The most useful diagnostic is what else works. Hot water gone but heating fine points at the cylinder, its coil, its thermostat or the diverter valve. Both gone points at the boiler or the controls. Hot water going cold quickly is a different fault again.
Answer these three questions first
Before anybody comes out, these three answers narrow the fault to a handful of possibilities, and it is worth having them ready when you call.
- Does the heating still work? If the radiators heat up but there is no hot water, the boiler is firing and the problem is downstream of it. If neither works, the boiler or the controls are the place to look.
- Do you have a hot water cylinder, or a combi? A combi heats water on demand and has no store. A cylinder system stores it. The fault lists are almost entirely different.
- Is it 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 a capacity, coil or thermostat issue – and on a combi it is usually a flow or scaling problem.
If you have a cylinder
| Symptom | Likely cause |
|---|---|
| Heating works, no hot water at all | Diverter or motorised valve stuck, cylinder thermostat failed, or wiring centre fault |
| Water lukewarm, never properly hot | Cylinder thermostat set low or failed, or a scaled coil transferring heat poorly |
| Hot water runs out very quickly | Cylinder too small for demand, thermostat low, or the boiler not reheating between draw-offs |
| Only the immersion heats it | Boiler is not reaching the cylinder – valve, thermostat or control fault |
| Only the boiler heats it, immersion dead | Failed element or thermostat. See immersion heaters |
| Water hot at the top of the cylinder, cold lower down | Normal during reheat. Persistent means a coil or circulation problem |
| Cylinder discharging, pressure dropping | Unvented system – almost always a failed expansion vessel |
The motorised valve is the single most commonly replaced component in this list. On a typical system a two-port or three-port valve directs boiler water either to the radiators or to the cylinder coil. When its actuator fails or its head sticks, one of the two stops working while the other carries on perfectly. That is exactly the ‘heating fine, no hot water’ pattern, and it is a modest repair rather than a boiler problem.
Most motorised valves have a small manual lever on the head. Moving it by hand and hearing the valve move tells you the body is not seized. It is a genuinely useful thirty-second test, and if the lever is stiff or the actuator hums without moving, you have found your fault.
If you have a combi
A combi has no store, so ‘no hot water’ means the boiler is not responding to the tap being opened, or is responding and not heating.
- Boiler shows a fault code or lockout. Note the code – it narrows the fault immediately. See understanding boiler fault codes.
- Heating works, hot water does not. Usually the diverter valve inside the boiler, or its plate heat exchanger. Both are known, routine faults on combis.
- Water is lukewarm and the flow is poor. In hard water, a scaled plate heat exchanger is extremely common. The flow drops and the temperature with it.
- Hot water fluctuates in temperature while showering. Usually a flow or scaling issue, sometimes another outlet drawing water simultaneously.
- No response at all when a tap is opened. The flow sensor or switch is not detecting demand – sometimes a sensor, sometimes a genuine flow problem upstream.
- Boiler pressure too low. Most combis will not fire at all below a minimum pressure. Check the gauge before anything else.
The plate heat exchanger scaling problem is the defining combi fault in a hard water county. It is the strongest practical argument for a scale inhibitor or softener on a combi property – see scale inhibitors.
Things you can check before calling
- Boiler pressure gauge. Below about one bar and most boilers will not fire.
- The programmer and thermostat. Flat batteries in a wireless room or cylinder stat present exactly like a dead system. Check the display.
- The timer schedule. Especially after a power cut or a clock change – a great many ‘faults’ are a programmer that has reset itself.
- Fault codes on the boiler display, and whether pressing reset once clears them.
- The immersion heater, if you have a cylinder – it will at least get you hot water while the fault is dealt with.
- Whether the fuel supply is there – oil in the tank, gas at other appliances, electricity at the boiler spur.
No hot water but heating fine? That is usually a valve or a thermostat, and it is a modest job. Call and say so – it saves time at both ends.
Questions we get asked about this
Heating works but there is no hot water. What is it?
On a cylinder system, most often 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 four cases it is a component, not the boiler.
My hot water runs out much faster than it used to. Why?
On a cylinder, either the thermostat has drifted down, the coil has scaled so the boiler cannot reheat it quickly, or demand has grown. On a combi it is usually scaling in the heat exchanger reducing both temperature and flow.
The water is hot but the flow 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. See low water pressure.
Can I get hot water while I wait 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 single best reason to keep the immersion in working order even if you never normally use it.
Is no hot water an emergency?
Not usually, unless there are vulnerable people in the house or it is accompanied by no heating in cold weather. It is treated as urgent and we will get to it quickly, but it does not normally warrant an out-of-hours call-out charge.
The boiler pressure keeps dropping and then there is no hot water.
The pressure loss is the real fault and the hot water is a symptom. A sealed system should not lose water – find out where it is going. See leak detection.
What happens when you call
Call and describe it
You speak to an engineer, not a call centre. Most jobs can be narrowed down on the phone.
We look at it properly
A real diagnosis before a price, and a straight answer if it is not worth doing.
A written quote
Itemised, so you can see what you are paying for.
The job, and the paperwork
Certificates where the work needs them, and what will need attention next.
Where we do this work
Across Norwich and the whole of Norfolk. If you are on the edge of the county, call and ask — we will tell you honestly whether we are the right people.
- Norwich
- Wymondham
- Dereham
- Attleborough
- Aylsham
- North Walsham
- Cromer
- Diss
- Loddon
- Long Stratton
- Watton
- King's Lynn
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.