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Quick answer
First identify whether you have a hot-water cylinder, an immersion heater or a combi boiler. Check the normal hot-water programme and more than one tap. Strong but cold flow suggests a heating problem; weak flow points elsewhere. Boiler faults need a suitably qualified boiler engineer; John does not carry out boiler work.
Before you start: Keep appliance and electrical covers closed. Stop for leaks, overheating, burning smells, scorching, buzzing, damaged wiring or repeated electrical trips. Keep clear of wet electrics and arrange professional help. Never run an immersion when its cylinder may be empty.
On this page
Identify your hot-water system
A cylinder is an insulated tank, often in an airing cupboard. Use visible labels and normal controls to identify the arrangement; do not remove covers. Check more than one hot tap and note whether the water is completely cold, briefly warm or cooler than usual.
| System | What to check first |
|---|---|
| Combi, without a separate cylinder | Hot water is normally heated as a tap opens. Cylinder and immersion checks do not apply; a boiler fault needs a suitably qualified boiler engineer. |
| Cylinder heated by the heating circuit | Check the normal hot-water programme and allow the cylinder time to reheat. Its controls and coil can need attention even when radiators work. |
| Cylinder with an electric immersion heater | Check labelled user switches and timers only. Keep electrical covers closed and stop for leaks, overheating or repeated trips. |
Check the hot-water programme
- Check the programmer’s displayed day and time, then confirm hot water is set to on or timed for the current period. Heating and hot water can have separate schedules; a power cut or accidental press may change one.
- Use a normal boost or advance only if the user instructions provide it. Give a depleted cylinder its normal recovery time rather than turning every control to maximum.
- Check visible low-battery, lost-connection or receiver warnings. Record existing settings before any approved reset; do not factory-reset controls or repeatedly clear a returning fault.
- Use only clearly labelled, user-accessible switches. If the controller calls for hot water but the cylinder does not heat, the controls, heat source or circulation need investigation.
Strong flow or poor flow?
If all hot taps flow strongly but remain cold, the problem is more likely to be heating the water. A single affected tap may have a local fault. Weak, spluttering or absent flow also points towards supply, valves or distribution; note recent draining, plumbing work or loss of supply.
Do not connect hot and cold outlets with a hose, block a mixer outlet or force water backwards. Backflow can contaminate drinking-water supplies. Never loosen a pipe joint as an improvised vent. The airlock and poor-flow guide explains safe symptom checks.
Immersion heater not heating water?
Check the labelled user switch and timer. If a protective device has tripped, follow the user instructions; if it trips again after one reset, leave it off and arrange a registered electrician’s inspection. Do not remove the immersion cover or reset a safety cut-out as a repair.
An immersion must remain surrounded by water. Do not turn it on if the cylinder may be empty. Water near electrical connections, overheating, a distorted cover or heat damage needs safe isolation and professional attention.
Why an immersion can fail while radiators work
An immersion is a separate electric element in the cylinder, controlled by a thermostat, timer, switch and supply. A failure there can leave the taps cold while radiators still work. Electrical tests distinguish those causes.
Some cylinders have separate lower and upper elements. A boost may warm only the top volume when the lower element is not operating. The thermostat controls temperature; a click or operated safety cut-out does not prove the equipment is safe or functioning.
Cylinder controls and circulation
Use the system’s normal marked cylinder-thermostat setting and check whether it was turned down accidentally. Do not turn it to maximum. A dial clicking does not prove that wiring, valves or the heating coil are working.
Leave hot exposed pipes, pump settings and unfamiliar valve levers alone. If the cylinder is leaking, bulging, badly corroded or overheating, turn off the relevant heat sources when safe, leave warning pipes unobstructed and call for help.
Why a cylinder can stay cold
A coil transfers heat from the heating circuit into stored tap water. A cylinder thermostat, correctly positioned against the cylinder, calls for heat; controls and motorised valves must then provide flow. Radiators working does not prove the cylinder route is working.
A pump may run but provide too little flow, and deposits or a partly closed valve may restrict circulation. Grinding, rattling or repeated starts are useful observations; silence alone does not prove pump failure. Do not force a valve lever.
Air after draining can affect a coil circuit differently from domestic hot-water pipes. A professional follows the demand through timer, controls and valves and checks temperatures and flow. Report visible settings and the history rather than touching hot pipes to diagnose it.
Who to call
Tell the professional whether you have a cylinder, how it is heated, which taps are affected, what the controls show and what changed beforehand. Photograph visible labels without dismantling anything.
| Problem | Next step |
|---|---|
| Cylinder, pump, airlock or water-side circulation | John can assess water-side plumbing and heating; describe the system and symptoms first. |
| Immersion electrical fault, overheating or repeated trips | Leave it off when safe and arrange a registered electrician’s inspection. Keep covers and safety devices alone. |
| Boiler fault or returning appliance code | Use a suitably qualified boiler engineer. John does not carry out boiler work. |
A combi’s internal diverter valve, appliance pressure problem or returning fault code requires boiler-specific help, even if radiators still heat. Do not remove the appliance case or repeatedly clear the code.
For local water-side work, see tanks, cylinders and pumps. For urgent loss of essential hot water, call through the emergency plumbing page and describe the fault so the right next step can be agreed.
Questions
Why is my heating working but there is no hot water? +
The hot-water side may not be receiving a demand or transferring heat even though the radiator circuit works. Common water-side causes include immersion or cylinder thermostat faults, timer settings, airlocks, motorised valves and restricted circulation.
Can an immersion heater fail while the central heating still works? +
Yes. The immersion is a separate electric heat source inside the cylinder, so its element, thermostat, timer or supply can fail without stopping the radiators. Repeated electrical tripping needs professional investigation.
Can an airlock cause heating but no hot water? +
Yes. Trapped air can restrict flow through a cylinder coil or affect hot-water delivery, particularly after draining or plumbing work. The safe remedy depends on the system, so avoid improvised hoses or loosened joints.
Should I reset the hot-water controls? +
Use only the normal boost, advance or reset steps in the user instructions. Do not repeatedly reset a tripping circuit or returning fault, and record settings before any approved reset.
Who should I call if the fault is inside the boiler? +
Call a suitably qualified boiler engineer. John does not carry out boiler work, so appliance pressure problems, internal diverter valves and fault codes need boiler-specific help.
Sources
Need a repair?
John is based in Harlow and covers the surrounding towns. Call to describe the plumbing or water-side heating problem.