Losing hot water always feels like an emergency - especially mid-shower - but it's surprisingly often a quick fix. Before you book anyone, it's worth ten minutes working through the basics, because the most common causes are a knocked timer, a tripped switch, or boiler pressure that's drifted too low. What you check depends on the kind of system you have, so the first job is knowing whether you're on a combi boiler or a hot water cylinder.
If you can smell gas, don't touch any electrical switches. Leave the house, open doors on your way out, and call the gas emergency line on 0800 111 999.
First, Work Out How Much Is Affected
The pattern of what's working tells you where to look. Run a few taps and note what you find before you do anything else.
No hot water anywhere in the house points to the boiler or cylinder. Only one tap cold usually means a local tap or valve fault, not the heating.
Hot radiators but no hot water (or the reverse) narrows it to a specific part like a diverter valve, not the whole boiler.
Note any fault code, warning light or blank screen - it tells you, and the engineer, a great deal.
Know Your System: Combi or Cylinder
This is the single most important thing to establish, because the two systems fail - and get fixed - in completely different ways.
- Combi boiler
- One unit on the wall, no separate tank. It heats water instantly as you turn the tap. No hot water usually means a boiler fault, and there's no stored water to fall back on.
- Hot water cylinder
- A large insulated tank, often in an airing cupboard, heated by a boiler or an immersion heater. Problems here are often the timer, the thermostat, or a tripped immersion switch.
Common Causes of No Hot Water
Across both system types, a handful of culprits account for most callouts - and several you can check or fix yourself.
- Low boiler pressure (below 1 bar) stopping the boiler firing
- A timer or programmer knocked off schedule or reset by a power cut
- A thermostat turned right down
- A tripped fuse or switch on the boiler or immersion
- A frozen condensate pipe in cold weather
- A faulty diverter valve (heating works, hot water doesn't)
If You Have a Combi Boiler
Combi faults are usually about pressure, ignition or a stuck valve. Work through the safe, external checks before calling.
- Check the pressure gauge - if it's under 1 bar, top up to 1.5 with the filling loop
- Read any fault code and try one reset (hold the reset button a few seconds)
- In a cold snap, check for a frozen condensate pipe and thaw it gently with warm water
- If it locks out again straight after a reset, stop and call
If You Have a Cylinder & Immersion
With a stored-water system, the controls are the usual suspects. The good news is an immersion heater gives you a backup way to heat water even if the boiler's down.
- Check the immersion heater switch hasn't tripped - flick it back on
- Confirm the timer or programmer is actually calling for hot water
- Make sure the cylinder thermostat is set to around 60°C
- No hot water for hours despite all this points to a failed immersion element or thermostat
When to Stop and Call
- The boiler shows a fault code that returns after a reset
- Pressure keeps dropping - there may be a hidden leak
- Water is leaking from the boiler or cylinder
- You've worked through the checks and still have no hot water
If it's pointing squarely at the boiler, our boiler not working checklist goes a step deeper, and the boiler repair cost guide shows what a call-out typically runs to. No hot water and checks exhausted? Get an emergency heating engineer on the way.