No Hot Water? Causes, Quick Checks and When It's an Emergency

Few household problems feel as urgent as turning on the tap and getting nothing but cold water. It disrupts washing, cooking and the morning routine, and in the middle of a London winter a total loss of heat and hot water quickly moves from inconvenient to genuinely uncomfortable. The good news is that a fair proportion of no-hot-water faults have a simple, safe explanation that you can identify in a few minutes. The less welcome news is that some point to a fault only a qualified engineer should touch.

This guide walks through the common causes by system type, the checks a homeowner can safely carry out, what needs a Gas Safe registered engineer, and the moments when a loss of hot water should be treated as an emergency rather than something to leave until next week. Throughout, the aim is to help you narrow down the problem so that whether you fix it yourself or call us, you are not left guessing.

First, work out which system you have

Before you diagnose anything, it helps to know what is actually heating your water, because the causes and the checks differ. Most London homes fall into one of three broad categories.

Combi boilers

A combi (combination) boiler heats water on demand, straight from the mains, with no separate hot water cylinder or loft tank. If you have no airing cupboard tank and hot water arrives more or less instantly when you open a tap, you almost certainly have a combi. These are the most common boilers in flats and smaller houses across the capital.

System and heat-only boilers with a cylinder

A system or heat-only (regular) boiler heats water and stores it in a hot water cylinder, usually in an airing cupboard. Heat-only setups often also have a cold water tank in the loft. Here, hot water is stored rather than made on demand, so you can run out if the cylinder has not reheated. Timers, programmers and a cylinder thermostat all play a part.

Immersion heaters

An immersion heater is an electric element inside a hot water cylinder, controlled by its own switch and often a separate thermostat. Some homes rely on immersion as their only hot water source; many system-boiler homes keep one as a backup. Because it runs on electricity rather than gas, its failure modes are different again.

Common causes of no hot water by system type

The table below is a quick reference to match what you are seeing against the most likely culprit. Use it as a starting point, not a final diagnosis, then move on to the safe checks in the next section.

Safe checks you can do yourself

None of the checks below involve removing boiler covers, touching gas components or working on electrical wiring. They are the same first steps a good engineer runs through, and doing them before you call can save you a visit or at least speed up the diagnosis. If at any point something looks scorched, smells of burning or gas, or you feel unsure, stop and call a professional.

1. Check the boiler is powered and switched on. Look for a blank display. Check the fused spur or plug that feeds the boiler, and check your consumer unit (fuse box) for a tripped switch. If a switch has tripped to the off position, reset it once. If it trips again immediately, leave it off and call an engineer, as repeated tripping points to an electrical fault.
2. Read the pressure gauge. On a combi or system boiler there is a pressure gauge on the front, usually a dial or a digital reading. Cold, it should sit around 1 to 1.5 bar. If the needle is in the red or near zero, low pressure is likely stopping the boiler from firing. You can often top this up yourself; our guide on how to repressurise a boiler walks through it step by step.
3. Check the thermostat and any hot water temperature dial. Make sure the room thermostat is calling for heat and, on the boiler, that the hot water dial has not been knocked down low. A dial turned to minimum, or a thermostat with a flat battery, is a genuinely common cause of lukewarm or absent hot water.
4. Check the timer or programmer. If you have a stored-water system, hot water only heats during programmed periods unless set to constant. A power cut can reset a programmer to its default schedule, or the clock can drift so the times no longer match your routine. Set it to an on period or constant and give the cylinder time to reheat.
5. Look for a pilot light or ignition issue. Older boilers have a standing pilot light that can blow out; if you can see a small flame window and it is dark, that is a clue. Modern boilers ignite electronically and instead show a fault or lockout on the display. Do not attempt to relight anything if you smell gas.
6. Try a single reset. If the boiler shows a fault code or lockout, most models have a reset button. Press it once and wait. If the boiler locks out again, do not keep resetting it repeatedly, as the lockout is a safety feature and the underlying fault needs attention.
7. Test the immersion heater if you have one. If your hot water relies on or backs up to an immersion, switch it on and check its circuit at the consumer unit. Give it 30 to 60 minutes and test a tap. No change usually means a failed element or thermostat, which is an electrical repair rather than a DIY fix.

Understanding the diverter valve

On a combi boiler, the diverter valve is a small mechanical component that decides whether heated water goes to your radiators or to your taps. When you open a hot tap, it should divert priority to hot water. When it sticks, you get one of two tell-tale patterns: heating works fine but hot water is cold or barely warm, or hot water only arrives when the heating is also running. If that matches what you are seeing on a combi, a sticking or failed diverter valve is a strong candidate. It is a common wear item, but replacing or freeing it is a job for an engineer, not a homeowner check.

What needs a Gas Safe registered engineer

By law, any work on the gas-carrying parts of your boiler must be carried out by a Gas Safe registered engineer. That is not red tape for its own sake; it is the line between a safe repair and a carbon monoxide or gas risk. Call an engineer, and stop your own investigation, if you encounter any of the following.

The boiler repeatedly locks out or shows a fault code after a single reset. Repeated ignition failure, sensor faults and lockouts almost always need diagnostic tools and genuine parts. The diverter valve is suspected, since it sits within the sealed boiler. Pressure keeps dropping after you top it up, which suggests a leak somewhere in the system rather than a one-off loss. There is any scorching, sooty marking around the boiler, or the flame burns yellow or orange rather than crisp blue. And of course, any smell of gas or any concern about fumes.

A qualified engineer will also confirm whether the fault is genuinely with the boiler or with a related control, valve or the cylinder, so you are not paying to replace the wrong thing. If you are in the capital and need someone quickly, our emergency plumber in London team covers boiler and hot water faults alongside general plumbing.

When no hot water is an emergency

Not every cold tap is a crisis. A combi that has simply lost pressure and can be topped up is a minor issue. But there are situations where waiting is the wrong call.

A complete loss of heating and hot water in winter is the clearest example. For a household with young children, elderly residents or anyone unwell, a cold home in a London January is a health issue, not just a comfort one, and it warrants a same-day response. A total shutdown that will not restart, especially overnight in cold weather, falls into the same bracket.

The second genuine emergency is anything involving gas or fumes. If you smell gas, or a carbon monoxide alarm sounds, do not investigate the boiler yourself. Do not switch anything electrical on or off, open windows, and leave the property, then contact the National Gas Emergency Service. Carbon monoxide is odourless, so treat any alarm as real. This takes priority over restoring hot water every single time.

The third is a suspected leak. If your pressure keeps falling no matter how often you top it up, water is escaping somewhere. Left alone, a hidden leak can damage floors, ceilings and electrics long before you find the source, and it will keep your boiler from working reliably.

When it points to a leak or pressure loss

Persistent pressure loss deserves its own attention because it is so often misread as a boiler fault. If you repressurise the system and the gauge drops again within hours or days, the water has to be going somewhere. Sometimes it is a visible drip under a radiator valve or from the boiler itself. Often it is not visible at all, hidden under floors, behind walls or within pipework buried in a screed.

This is where specialist help matters. Our engineers are also trained in central heating leak detection in London, using non-invasive methods to trace the loss rather than lifting every floorboard on a hunch. Finding the leak precisely means a smaller, cheaper repair and far less mess. If you have topped up more than once in a week, that is the signal to stop guessing and have the system properly traced.

What the forums say, honestly

If you search for no-hot-water advice, you will land on threads across communities such as r/DIYUK, DIYnot and MoneySavingExpert. The general consensus in those spaces is worth knowing, because it broadly matches what we see on the ground.

The recurring theme is that homeowners should exhaust the safe, obvious checks first: power, pressure, thermostat and timer settings account for a surprising share of panicked posts that turn out to be a knocked dial or a tripped switch. Regular contributors are consistent on the boundaries, too. The prevailing view is that gas work is strictly for Gas Safe engineers, that repeatedly hitting the reset button on a locking-out boiler is a bad idea, and that a boiler losing pressure again and again means a leak that needs finding rather than endless topping up. There is also healthy scepticism about being upsold a whole new boiler when a specific part, such as a diverter valve or a thermostat, is the actual fault. That last point is a fair one, and a good engineer should be able to show you what has failed and why.

A note on costs

Repair costs vary with the fault, the parts and the access involved, so any figure you see online is only ever a guide. As typical UK trade cost-guide ranges, a diverter valve replacement, a thermostat swap or an immersion element are all mid-range parts jobs, while a full leak trace and repair depends heavily on where the leak turns out to be. We will not pretend to price your specific job before we have seen it. What we will do is agree a price with you before we travel, so there are no surprises on the doorstep.

How we approach a no-hot-water call

When you call us about lost hot water, we give you an honest arrival window rather than a vague promise, and we confirm the price before we set off. On arrival, the first priority is always to isolate and make safe, particularly where there is any sign of a leak or an electrical fault. Only then do we diagnose, explain what has failed and carry out the repair. For pressure-loss cases, our leak-detection specialists can trace the problem accurately instead of guessing, which keeps the eventual repair as small as possible. The goal is a system you can trust again, not just a tap that runs warm for a week.

Whatever is behind your cold tap today, working through the checks above will tell you a great deal about whether it is a five-minute fix or a job for an engineer. And if it turns out to be the latter, at least you will know what you are dealing with before anyone knocks on the door.