Heating On But No Hot Water? Causes and Fixes

First identify how your hot water is heated

Heating on but no hot water means the space-heating side is operating, at least partly, while heat is not reaching the water used at taps and showers. The useful first step is to identify whether you have a hot-water cylinder. A cylinder is a tall insulated tank, often in an airing cupboard. It may be heated by water passing through an internal coil, by one or more electric immersion heaters, or by both.

A combi arrangement normally heats water as a tap opens and has no separate stored-water cylinder. That distinction matters because immersion heaters, cylinder thermostats and cylinder coils do not apply to a home without a cylinder. Do not remove covers to identify equipment. Labels, visible controls and the presence of a cylinder are usually enough for the first conversation with a professional.

Check more than one hot tap. A single cold outlet can have a local fault, while every outlet being affected points towards hot-water generation, storage or distribution. Note whether the water is completely cold, briefly warm or cooler than usual.

Allow a depleted cylinder its normal reheating time. Burning smells, scorching, buzzing, leaking or damaged wiring require safe electrical isolation and professional help.

Check the immersion heater and its thermostat

An immersion heater is an electric element fitted into a hot-water cylinder. It heats the stored water directly, much like an element in a kettle. Some cylinders have one immersion, while others have separate lower and upper elements for normal and boost heating. If the heating system works but an immersion-fed cylinder stays cold, the element, its thermostat, timer, switch or electrical supply may be at fault.

Start with user controls only. Check that the labelled immersion switch is on, that any timer is calling for heat and that a consumer-unit breaker or residual-current device has not tripped. If a protective device trips again after one reset, leave it off. Repeatedly resetting an electrical fault is unsafe and can hide a failing element, damaged cable or moisture problem.

The immersion thermostat limits cylinder temperature and switches the element off when the set point is reached. A failed thermostat may leave the element without power, or its safety cut-out may operate if the water became too hot. Some cut-outs can be reset, but this usually involves access near mains wiring. Do not remove the immersion cover unless you are competent, have isolated and proved the supply dead, and the maker's instructions explicitly support the check.

If a boost warms only the top of the cylinder, an upper immersion may be heating a limited volume while the lower element is not operating. Proper electrical tests can distinguish the supply, thermostat and element.

Never switch an immersion on when the cylinder may be empty. The element depends on surrounding water to carry heat away and can fail rapidly if dry. Signs of a leaking immersion boss, water around electrical connections, heat damage or a distorted cover require immediate isolation and professional attention.

Check the cylinder thermostat and heating coil

In many stored-water systems, hot water from the heating circuit passes through a coil inside the cylinder and transfers heat to the stored tap water. A thermostat strapped to the outside of the cylinder measures temperature and asks the controls for more heat. The radiators can still work if that cylinder demand is missing or if flow through the coil is restricted.

Check that the cylinder thermostat has not been turned down accidentally and is firmly positioned against the cylinder. Use the normal marked setting or the system instructions rather than turning it to maximum. If moving the dial produces a click, that only shows the thermostat mechanism changed state. It does not prove the control, wiring, valve or coil is operating correctly.

During a hot-water demand, the pipes serving the cylinder coil should normally become hot. Touch exposed pipes cautiously because they can burn. Hot flow pipework but little temperature change in the stored water can indicate poor transfer, restricted coil flow or heavy scale inside the cylinder. Cold coil pipes while radiators heat can indicate a control, valve, air or circulation issue preventing that circuit from opening.

Diagnosing a cylinder coil involves temperatures, control signals and circuit flow. If the cylinder is leaking, bulging, badly corroded or overheating, turn off the relevant heat sources, do not obstruct warning pipes and call a plumber.

Look for airlocks in the hot-water or heating circuit

Air can stop water moving through pipework even when the rest of the system appears to work. An airlock may follow draining, plumbing alterations, a supply interruption or a cylinder running low. Depending on the design, it can affect flow from hot taps or prevent heating water circulating properly through the cylinder coil.

Clues include spluttering taps, irregular flow, gurgling and a hot-water problem that began immediately after work or loss of supply. If hot-tap flow is strong but cold, the issue is more likely to be heating the water than delivering it. If flow is weak or absent as well as cold, a distribution airlock, valve or supply problem becomes more plausible.

Online advice sometimes suggests joining hot and cold taps with a hose to force mains water backwards. That is not suitable for every system and can contaminate supplies, damage fittings or over-pressurise equipment. Do not improvise cross-connections. A plumber can identify the system type, isolate the correct route and clear trapped air without guessing.

Air in a coil circuit differs from air in domestic hot-water pipework. Never loosen a compression joint as an improvised vent. Record when the symptom began and any recent draining or plumbing work.

Circulation, pump and valve problems

Some systems use a pump to move heated water around both the radiator circuit and the cylinder coil. The radiators working does not prove that every route has adequate circulation. A motorised valve may direct flow towards heating, hot water or both. If the hot-water route does not open, radiators can warm normally while the cylinder receives no useful heat.

New pump grinding, rattling or repeated starting and stopping is useful evidence, but silence alone does not prove failure. A pump can also run while worn, obstructed or unable to circulate enough water through one route.

Motorised valves often have a visible lever or position indicator, but forcing one can damage the mechanism or mask the original fault. Use only normal user controls unless you know the specific valve and procedure. A plumber can check whether the valve receives a demand, moves to the correct position and allows flow towards the cylinder.

Deposits, a partly closed valve or poor balancing can also restrict circulation. Note if coil pipes warm slowly while radiator pipes heat quickly. Diagnosis should follow demand from the timer through the controls and then confirm flow.

Timer and controls set for heating but not hot water

A common explanation is simply that the programmer is calling for heating but not hot water. Many controllers have separate schedules, override buttons or modes for the two functions. A power cut, clock change or accidental button press can alter one schedule without affecting the other. Check the displayed day and time, then confirm that hot water is set to on or timed for the current period.

Use a temporary hot-water boost or advance function if the instructions provide one. Give a cylinder its normal recovery time before judging the result. If the controller shows a hot-water demand but the cylinder does not heat, the fault lies further along the chain. If the display is blank, first check its normal supply and any clearly labelled user-accessible switch.

Wireless controls add another failure point. Check for low-battery, lost-connection or receiver warnings, then follow the user instructions. Do not factory-reset controls before recording their settings.

The programmer permits timed operation while the cylinder thermostat stops demand at temperature. Check that chain in order instead of turning every control to maximum.

When the fault is on the boiler side

Boiler-side faults need a suitably qualified boiler engineer. John does not carry out boiler work. If your home has no hot-water cylinder, or the appliance shows a fault code, loses pressure or fails when a hot tap opens, stop at the user checks in its instructions and arrange boiler-specific help.

A diverter valve inside a combi arrangement can fail to direct heat towards tap water even while radiators work. Low system pressure and internal faults can produce a similar symptom. These are not DIY repairs. Do not remove the appliance case or repeatedly clear a returning fault code.

When to call for plumbing help

Call a plumber when user-level control checks do not restore hot water, when an immersion protective device trips repeatedly, or when there are signs of leakage, overheating or poor circulation around the cylinder. Isolate electricity safely if water is near an immersion connection or other electrical equipment. Do not work live or remove covers simply to confirm a suspicion.

Tell the plumber whether you have a cylinder, which heat source it uses, whether every hot outlet is affected and what changed beforehand. Photograph labels and displays without dismantling anything, and note whether coil pipes warm.

John can help with immersion heaters, cylinder controls, coils, airlocks, pumps and water-side circulation where the job is within scope. See the tanks, cylinders and pumps service for the local service route. If a winter evening leaves the household without essential hot water and you need prompt advice, use the emergency plumber in Harlow page and call John. Boiler work must go to a suitably qualified boiler engineer.