No Hot Water? 7 Causes and What to Do

Waking up to no hot water is one of the most disruptive household problems — especially in a London winter. The good news is that many causes are easy to diagnose and some can be fixed in minutes without calling a plumber. Here are the 7 most common causes, and exactly what to do for each.

Quick Check First: Is Your Boiler Showing an Error Code?

Before doing anything else, look at your boiler's display panel. Most modern boilers show a fault code when something goes wrong — look yours up in the manufacturer's manual or search "[boiler brand] error code [number]" online. This will often tell you exactly what's wrong.

Cause 1: Boiler Has Gone into Lockout

Symptoms: No heating or hot water, boiler display showing fault code, boiler completely unresponsive.

What to do: Press the reset button on your boiler (usually marked with a circular arrow or flame symbol). Hold it for 3–5 seconds. The boiler should restart and fire up within 30–60 seconds. If it locks out again within a few minutes, there's an underlying fault — call a Gas Safe engineer.

Cost to fix (if engineer needed): £80–£300 depending on the fault.

Cause 2: Low Boiler Pressure

Symptoms: No hot water or heating, boiler pressure gauge reading below 1 bar (the green zone is usually 1–1.5 bar).

What to do: You can repressurise most boilers yourself using the filling loop (a small flexible hose connected to the pipework underneath or beside the boiler). Consult your boiler manual — most have step-by-step instructions. Slowly open the valves until the pressure reaches 1.0–1.5 bar, then close them again. Reset the boiler.

Important: If pressure keeps dropping, there is a leak somewhere in the system. Call a plumber to find and fix the source.

Cause 3: Pilot Light Has Gone Out (Older Boilers)

Symptoms: No hot water or heating, no flame visible through the boiler viewing window, older boiler model.

What to do: Check your boiler manual for pilot light relighting instructions — most involve holding down the pilot button and using a long match or lighter. If the pilot won't stay lit, the thermocouple (a safety device) may be faulty and will need replacing by a Gas Safe engineer.

Note: Most boilers installed after 2005 don't have a standing pilot light — they use an electronic ignition instead.

Cause 4: Frozen Condensate Pipe

Symptoms: No hot water or heating in cold weather, boiler showing a fault code, gurgling sound from the boiler.

What to do: The condensate pipe runs from your boiler to an external drain and can freeze in cold weather. Find the plastic pipe (usually white, running outside through an external wall). Pour warm (not boiling) water over the frozen section, or wrap it with a hot water bottle. Once thawed, reset the boiler.

Prevention: Ask a plumber to insulate the external section of the condensate pipe before winter.

Cause 5: Faulty Diverter Valve (Combi Boilers)

Symptoms: Heating works but no hot water from taps, or hot water works but no heating.

What to do: This is not something you can fix yourself. The diverter valve directs hot water between your radiators and hot water taps. When it fails, it gets stuck in one position. A Gas Safe engineer needs to replace it — typically costing £150–£300 in London.

Cause 6: Thermostat or Timer Issue

Symptoms: Hot water works at certain times but not others, or boiler seems to be working but water stays cold.

What to do: Check your programmer or room thermostat. Make sure the hot water schedule is set correctly — especially after a power cut, which can reset the clock. Some thermostats need new batteries. If your programmer is faulty, replacement typically costs £100–£200.

Cause 7: Hot Water Cylinder Fault (For Homes with a Cylinder)

Symptoms: No hot water but heating works fine, you have a separate hot water cylinder (usually in an airing cupboard).

What to do: Check the cylinder thermostat setting (should be around 60°C). Check whether the immersion heater element has failed — an electrician or plumber can test this. Also check that the cylinder's heat source (boiler indirect coil or electric immersion) is switched on.

Immersion heater replacement cost in London: £100–£200.

When Is No Hot Water an Emergency?

In winter, having no heating or hot water for more than a few hours is a genuine emergency — especially for households with young children, elderly residents, or medical needs. London's emergency plumbers are available 24/7 for urgent boiler faults.

See also: How much does a boiler repair cost in London?

Frequently Asked Questions

My boiler turns on but shuts off after a few minutes — why?

This is called "short cycling" and is usually caused by a faulty thermostat, a blocked heat exchanger, or low water pressure. It can also be caused by an oversized boiler relative to your property's heating demand. A Gas Safe engineer can diagnose the cause in a single visit.

How often should I service my boiler to prevent hot water problems?

Once a year is the recommended frequency. Annual servicing catches developing faults before they become breakdowns, keeps your boiler running efficiently, and maintains the manufacturer's warranty. A London boiler service typically costs £60–£100.