Few things unsettle a morning like a shower that runs cold. The routine you have built around a hot wash, a quick breakfast and an on-time departure falls apart in a single yelp. If you live in a home with a combination boiler, the good news is that a lack of hot water has a short list of likely causes, and a few of them can be checked safely without tools. Here is a practical outline of what to check when the hot water stops, written with guidance from the Gas Safe registered team at Olia’s Plumbing & Heating, and a calmer way to approach the problem than standing under the showerhead hoping it warms up.
How a combination boiler makes hot water
A combination, or combi, boiler heats water on demand. When you open a hot tap, a flow sensor detects the water moving, the boiler fires, and the heat is transferred to the mains water through a plate heat exchanger. There is no tank of stored hot water. That makes the system compact and efficient, but it also means that a fault anywhere in this chain can leave you with no hot water while the radiators continue to work.
A key part is the diverter valve, which switches the boiler’s output between the central heating circuit and the domestic hot water. When you open a tap, the valve directs heat to the hot water. If it sticks, hot water stops, or the heating and hot water get confused.
Start with the simple things
Check that other taps and the pressure are normal
Run a hot tap elsewhere in the house. If none of them produce hot water, the issue is with the boiler or its supply. If only one tap is cold, the problem may be a blocked aerator, a faulty mixer cartridge or a thermostatic shower valve.
Next, look at the pressure gauge on the boiler. Low pressure will often stop the boiler from firing at all. If the gauge is below the cold range in the manual, top it up using the filling loop according to the manufacturer’s instructions.
Check the boiler’s display and settings
Look at the display for a fault code and confirm the boiler is on the correct mode. Some boilers have separate settings for hot water, a “winter” mode or a hot water temperature dial that may have been turned down. Check that the timer or programmer has not turned the hot water off, and check any smart controls for an override.
Check the mains water supply
A combi boiler needs a decent flow of cold mains water to trigger. Check whether cold taps are running normally. If the stopcock has been partly closed or the water supply is interrupted, the boiler may not fire. Some boilers also need a minimum flow rate to start, so a very weak flow at the tap can mean no hot water.
Common reasons for no hot water with working heating
Diverter valve faults
A diverter valve that is stuck, worn or blocked by debris can fail to switch over, leaving the heating working but the hot water cold. Typical signs include a boiler that fires but sends heat to the radiators when a hot tap is open, or radiators getting warm when you run a bath. Replacing a diverter valve involves opening the boiler and is a job for a registered engineer.
Plate heat exchanger scaling or blockage
In hard water areas, limescale can build up inside the plate heat exchanger, reducing the transfer of heat. The hot water may run lukewarm, fluctuate in temperature or run out quickly. Descaling or replacing the part is a task for an engineer. A water softener or scale inhibitor can reduce the problem.
Flow sensor or turbine problems
If the boiler does not detect water flowing, it will not fire for hot water. Debris on the sensor can interrupt the signal. This can happen after plumbing work or a supply interruption.
Thermistor and sensor faults
Boilers use temperature sensors, called thermistors, to monitor hot water temperature. A faulty sensor may cause the boiler to shut down or to deliver lukewarm water. The control board may show an error code.
Is it the boiler or the hot water cylinder?
Not every home has a combi boiler. Systems with a hot water cylinder rely on a stored volume. If there is no hot water there, the thermostat on the cylinder, the immersion heater, a motorised valve or the timer may be the cause. If your house has a cylinder, check that the cylinder thermostat is set to a suitable temperature and that the programmer has hot water switched on.
Safe things to do at home
- Check pressure, display, settings and controls.
- Run several hot taps to see whether the problem is local.
- Check the cold water supply.
- Reset the boiler once if the manual allows.
- Note any fault codes and when the fault began.
It is wise to avoid:
- removing boiler covers;
- adjusting gas or water valves inside the boiler;
- adding chemicals to the system without advice;
- repeatedly resetting a boiler that locks out again.
Staying comfortable in the meantime
If the boiler cannot be repaired immediately, a kettle and bowl cover the basics, and a shower at a gym or a friend’s home is a practical stopgap. Be careful with any temporary heater or kettle on a wet floor, and keep children and vulnerable people away from scalding water.
Safety notes
Hot water faults are usually an inconvenience, not an emergency, but a few signs call for caution. If you smell gas, avoid flames and switches, open windows and doors and call the National Gas Emergency Service free on 0800 111 999. If the boiler produces a strange odour, soot or visible scorching, stop using it and have it inspected. Gas work in Great Britain must be done by an engineer on the Gas Safe Register, who will carry an ID card.
Keeping hot water reliable
- Service the boiler every year so the diverter valve, sensors and heat exchanger are checked.
- Consider a scale reducer in hard water areas.
- Keep the filling loop and pressure in order.
- Flush the system and fit a filter if sludge is a problem.
- Replace ageing boilers before they become unreliable. As a rough guide, many boilers last around 10 to 15 years.
Summary
When the hot water stops, take a breath and work through a short sequence: check whether the problem is at one tap or all of them, look at the pressure and display, check settings and the cold water supply, and reset once. Persistent problems, such as diverter valve faults, scaling or sensor failures, belong with a Gas Safe registered engineer. With the right checks, you can narrow the cause quickly and spend less time standing in a cold bathroom.
