No hot water is one of the most commonly reported plumbing emergencies, but it rarely involves an urgent leak or flood risk — which means there is usually time to diagnose the problem methodically before calling anyone. The right approach depends entirely on what type of system you have. A combi boiler, a system boiler with a hot water cylinder, and an unvented cylinder all fail in different ways.
Combi Boilers — the Most Common Type
A combi boiler heats water directly from the mains as you draw it. If you have no hot water from a combi, the boiler itself is the first place to look. Check the boiler display for a fault code — modern combis (Vaillant, Worcester Bosch, Ideal, Baxi) all show codes on the display and the manufacturer's website lists what each code means. Common causes include: loss of mains gas pressure; the boiler's condensate pipe frozen (in very cold weather); low system water pressure (below 1 bar on the gauge); or a failed diverter valve — the internal valve that switches the boiler between heating and hot water.
"A frozen condensate pipe is one of the most common reasons a combi boiler stops working in cold weather. The condensate pipe runs outside on many installations and can freeze in temperatures below -2°C. Homeowners can often thaw it themselves with warm (not boiling) water poured over the pipe."— Worcester Bosch — Frozen Condensate Pipe Guidance
System Boilers with a Hot Water Cylinder
A system boiler heats water and stores it in a separate insulated cylinder. If you have heating but no hot water, the most likely causes are: the programmer or timer not set to hot water; the cylinder thermostat set too low or failed; the motorised valve that directs hot water to the cylinder stuck or failed; or the immersion heater element failed (if you have one as backup). Check the programmer first — it is surprisingly common for the hot water schedule to get reset after a power cut.
Unvented (Mains Pressure) Cylinders
An unvented cylinder — Megaflo is the best-known brand — stores hot water at mains pressure. These systems have multiple safety devices that can trigger and cut off hot water: the pressure relief valve, the temperature relief valve, and the expansion vessel. If the expansion vessel fails, the pressure relief valve will discharge regularly — you will notice water dripping from the discharge pipe outside. An unvented cylinder must be serviced annually and any fault diagnosis must be carried out by a plumber with G3 Unvented Hot Water qualification — it is not a DIY system.
"Unvented hot water systems must only be installed, maintained, and repaired by an operative holding a current G3 Unvented Hot Water qualification. Attempting to work on safety components without this qualification is a Building Regulations offence."— GOV.UK — Building Regulations Approved Documents
Legionella and Cylinder Temperature
Hot water cylinders should be set to store water at 60°C to prevent Legionella bacteria from multiplying. Below 50°C, conditions become favourable for growth. If your cylinder has been consistently running below temperature — perhaps because the thermostat has drifted — running the hot taps for several minutes before use is a simple precaution. Have the thermostat checked at the next service visit.
When to Call a Gas Safe Engineer
Any work on gas components of a boiler must be carried out by a Gas Safe registered engineer. You can check registration at gassaferegister.co.uk. Do not attempt to adjust the gas valve, burner, or heat exchanger. If you smell gas, do not attempt to diagnose the boiler — leave the property, open windows, and call the National Gas Emergency Service on 0800 111 999.
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