You usually notice it at the worst possible moment – the shower goes cold, the taps never heat up, or the cylinder seems to be doing nothing at all. If you are asking, “why is my hot water not working”, the cause can range from a simple setting issue to a boiler, cylinder or electrical fault that needs professional attention.
The good news is that no hot water does not always mean the whole system has failed. In many homes, the problem comes down to one component, one control, or one recent change that has gone unnoticed. The key is knowing what is safe to check yourself and what should be left to a qualified engineer.
Why is my hot water not working if the heating still works?
When the heating is fine but the hot water is not, that tells you something useful straight away. It often means the issue is not a full power loss or total boiler breakdown. Instead, it may be linked to how your system switches between heating and hot water, or to the part of the system that stores or delivers domestic hot water.
On a combi boiler, a common cause is a faulty diverter valve. This valve directs hot water either to your radiators or to your taps and shower. If it sticks or fails, the boiler may still heat your home but struggle to supply hot water on demand.
On a conventional or system boiler with a hot water cylinder, the issue may sit with the cylinder thermostat, motorised valve, programmer or immersion heater. In those setups, hot water and heating often run through different controls, so one can stop working while the other keeps going.
If neither heating nor hot water is working, the fault is more likely to be with the boiler itself, the power supply, gas supply, pressure, or a safety lockout.
The most common reasons hot water stops working
There is no single answer to why your hot water is not working because different properties use different systems. Still, a few faults come up again and again.
Low boiler pressure is one of the first things to check on sealed systems. If the pressure has dropped too far, the boiler may stop operating properly or lock out completely. Many boilers work best around 1 to 1.5 bar when cold, but you should always follow the manufacturer guidance for your model.
A boiler fault code is another clear sign. Modern boilers are designed to shut down or limit operation if they detect an unsafe or abnormal condition. That could be related to ignition, overheating, circulation, sensors or fan problems. Resetting the boiler may help in some cases, but repeated lockouts usually mean there is an underlying fault that needs diagnosing.
Frozen condensate pipes are also common during colder weather. If the external condensate pipe freezes, many condensing boilers will shut down as a safety measure. This often happens suddenly after a frost and can look like a major breakdown when it is actually a weather-related issue.
For homes with a cylinder, a failed immersion heater can leave you without stored hot water, especially if the boiler side of the system is not heating the cylinder as it should. Likewise, if the hot water programmer has been switched off, the timer is incorrect, or the thermostat is set too low, you may simply not be heating water when you think you are.
What you can safely check at home
Before arranging a repair, there are a few practical checks worth making. These will not solve every problem, but they can help rule out the obvious.
Start with your boiler display. If there is an error code, note it down. That information can help an engineer diagnose the issue more quickly. Check whether the boiler has power, whether the pressure is in the normal range, and whether it has gone into reset mode.
Next, look at your controls. It is surprisingly common for timers, programmers or thermostat settings to be changed by accident, especially after a power cut or when the clocks change. Make sure hot water is actually scheduled to come on, and check whether the cylinder thermostat is turned up if you have a stored hot water system.
If you have a combi boiler, try running the hot tap and see whether the boiler fires up. If it does not respond at all, that can point towards a flow sensor or internal boiler fault. If it fires but the water stays lukewarm, the issue may be with heat exchange, scale build-up or a failing component.
You should also check whether the issue affects every tap or just one. If the kitchen tap gets hot but the bath does not, you may be dealing with a local plumbing issue rather than a boiler problem.
When the problem is electrical rather than plumbing
Hot water faults are not always caused by the boiler. In some properties, especially those with immersion heaters or electric water heating, the problem may be electrical.
A tripped fuse spur, faulty thermostat, failed element or wiring issue can stop hot water production entirely. If your hot water cylinder relies on an immersion heater as a backup, it may not work if the electrics to that circuit have failed.
This is where proper diagnosis matters. A heating fault and an electrical fault can look very similar from the customer side – no hot water is no hot water. But the repair approach is completely different. If there is any sign of burning smells, tripping electrics, or intermittent power, stop using the system and arrange a professional inspection.
Why hot water can be unreliable rather than fully off
Sometimes the problem is not that hot water has stopped completely. It may come and go, run hot then cold, or take far too long to heat up. That usually points to a component beginning to fail rather than a total shutdown.
On combi boilers, fluctuating hot water can be caused by plate heat exchanger issues, sensor faults, low flow rates or scale build-up. In hard water areas, limescale is a frequent cause of poor hot water performance because it restricts heat transfer and flow.
With cylinders, intermittent hot water can suggest thermostat issues, motorised valve faults, airlocks, sludge in the system or poor circulation. In older systems, more than one fault may be contributing at the same time.
This is why a quick patch is not always the best answer. A reset might get hot water back for a day or two, but if the root cause is left alone, the fault usually returns – often at a more inconvenient time.
When to call a qualified engineer
If you have checked the obvious settings and still have no hot water, it is time to bring in a professional. That is especially true if the boiler keeps locking out, you can smell gas, water is leaking, the electrics are tripping, or the hot water problem is part of a wider heating issue.
Gas appliances should only be worked on by a Gas Safe registered engineer. Fault-finding on boilers, valves, cylinders and controls needs the right testing equipment and the right experience. Guesswork can lead to unnecessary cost at best and serious safety risks at worst.
A proper visit should focus on diagnosis first, not just swapping parts. The aim is to identify whether the issue lies with the boiler, controls, cylinder, pipework or electrical supply, then recommend the most sensible repair. In many cases, an experienced engineer can get you back up and running without replacing major components.
For landlords and property managers, speed matters even more. No hot water can quickly become a tenant issue, a habitability concern and a scheduling problem if left too long. Fast attendance and clear communication make a real difference.
How to reduce the risk of hot water problems
Some hot water failures are sudden and unavoidable, but many are linked to lack of maintenance. Boilers and hot water systems work hard throughout the year, and small issues often show up before a full breakdown happens.
An annual boiler service helps pick up wear, pressure issues, safety concerns and performance problems early. If your home has a hot water cylinder, that also benefits from periodic checks, especially where controls, valves or immersion heaters are involved. In systems with dirty water or circulation issues, powerflushing or targeted system cleaning may improve reliability and efficiency.
If your boiler is older, frequently losing pressure, struggling to meet demand or breaking down repeatedly, it may be more cost-effective to look at replacement rather than ongoing repair. That decision depends on the age of the appliance, parts availability and the overall condition of the system.
For homeowners across London and the South East, CKT Boilers often sees hot water problems that started as minor faults and gradually became major ones. Getting them checked early is usually the less stressful option.
No hot water can throw off the whole day, but the cause is often more straightforward than it first appears. Start with the safe checks, avoid risky DIY repairs, and if the problem persists, get a qualified engineer to pinpoint it properly so your home can get back to normal quickly.
