No Hot Water but Heating Works: Safe Checks and Repairs

If you have no hot water but heating works, the fault usually concerns hot water controls, demand detection or heat transfer. First identify whether you have a combi boiler, cylinder or communal supply, then compare hot outlets and check accessible settings. Leave internal inspection and component repairs to a qualified professional.

For Greater London households, identifying the system matters before arranging a visit. A combi boiler, a cylinder in an airing cupboard and a communal heat supply can produce similar symptoms while needing different repairs.

A useful report describes what happens at the taps and on the display. Ordering a diverter valve because the radiators still work can lead to unnecessary expense when the actual fault is a shower mixer or a disabled cylinder programme.

Why is there no hot water but heating works?

Working radiators show that space heating is operating, but hot water has its own controls and components that need checking separately.

Keep checks external. Never remove boiler covers, touch potentially hot pipework, force valves or attempt internal repairs. If you smell gas, hear a carbon monoxide alarm or see dangerous overheating, stop troubleshooting and follow the emergency advice below.

Identify how your home heats its water

Use the appliance manual, existing paperwork and visible equipment. You do not need to open anything to establish the basic system type.

A combi generally heats domestic hot water, meaning water for washing and bathing, as it is needed. A cylinder stores it and can receive heat from a boiler, electricity or another heat source.

Some London flats have a heat interface unit, which transfers heat from a communal network into the home's heating and hot water systems. Report faults through the building's arrangements before booking a private boiler repair.

Compare a second hot tap

Check whether the kitchen tap, basin and shower are affected in the same way. Avoid putting your hands under potentially scalding water.

Record whether the water stays cold, becomes lukewarm, alternates between hot and cold, or has weak flow. No water emerging from a hot tap is a different symptom from water running cold and needs a water-supply or plumbing investigation.

If ordinary hot taps work but one shower stays cold, a local mixer or cartridge fault becomes more likely. An electric shower heats water independently of the boiler, so establish the shower type before attributing its fault to the heating system.

No hot water but heating works: what can you safely check?

Limit homeowner checks to normal user controls, visible displays and observations permitted by the appliance manual.

Viessmann's homeowner guidance includes checking fault codes and boiler controls before investigating components. The useful principle is to gather evidence without entering the appliance.

Check settings, schedules and fault codes

On a combi, check that domestic hot water is enabled and its temperature setting matches the instructions. On a cylinder system, check the separate hot water programme. Turning up the room thermostat does not normally request stored hot water.

Look for a changed clock, disabled schedule or altered setting after a power interruption. If an app controls the system, compare its hot water programme with the physical controller.

Photograph any fault code before resetting. Observe whether the combi display changes when a hot tap opens, using the manual to interpret symbols. No response is useful information, but it does not prove the flow sensor has failed.

Know when to stop checking

Read the pressure gauge if fitted and compare it with the manufacturer's stated cold range. Heating-circuit pressure is different from mains water pressure and tap flow. Some open-vented systems have no homeowner-adjustable sealed-system pressure arrangement.

A reading below about 1 bar can be associated with low-pressure lockout on some boilers, but it is not a universal fault threshold or a target for topping up.

Low pressure does not automatically explain a hot-water-only fault. Leave repressurisation to an engineer if the manual, filling arrangement or reason for pressure loss is unclear. Repeated topping up conceals a problem that needs investigation.

Only reset once if the manual permits it and there are no warning signs. Stop if the appliance locks out again, overheats, shows scorching or leaks near electrics. For relevant models, our Vaillant fault code guide can help you describe the displayed warning.

Combi boiler heating working but no hot water: likely causes

A combi can continue heating radiators while a fault prevents it detecting hot water demand, directing heat correctly or transferring enough heat to the taps.

Four common component suspects are the diverter valve, flow sensor, plate heat exchanger and temperature sensor. This is a diagnostic shortlist, not a reason to replace all four.

Internal inspection belongs to a qualified professional. Your role is to describe the symptoms, including whether they occur at every outlet and whether they change when other taps are used.

Cold water and no apparent hot water demand

A flow sensor detects water movement when a hot outlet opens. If demand is not recognised, the boiler may continue responding to heating requests without producing hot water.

Possible explanations include inadequate water flow, a sensor fault or a control problem. Report whether cold mains flow also seems reduced. The engineer needs to distinguish a supply issue from a failure inside the appliance.

A diverter valve directs boiler heat between space heating and domestic hot water. If it sticks or passes heat in the wrong direction, hot water performance can suffer even though radiators work.

If you have already noticed radiators warming during hot water use, mention it. That observation can support investigation of the valve, but it does not establish the diagnosis. Do not attempt a pipe-touch test.

Water that stays lukewarm or changes temperature

A plate heat exchanger transfers heat between separate heating-water and domestic-water passages. Restricted heat transfer can produce lukewarm or fluctuating water.

Heating-side magnetite sludge, a corrosion product, and domestic-water-side limescale can both restrict performance. Hard water is relevant across much of Greater London, but water hardness alone does not diagnose a blocked exchanger.

Other possibilities include valve leakage, temperature regulation faults or demand exceeding the appliance's available output. A temperature sensor helps the boiler regulate its operation; an incorrect reading can affect delivery at the tap.

Compare a mixer shower with an ordinary hot tap before assuming the boiler is responsible. For example, a cold shower alongside consistently hot basin water points the investigation towards the shower and its supply.

During a boiler repair investigation, demand recognition, measured temperatures, fault history and component operation should be checked before replacement parts are recommended. After repair, operation needs testing under hot water demand.

That test can involve running a hot tap for several minutes while the engineer monitors outlet temperature and boiler operation, rather than accepting an initial burst of warm water as proof of a lasting fix.

Why is the hot water cylinder not heating?

A cold cylinder with working radiators points towards its hot water programme, thermostat, motorised valve or heat source, rather than automatically indicating a combi-style fault.

Check the programme before suspecting a valve

Confirm that hot water is enabled separately from central heating. If the controller provides a normal boost function, use it according to the system instructions.

After heavy use, allow the cylinder its specified recovery time. Stored hot water takes time to replenish, so a cold shower immediately after several baths does not by itself confirm a failed component.

The cylinder thermostat requests heat when the stored water falls below its set temperature. On a boiler-heated system, external motorised valves control the flow of boiler water through the cylinder's heating coil.

Two common arrangements are:

• S-plan: separate two-port zone valves control heating and cylinder circuits.
• Y-plan: a three-port mid-position valve directs flow towards heating, hot water or both.

These descriptions help you understand an engineer's diagnosis. They are not instructions to move valve levers or access wiring.

Report whether the boiler responds when the normal hot water programme requests heat. The engineer can then investigate the thermostat, valve and control circuit without assuming the whole boiler has failed.

Use an immersion backup only if it is suitable

An immersion heater is an electric heating element fitted to a cylinder. Some systems have a usable immersion backup; others do not.

Use only a known, functioning immersion heater through its normal labelled controls and in accordance with its instructions. Avoid using it where there are leaks, electrical faults or overheating concerns. An immersion or electrical fault may require an appropriately qualified electrician or cylinder specialist.

Do not remove thermostat or immersion covers, reset concealed thermal cut-outs or operate safety valves. Follow the system's storage-temperature instructions rather than turning settings down to compensate for a fault; stored-water hygiene and outlet scald protection both matter.

An unvented cylinder stores water under pressure and requires an appropriately competent person for relevant work, commonly described as holding G3 qualification. Tell the repair provider you have an unvented cylinder when arranging assistance so the correct competence can be confirmed.

Arrange the right hot water repair in London

If accessible settings do not restore hot water, choose a professional suited to the system and agree diagnostic charges before the visit.

For gas boiler work, check the engineer's Gas Safe registration and relevant appliance categories. Cylinder work and electrical repairs can require additional competence. Communal-supply faults should go through the managing agent or heat supplier.

Gas Safe Register is the named registration body to check for gas work, rather than treating a general plumbing qualification as equivalent.

We provide both boiler repairs and general plumbing services. When describing your fault to InstaGasWorks, explain whether every hot outlet is affected or only one fitting. That distinction helps identify whether the enquiry concerns the appliance or a local plumbing problem.

Separate an urgent repair from an emergency

Cold taps alone, without warning signs, usually require a repair appointment. An absence of hot water is not evidence of a gas leak.

Suspected gas leak: leave the affected area and call the National Gas Emergency Service on 0800 111 999 from a safe location. Avoid flames and electrical switches.
Carbon monoxide alarm or suspected exposure: move outside and seek emergency advice. Call 999 for severe symptoms or immediate danger.

Dangerous overheating or water leaking near electrics also means troubleshooting must stop. Do not obstruct discharge from a cylinder safety pipe; seek urgent qualified help.

If you rent, report the problem promptly to your landlord or managing agent. Mention vulnerable occupants when requesting a response.

Agree the scope and price before repairs

Prepare this information for the booking:

• Your postcode and access arrangements.
• System type and appliance model.
• Which outlets are affected.
• Cold, fluctuating, weak-flow or no-flow symptoms.
• Fault code and visible pressure reading, if available.
• When the fault started and checks already completed.

Ask whether the charge includes VAT, call-out or diagnosis, labour, parts, parking, congestion-related charges and out-of-hours premiums. Clarify whether the diagnostic charge remains payable if you decline further work.

One advertised West and North West London example in the research lists £120 for a weekday diagnostic visit, with repairs priced separately after diagnosis. This is not an InstaGasWorks price, a London-wide average or a confirmed total repair cost; the excerpt does not establish whether VAT is included.

Request the diagnosed fault, proposed repair, expected total and repair warranty in writing. Any preliminary figures are indicative until the scope is confirmed; ask InstaGasWorks for a quote based on your system and symptoms.

Consider replacement only after diagnosis, taking account of appliance condition, parts availability and recurring faults. A successful repair should be checked with hot water running, including the symptom you originally reported.

Keep a clear record of the hot water fault

Record the affected outlets, photograph the display and arrange suitable help if the fault remains, giving the engineer your system type, symptoms, onset, fault code and completed checks. Once the immediate problem is resolved, follow the appliance's servicing and cylinder-maintenance requirements, while recognising that maintenance cannot prevent every breakdown.

For help identifying the appropriate next step, contact InstaGasWorks about your hot water fault with those details rather than attempting an internal repair.