set low or drifted, or a scaled coil | In hard water, the coil is a real candidate |
| Runs out very quickly | Cylinder too small for demand, thermostat low, or poor reheat | Check the thermostat before assuming size |
| Only the immersion heats it | Boiler not reaching the cylinder – valve, stat or control | The immersion working proves the cylinder itself is fine |
| Only the boiler heats it, immersion dead | Failed element or immersion thermostat | See immersion heaters |
| Hot at the top, cold lower down | Normal during reheat | Persistent means a coil or circulation problem |
| Cylinder discharging, pressure dropping | Failed expansion vessel on an unvented system | See expansion vessel failure |
| Hot water only when the heating is on | Control or wiring – the two circuits are not being called independently | Common after a control change |
Most two-port and three-port valves have a small manual lever on the actuator head. Move it by hand. If it moves freely and you can hear or feel the valve operate, the body is not seized and the fault is electrical. If the actuator hums without moving, or the lever is solid, you have found it. A thirty-second test that identifies the single most common cylinder-system fault.
Combi boilers
No store, so no hot water means the boiler is not responding or not heating.
- Fault code displayed. Write it down exactly before resetting. It narrows the fault immediately.
- Heating works, hot water does not. Usually the internal diverter valve, or the plate heat exchanger. Both are known, routine faults.
- Lukewarm with poor flow. In hard water, a scaled plate heat exchanger is extremely common – it insulates and restricts at the same time.
- Temperature fluctuates while showering. Flow-related, or another outlet drawing simultaneously, or a scaled exchanger.
- No response at all when a tap opens. The flow sensor is not detecting demand – sometimes the sensor, sometimes a genuine flow problem upstream.
- Boiler pressure too low. Most combis will not fire below a minimum. Check the gauge first.
- Hot water fine, heating not. The opposite fault, and it points at the same diverter valve or at the heating side controls.
Before calling: the free checks
- Boiler pressure gaugeShould be in the normal band, typically around one bar cold. Below that, most boilers will not fire at all.
- Programmer and thermostatFlat batteries in a wireless stat present exactly like a dead system. Check the display. Check the schedule has not reset after a power cut.
- Fault codeWritten down exactly, before pressing reset.
- FuelOil in the tank, gas at other appliances, electricity at the boiler spur.
- The motorised valve leverOn a cylinder system. Thirty seconds.
- Cylinder thermostat settingIt is a dial strapped to the cylinder under the insulation. It can be knocked, and it can drift.
- Immersion heaterOn a cylinder system it will get you hot water within the hour while the fault is dealt with.
- Condensate pipeIn cold weather, on a condensing boiler.
What it is usually not
Worth stating, because these are the conclusions people jump to:
- Not usually the boiler needing replacement. Heating working and hot water not is a component fault – a valve, a thermostat, a diverter – not an appliance at the end of its life.
- Not usually the cylinder. A cylinder is a tank. Unless it is leaking or the coil is scaled, it does not fail in a way that stops hot water.
- Not usually a blockage. Poor flow is a plumbing problem; no hot water with normal flow is a heating problem.
- Not usually the pump, unless the heating is also affected.
A great deal of money is spent on boilers whose actual fault was a stuck valve.
Hot water that runs out too quickly
A different diagnosis from no hot water at all, and worth separating.
- Check the cylinder thermostat setting first. A stat that has drifted down means the cylinder holds less usable heat. Free to check.
- Check the reheat. If the boiler is reheating the cylinder slowly, you run out faster. A scaled coil is the usual cause in hard water.
- Check whether demand has changed. An extra bathroom, a bigger shower, another person in the house. The cylinder may simply be undersized now.
- Check the coil, not just the volume. Two cylinders of the same litre capacity reheat at very different rates depending on coil surface area – see sizing a hot water cylinder.
- Check the hot water schedule. If it only heats at set times and you have moved the shower to a different hour, you may be drawing on a cylinder that has been cooling since morning.
- On a combi, check for scale. Reducing flow and temperature together is the signature.
Heating fine, no hot water? Try the motorised valve lever before anything else. Thirty seconds, and it is the most common cause.
Questions we get asked about this
Heating works but there is no hot water. What is it?
On a cylinder system, usually a stuck motorised valve or a failed cylinder thermostat. On a combi, usually the internal diverter valve or a scaled plate heat exchanger. In all cases a component, not the boiler.
Can I get hot water while waiting for a repair?
If you have a cylinder with a working immersion heater, yes – switch it on and you will have hot water within the hour. It is the best reason to keep the immersion working even if you never normally use it.
My hot water is lukewarm.
Check the cylinder thermostat setting, and then consider a scaled coil – in hard water the coil transfers heat progressively less well, so the cylinder never quite gets there.
Is no hot water an emergency?
Not usually on its own, unless there are vulnerable people in the property or it comes with no heating in cold weather. It is treated as urgent but rarely warrants an out-of-hours premium.
Do I need a new boiler?
Almost certainly not for this. Heating working and hot water not is definitionally a component fault, because the boiler is demonstrably firing and producing heat.
The hot water is hot but the pressure is poor.
That is a flow problem rather than a heating one – scaled fittings, a partly closed valve, or on a combi a scaled heat exchanger. Different diagnosis entirely.
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.