No hot water but heating works? Common causes explained

If your radiators are warming up nicely but the hot water has given up, it is frustrating and confusing. The good news is that this pattern usually points to a fairly specific set of boiler or control issues that an engineer can track down quickly.

First checks when heating works but hot water does not

Before assuming the boiler has failed completely, it is worth running through a few simple checks. These can often rule out easy-to-fix settings problems and help you describe the issue more clearly to an engineer.

Start by confirming whether the problem affects every hot tap and shower. If only one outlet runs cold, you may have a localised mixer or tap issue rather than a boiler fault.

• Check the programmer/timer is set to provide hot water, not just heating
• Make sure the hot water thermostat is turned up above lukewarm
• Look at the boiler pressure gauge (on sealed systems it should usually be around 1.0–1.5 bar when cold)
• Note any error codes or flashing lights on the boiler display
• Listen for the boiler firing when you open a hot tap

If you have checked these basics and still have no hot water, the next step is to think about which type of boiler you have, as the common faults are slightly different.

Comparing symptoms: hot water gone vs heating gone

Many homeowners see similar problems the other way round, with plenty of hot water but cold radiators. The pattern of what works and what does not can be a big clue.

In simple terms, if the heating is fine but hot water is not, the fault often sits with components that prioritise hot water over heating. If hot water works but radiators stay cold, it is more likely to involve motorised valves, room thermostats or programmer settings for heating.

Combi boilers: heating but no hot water

Combi boilers heat water on demand, so there is no hot water cylinder. If your radiators are working but hot taps run cold, the boiler is usually fine at heating water, but something is stopping it from switching into hot water mode correctly.

Common combi boiler culprits

Several components are frequent offenders in this situation. An engineer will typically focus on:

Diverter valve issues. The diverter valve directs hot water from the boiler either to the radiators or to your taps. If it sticks in the heating position, your radiators will warm but hot water will not be produced properly, or may only go lukewarm.

Plate heat exchanger problems. This part transfers heat from the boiler water to your tap water. If it becomes blocked with limescale or sludge, you might get weak hot water, water that swings between hot and cold, or taps that never really heat up.

Flow sensor or flow switch faults. Combi boilers only fire for hot water when they sense water movement. If the flow sensor fails, the boiler does not ‘notice’ you have opened a tap, so it never starts the hot water process.

Homeowner checks for combi boilers

You can safely check the boiler display for any fault codes and see whether the flame symbol appears when you run a hot tap. You can also try different taps in the property and see if the boiler reacts to any of them.

What you should not do is try to manually move valves, open the boiler case or adjust internal parts. These jobs are for a Gas Safe registered engineer only.

System and regular boilers with hot water cylinders

If you have a separate hot water cylinder, the picture is slightly different. Your boiler heats water which is then stored in the cylinder, ready for taps and showers.

Motorised valve and controls issues

Motorised valve faults. System and regular boilers often use one or more motorised valves to direct hot water to either the heating, the hot water cylinder, or both. If the valve that should feed the cylinder fails, you will get good heating but the cylinder never heats up.

Cylinder thermostat problems. The cylinder stat tells the boiler when the stored water has reached temperature. If it is set too low or has failed, the boiler may not be asked to heat the cylinder at all.

Programmer or timer settings. On stored hot water systems the programmer usually has separate settings for heating and hot water. If the hot water channel is off, on the wrong schedule, or stuck on ‘timed’ when you expect ‘constant’, you may run out of hot water.

Immersion heater and backup checks

Many cylinders have an immersion heater as a backup or for use with off-peak electricity. If your boiler is not heating the water, you might be relying on an immersion heater without realising it, or that immersion may have failed.

You can safely check the immersion heater switch or fused spur is on, and that any timer controlling it is set correctly. If it still does not heat the cylinder, an electrician or heating engineer should test it properly.

Safe checks you can do without tools

There are a few more safe, non-invasive checks that can be very helpful for the engineer:

• Confirm whether the issue is constant or only at busy times (mornings/evenings)
• Note if hot water starts warm then quickly goes cold, or never heats at all
• Check for any recent work on the system, such as new radiators or controls
• Make sure all isolation switches for the boiler and cylinder are turned on

Write down anything you spot, along with any fault codes from the boiler display. Sharing this with the engineer often speeds up diagnosis.

Safety: what you should never attempt yourself

Modern boilers are gas appliances with serious safety risks if tampered with. There are a few clear red lines for homeowners.

Do not remove the boiler case to look inside, even out of curiosity. On most boilers this forms a safety seal and should only be removed by a Gas Safe registered engineer.

Never try to adjust gas valves, burner settings or internal boiler components. The same goes for attempting to refill expansion vessels directly or dismantling flues and condensate pipes.

If you smell gas, see signs of scorching around the boiler, or the flue looks damaged, switch the appliance off and contact the gas emergency number immediately, then arrange a professional visit.

What a heating engineer will typically test

When you book a repair, the engineer will not just swap parts at random. They will work through a structured set of checks to find the real cause.

On combi boilers, this usually includes checking hot water flow rate, testing sensors and switches electrically, inspecting the diverter valve and plate heat exchanger, and confirming correct burner operation in both heating and hot water modes.

On systems with cylinders, they will test the cylinder thermostat, motorised valve operation, wiring centre connections and programmer outputs. They may also check the cylinder temperature directly and confirm that the boiler is receiving the correct demand signal.

A good engineer will explain their findings in plain language and offer clear repair options, including any preventative maintenance that could reduce the risk of repeat failures.

Next steps: arrange a professional boiler repair

If you have run through the safe checks and still have radiators working but no hot water, it is time for expert help. Leaving faults unresolved can put extra strain on your boiler and may lead to more expensive repairs later.

Parks Plumbing and Heating Ltd can investigate the fault, replace failed components and carry out any overdue boiler maintenance to keep your system running reliably. To book a boiler repair visit, call Parks Plumbing and Heating Ltd on 07889 370789.

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