A cold shower is an unwelcome way to find out there is no hot water from boiler. Whether it happens before the school run, during a busy morning in a rental property or at the start of a working day, the priority is clear: establish whether there is a simple setting or pressure issue, then get the right repair without taking risks.
Some checks are safe for a homeowner or property manager to make. Others need a Gas Safe registered engineer, particularly where gas appliances, sealed heating systems or hot-water cylinders are involved. Knowing the difference can save time, prevent further damage and help get the hot water back on sooner.
First, work out what has stopped working
Start by checking whether your heating is working as normal. If you have heating but no hot water, the fault may relate to the boiler’s hot-water function, a diverter valve, a programmer setting or, in homes with a cylinder, the hot-water controls or immersion heater. If you have neither heating nor hot water, the issue could be a loss of power, low boiler pressure, no gas supply, a boiler lockout or a wider boiler fault.
It also helps to know which system you have. A combi boiler heats water directly when you open a hot tap, so it has no separate hot-water cylinder. A system or regular boiler usually works with a cylinder, often located in an airing cupboard, and may have a separate timer, thermostat and immersion heater. The correct next step depends on that setup.
Safety comes before boiler troubleshooting
Do not remove a boiler casing, interfere with internal components or attempt a gas repair yourself. These jobs require a suitably qualified Gas Safe engineer. If you can smell gas, feel unwell with symptoms such as a headache or dizziness, or suspect a carbon monoxide issue, turn off the gas at the emergency control valve if it is safe to do so, open doors and windows, leave the building and call the National Gas Emergency Service on 0800 111 999.
A boiler leak also needs attention. A small drip can lower the system pressure and eventually stop the boiler working, while water near electrical parts is never something to ignore. Place a towel beneath a minor external drip if safe, but do not dismantle anything to find its source.
No hot water from boiler: safe checks to make first
Before arranging a repair, there are a few straightforward things worth checking. They can rule out a simple cause and give an engineer a clearer picture of the fault.
- Check the boiler display and electrics. Is the screen lit, or does it show an error code? Check that the boiler isolator switch has not been turned off and that the relevant fuse or circuit breaker has not tripped. Write down any fault code before resetting anything.
- Check the controls. Make sure the programmer is set to provide hot water, the clock is correct after a power cut and any hot-water temperature setting has not been turned down. On a combi boiler, try more than one hot tap to rule out a problem with a single tap or shower mixer.
- Check boiler pressure. Most sealed systems need pressure around 1 to 1.5 bar when cold, although the correct range is shown in the manufacturer instructions. If the gauge is very low, the boiler may lock out. Only repressurise it if you understand the filling loop and your boiler instructions clearly. Repeated pressure loss is a fault, not a routine inconvenience.
- Check your gas and water supply. If you use a prepayment meter, confirm that there is credit. See whether other gas appliances work, if you have them. Check that cold water is available from taps too, as a supply issue can sometimes be mistaken for a boiler problem.
A single reset may be appropriate if the boiler instructions allow it. Do not keep resetting a boiler that continues to lock out. Repeated resets can hide an underlying problem and delay the repair it needs.
Common reasons a boiler stops producing hot water
For combi boilers, a failed diverter valve is a common cause of heating without hot water. This internal valve directs heat either to radiators or to the hot-water heat exchanger. When it sticks or fails, the boiler may still run the heating but cannot correctly provide hot water at the tap. This is an engineer repair.
Low pressure is another frequent issue, especially following radiator bleeding, a visible leak or a slow hidden leak elsewhere on the heating system. Topping up the pressure may temporarily restart the boiler, but pressure that drops again needs proper fault-finding. The cause could be a leaking radiator valve, pipework issue, faulty expansion vessel or pressure relief valve. Simply refilling it over and over can make matters worse.
A frozen condensate pipe can stop a modern condensing boiler during a cold spell. This pipe takes mildly acidic wastewater from the boiler to an outside drain. If it freezes, the boiler may display a lockout code. Where the external pipe is safely accessible, gentle warming with warm water, never boiling water or a naked flame, may clear it. If access is awkward, the pipe is frozen repeatedly or you are unsure, arrange professional help instead.
Limescale and debris can also affect hot-water performance. In hard-water areas, scale can restrict flow through a plate heat exchanger in a combi boiler. You may notice that water is only lukewarm, gets hot and cold in cycles, or is hot only when the tap is turned on slowly. Sludge and corrosion in the heating system can contribute to component wear too. The solution depends on the diagnosis: it may involve cleaning or replacing a part, and in some cases system treatment or power flushing.
If your property has a hot-water cylinder, the boiler may be working correctly while the cylinder is not being heated. A failed motorised valve, faulty cylinder thermostat, programmer problem or circulation pump issue can prevent boiler-heated water reaching the cylinder coil. If the boiler side is unavailable, a faulty immersion heater may be the reason there is no backup hot water either. Unvented cylinders require particularly careful handling and must be serviced or repaired by an appropriately qualified engineer.
When to call an engineer
Call for professional help promptly if the boiler displays a recurring fault code, loses pressure, leaks, makes unusual banging or whistling noises, or has stopped providing both heating and hot water. The same applies if you have reset it once and it locks out again, or if the boiler is old enough that replacement parts may be a consideration.
For landlords and property managers, a loss of hot water can affect tenant welfare and should be dealt with quickly. Record the boiler make and model, the displayed code, whether heating still works and any changes such as low pressure or leaking water. That information helps an engineer arrive prepared and diagnose the problem efficiently.
In Harpenden, St Albans and nearby Hertfordshire areas, IGM Plumbing & Heating Ltd can investigate boiler and hot-water faults with a practical, no-nonsense approach. With 30 years of experience, Gas Safe registration and unvented hot-water certification, the focus is on identifying the actual cause rather than guessing at a replacement part.
Reducing the risk of another hot-water failure
Annual boiler servicing is the most useful starting point. It allows an engineer to check safe operation, identify worn components and spot issues such as poor pressure control before they cause a complete loss of hot water. Keep the boiler instructions somewhere accessible, and take a photo of the normal pressure reading so you know what looks right for your system.
If pressure has fallen more than once, radiators need frequent bleeding or you have noticed damp patches, do not assume these are separate problems. A concealed leak can affect boiler performance as well as damage a property. Early investigation is usually cleaner and less disruptive than waiting for a small fault to become an emergency.
Cold water from the tap is frustrating, but it is also useful evidence. Make a note of what still works, avoid repeated resets, and have the fault properly diagnosed so the next shower is hot for the right reason.
