No Hot Water but Heating Works? Causes, Checks and Fixes

No hot water but heating works usually means the boiler’s hot-water side has failed while the heating side still works. A combi boiler most often has a stuck diverter valve, a faulty flow sensor or a blocked plate heat exchanger, and a system or regular boiler has a wrong programmer setting, a faulty cylinder thermostat or a stuck motorised valve.

Which of these applies depends on your boiler type and what you see at the tap. If you have heating working but no hot water, this guide covers how to tell your boiler type, match your symptoms to the likely cause and work through the safe checks. It also explains when a Gas Safe registered engineer is needed.

Why Is My Central Heating Working but No Hot Water?

Heating and hot water share the same boiler, but it has different components. If the radiators are heating normally, then the burner, pump and gas supply are probably fine, and the problem is on the hot-water side. If there is hot water but no heating, this has a different fault and requires a different check.

When a hot tap is opened, a combi boiler detects the water in motion. The heat from the boiler is then transferred to the plate heat exchanger and the mains water is heated. For a system or regular boiler, the programmer, cylinder thermostat and motorised valve determine when heat reaches the coil of the cylinder and the stored water is heated.

Combi or System Boiler? How to Tell in 10 Seconds

Check whether your boiler has a hot water cylinder, which is normally a tall, insulated tank in the airing cupboard. If you find one, then it is a system or regular type of boiler. A combi boiler does not have a cylinder, and hot water is only produced when a tap is turned on. A regular boiler also has a cold-water tank in the loft, which a system boiler does not.

The causes differ between the two types, so the table below matches each symptom to the boiler it applies to.

No Hot Water but Heating Works: Match Your Symptom

Check which row below matches your taps and radiators. The boiler type is in brackets, since combi and system boilers fail in different ways.

Heating but No Hot Water: What to Check First

Always check the basic causes before suspecting a boiler fault. These checks cover the most common reasons for no hot water from a boiler but heating working.

1. Check the Hot Water Setting and Timer

On a combi boiler, ensure that hot water is on, the temperature dial is not down, and no eco or preheat mode is active. On a system or regular boiler, make sure the programmer’s hot water channel is on and that its clock and on-times are in order. They can be knocked out by a power cut, a flat back-up battery or the clocks changing for British Summer Time. If the settings are correct and you have a boiler not responding to the thermostat, the programmer or thermostat may be faulty and needs an engineer.

2. Check the Boiler Pressure and Fault Code

Normally the heating will also be stopped by the low pressure, but the gauge takes seconds to read. When the boiler is cold, it should be between 1 and 1.5 bar. If it is lower, repressurise by using the filling loop, only if your boiler manual allows it. If the boiler keeps losing pressure, then there can be a leak or a failing component. Note any error on the display too, since the meaning differs by make, and check the fault code for Ideal, Worcester Bosch, Vaillant or Baxi.

3. Check Power, Gas and Prepayment Meter Credit

Warm radiators show the boiler has power and gas, but a power cut can still reset the hot water settings. On a prepayment meter, check the credit has not run out, because radiators stay warm for a while after the gas stops.

4. Check Cold Water Flow, Stop Tap and Isolation Valve

A combi boiler needs a minimum flow of mains water before it fires, so weak flow can mean no hot water from taps but heating works as normal. Each model has its own minimum combi boiler flow rate. Check that the stop tap and the isolation valve on the boiler’s cold supply are fully open. If the cold taps run weakly too, the fault is in the water supply and not the boiler.

5. Check for a Frozen Condensate Pipe in Winter

In freezing weather, a frozen condensate pipe can lock the boiler out. It usually stops the heating too, so check it only if the boiler shows a fault. Thaw the outside section with warm water, never boiling.

6. Reset the Boiler Safely

Reset only after the checks above, since a reset clears a lockout but not the fault behind it. The reset steps vary by make, from an Ideal Logic combi to a Worcester boiler, so follow your manual. Press reset once, then run a hot tap and watch the boiler for 10 to 15 minutes. If the fault returns, stop resetting. A reset button inside the casing is for engineers only, so never remove the casing.

Combi Boiler No Hot Water: Common Causes

If a combi boiler is working but there’s no hot water, the issue is likely in one of these three internal components.

Diverter Valve

On a combi boiler, the diverter valve sits inside the boiler and switches the heat between the radiators and the hot water. If it is stuck in the heating position, heat will continue to flow to the radiators even when the tap is cold or lukewarm. It can only be fixed or replaced by a Gas Safe registered engineer.

Blocked Plate Heat Exchanger

The plate heat exchanger can become covered with limescale in hard water areas and clogged by sludge in the heating circuit, which a boiler service can help catch early. The water then absorbs insufficient heat, and the taps are lukewarm or hot for a few seconds, then cold.

Faulty Flow Sensor, Thermistor or PCB

When a hot tap is opened and the boiler fails to react, the flow sensor is probably the cause. A thermistor reads the water temperature. If its reading is wrong, the water can come out too cool, or the boiler can switch itself off. A fault on the PCB, the main circuit board, can cause either problem, and it can happen on any make of boiler.

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System or Regular Boiler: Common Causes of No Hot Water

Warm radiators with a cold cylinder can mean that either the heat is not reaching the cylinder, or the cylinder is not telling the boiler to heat the water.

• Cylinder thermostat or programmer: the cylinder thermostat should be set around 60°C. If the setting of the thermostat is low, or if it fails, the cylinder does not request heating, and with a faulty programmer, the hot water channel can remain off.
• Stuck motorised valve: a stuck 2-port or 3-port mid-position valve stops heat reaching the cylinder coil. It should only be replaced by a Gas Safe registered engineer.
• Cylinder coil, airlock or loft tank: An airlock in the coil will prevent hot water from passing through it. If the float valve in the loft tank is faulty on a standard boiler, it may leave the cylinder empty.
• Immersion heater: If your cylinder has an immersion heater, use it as a temporary solution until the boiler is fixed.

No Hot Water From Just One Tap or Shower?

If all the other taps are hot, and the heat is not reaching only one tap or shower, then the fault is not in the boiler. A running cold single tap is typically due to a partially closed isolation valve or a worn cartridge. A mixer shower can lose hot water due to a malfunctioning thermostatic cartridge or a blocked inlet filter, and in an electric shower the water is heated by the shower itself, so the boiler has nothing to do with it. A cold shower with no hot water while the other taps run hot is a plumbing fault, and a plumber can fix it. A Gas Safe registered engineer is not needed.

When Should You Call a Gas Safe Registered Engineer?

Call a Gas Safe registered engineer if:

• the diverter valve, plate heat exchanger, flow sensor or PCB is the likely fault
• the fault code comes back after one reset
• the pressure drops again within a few days
• the cylinder stays cold after the programmer and cylinder thermostat are set correctly

If you smell gas, leave the property and call the National Gas Emergency Service on 0800 111 999 (Great Britain) first. Never take the boiler casing off. To get the fault found and fixed, book a boiler repair.

Need a Boiler Repair? Book Here

Heating works but no hot water? A Gas Safe registered engineer will inspect your boiler and help you with the next step.

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Conclusion

If there is still no hot water but heating works after these checks, the fault is inside the boiler or in the cylinder controls and needs an engineer. A one-off boiler repair from 24│7 Home Rescue is carried out by a Gas Safe registered engineer, who finds and can fix the fault, with parts and labour included (subject to terms and conditions). Once the hot water is back, boiler cover is available to protect against unexpected repair bills from breakdowns throughout the year. Terms and conditions apply.

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