If your radiators are cosy but your taps are stone cold, it is usually a sign your boiler or controls are not sending hot water where it should go. The good news is that a few simple checks can often narrow down the problem before you call an engineer.
First check: how your system is set up
Before doing anything else, work out what type of system you have. This makes the fault much easier to understand and explain if you need help.
If you have a combi boiler, it heats water on demand with no hot water cylinder. If you have a system or regular boiler, you will usually have a hot water cylinder in an airing cupboard.
Take a moment to notice exactly what is and is not working. Are only the radiators hot? Do you get hot water at any tap or shower, or none at all? These details are important clues.
Quick checks you can safely do now
There are a few basic things most homeowners can check without any tools. These can rule out simple setting issues before you assume a major fault.
Check boiler mode and temperature: Make sure the boiler is switched on, not in summer/heating-only mode, and that the hot water temperature is set high enough.
Check programmer or timer: Confirm the hot water is set to "On" or "Continued" and has not been accidentally set to "Off" or holiday mode.
Check room thermostat vs hot water control: Remember the room stat only controls heating. Look for a separate hot water control or cylinder thermostat if you have a cylinder.
Look at boiler pressure: On sealed systems, the pressure gauge should usually be around 1 to 1.5 bar when cold. Very low pressure can stop hot water.
Check for error codes or warning lights: Make a note of any codes shown on the display, as these really help an engineer diagnose quickly.
Common causes when radiators work but hot water does not
Once you have checked the basics, the fault is often due to a component that directs or senses hot water. The exact parts differ slightly between combi and system boilers.
Combi boiler: diverter valve and related faults
In a combi, a part called the diverter valve sends heat either to the radiators or to your taps. If it sticks towards heating, the radiators get hot but your hot water stays cold or lukewarm.
You might notice the boiler firing when you run a tap, but the radiators warming up instead of the water at the tap. This nearly always points to a diverter valve problem and needs a Gas Safe engineer.
Another frequent combi issue is a scaled plate heat exchanger. Hard water can build up limescale inside, reducing the heat transfer. Your hot water may run warm then quickly go cool, or only get hot at low flow rates.
A faulty NTC thermistor or temperature sensor can also confuse the boiler into thinking the water is hotter or cooler than it is. This causes unstable temperatures or the boiler shutting down the hot water too soon.
System or regular boiler: motorised valves and cylinder controls
If you have a hot water cylinder, the boiler heats water which is stored in the tank. Flow is usually controlled by one or more motorised valves in the airing cupboard or near the boiler.
On older Y-plan systems there is typically one 3-port valve that directs heat to hot water, heating, or both. On S-plan systems, there are usually two separate 2-port valves, one for heating and one for hot water.
If the hot water valve is stuck closed, or the 3-port valve is stuck on heating, your radiators can work normally while the cylinder never heats up. A failed cylinder thermostat can have a similar effect, as it may not ask the boiler for heat even when the water is cold.
Problems that can affect any boiler type
Certain issues can crop up on both combi and system boilers. These often show up alongside the no-hot-water problem.
Low system pressure can prevent the boiler from firing properly. You may also hear gurgling in radiators where air has entered the system. Topping up pressure is sometimes safe, but make sure you follow the instructions in your manual.
Airlocks in the hot water pipework or cylinder coil can stop hot water from circulating. Symptoms include pipes that get hot on one side of a valve or component but stay cold on the other.
General sensor faults or wiring issues in the controls can also mean the boiler is not getting the right signals for hot water demand.
What you can safely check yourself
If you are comfortable doing so, there are a few extra checks you can usually carry out without touching gas or opening the boiler.
Confirm all isolating switches for the boiler, programmer and immersion heater (if fitted) are on.
Try turning your hot water setting up to maximum for a short test period.
Run a hot tap for at least a minute and feel if any pipes near the boiler start to warm.
Note any patterns: only certain taps affected, only at certain times, or linked to heating being on or off.
Things you should not do
Some tasks are unsafe or illegal for anyone who is not properly qualified. Avoid the following and call an engineer instead.
Do not remove your boiler case or any sealed covers. On most modern boilers this forms part of the flue system, and only a Gas Safe engineer should open it.
Do not repeatedly reset the boiler if it keeps locking out, especially if you smell gas, burning or notice scorch marks. Switch the boiler off and seek professional help immediately.
Do not tamper with sealed gas valves, combustion settings, or wiring inside the boiler or junction boxes. These need specialist tools and test equipment to set up safely.
When the problem is urgent
No hot water is always inconvenient, but some signs suggest you need rapid attention rather than a routine visit.
Gurgling, banging or kettling noises from the boiler or pipes can indicate serious circulation or overheating issues. This should be investigated quickly to avoid further damage.
Visible leaks, dripping from the boiler or cylinder, or a rapid pressure drop on the gauge need prompt attention to avoid water damage and possible electrical hazards.
If you see persistent error codes, or notice sooting or scorch marks, switch the boiler off and contact a professional straight away.
Quick glossary of common boiler terms
Diverter valve: A valve in combi boilers that sends heat either to radiators or to your taps, depending on what you need.
Plate heat exchanger: A compact component that transfers heat from the boiler water to your tap water without mixing the two.
NTC thermistor: A temperature sensor that tells the boiler how hot the water is so it can adjust the flame.
Motorised valve: An electrically operated valve that opens and closes to send hot water to heating, hot water cylinder, or both.
Cylinder thermostat: A control strapped to the side of a hot water cylinder that tells the system when the stored water is hot enough.
When to call Fixbot and how to speed things up
If you have checked the basics and still have no reliable hot water, it is time to bring in a professional. Many of the faults described, such as diverter valves, motorised valves and sensors, need tools, parts and safety checks.
When you contact Fixbot, having a few details ready will help the engineer diagnose the issue more quickly. Note the boiler make and model, any fault codes on the display, whether heating works, hot water works, both, or neither, and anything unusual you have noticed such as noises, leaks or fluctuating temperatures.
Fixbot can carry out Boiler Repairs to get your hot water back on, and Boiler Servicing to keep the system running smoothly and reduce the risk of repeat faults in future.
If your radiators are warm but your taps are not, or vice versa, and you are not sure what to do next, call Fixbot on 01234815101 for friendly, expert help.
