Waking up to no hot water is one of the most frustrating problems a home can have, and it tends to happen in winter when you need it most. The good news is that plenty of the causes have simple fixes you can try yourself before calling anyone. This guide covers the common reasons, the checks to run in order, and the point where you need a Gas Safe registered engineer.
Work out which problem you have
Start by working out which problem you actually have, because they point at different faults.
No hot water at all. No hot water from any tap or shower, and the heating may or may not be working. Usually a boiler fault, a pilot light out, or a power or gas supply problem.
Lukewarm only. The water is warm but never properly hot, or noticeably cooler than normal. Usually a thermostat setting, a diverter valve, or a heating element.
Hot then quickly cold. It starts fine but runs cold after a few minutes. Usually a cylinder issue, the boiler cycling, or insufficient capacity.
Heating works but taps are cold. The radiators get hot but the taps and showers do not. Almost always a diverter valve, a motorised valve, or the programmer.
Combi boilers
Combis heat water on demand rather than storing it, and they are the most common type of boiler in homes across Crawley and West Sussex. These are the usual culprits.
Low pressure. Below 1 bar many combis will not heat water at all. Check the gauge first.
Diverter valve fault. This valve switches between heating and hot water. Stuck, you get one and not the other.
Faulty flow sensor. It detects when you open a hot tap. If it fails, the boiler never knows to fire.
Blocked heat exchanger. Scale restricts flow. Common across West Sussex and Surrey, which are hard water areas.
Frozen condensate pipe. In winter the external pipe freezes and shuts the boiler down completely.
Of these, low pressure and a frozen condensate pipe are things you can check and often put right yourself. The diverter valve, flow sensor and heat exchanger are inside the boiler and need a Gas Safe engineer.
System boilers with a cylinder
A system boiler heats water and stores it in a cylinder, so the fault can be in the boiler, the cylinder or the controls.
Faulty motorised valve. Controls flow to the cylinder. Stuck closed, nothing gets heated.
Broken cylinder thermostat. Tells the boiler when the water is up to temperature. Faulty, and it never heats properly.
Programmer settings. If hot water is not scheduled on, the boiler will not heat it however well it is working.
Airlock. Trapped air stopping proper circulation to the cylinder.
Immersion heater failure. Relevant if you are relying on the backup. If the immersion has failed as well as the boiler side, you have no way to heat the water at all.
If the boiler is the problem and your cylinder has a working immersion heater, switching it on gets you hot water while you wait for the repair.
What to check, in order
Try these before calling anyone. Many no hot water problems turn out to have a simple fix.
1. The basics. Is the boiler switched on at the wall and at the fuse box? Do other gas appliances work? On older boilers, is the pilot lit? Any error code on the display? Is hot water actually scheduled on?
- Power. Check the switch next to the boiler and look for a tripped circuit at the consumer unit.
- Gas. If the hob will not light either, the problem is the gas supply rather than the boiler. On a prepayment meter, check there is credit.
- Pilot light. On older boilers it is a small blue flame visible through a little window. If it has gone out and will not stay lit, you need a Gas Safe engineer.
- Error codes. Write the code down. It narrows the diagnosis a great deal, and our boiler error codes guide explains the common ones.
- Timer. Check hot water is set to on, and that the programmer clock is still correct if there has been a power cut.
2. Boiler pressure. The single most common cause on a combi. Should be 1 to 1.5 bar cold. The gauge is usually on the front of the boiler and reads from 0 to 4 bar. If it is below 1, the system needs topping up, which we walk through step by step in boiler keeps losing pressure. If it reads over 2.5 bar when cold, that points to a fault that needs an engineer.
3. Temperature settings. Many boilers have a separate hot water control. It should be 50 to 60 degrees. Give it ten minutes after adjusting. On a combi the hot water setting is often a separate dial or menu option from the heating, so check you have turned up the right one.
4. Frozen condensate, in winter. The white plastic pipe running outside. Signs are gurgling, an error code, and no heat or hot water. Pour warm, not boiling, water slowly over the frozen section, then reset. The pipe usually runs from the boiler to an outside drain, and the frozen section is normally the exposed part outdoors. Use the reset method in your boiler manual.
5. Reset the boiler. Hold the reset button for three to five seconds and wait two or three minutes for it to run through startup. If you have to reset repeatedly, there is a real fault underneath. The button is usually on the front panel or control unit, and the manual gives the exact method for your model. Once it has restarted, run a hot tap to test.
Never attempt a repair on a gas boiler yourself. If the basic checks do not solve it, or you smell gas at any point, stop and call a Gas Safe registered engineer.
When to call
- Any smell of gas. Do not touch any switches, leave the property and call the National Gas Emergency Service on 0800 111 999 first. Our guide to smelling gas covers exactly what to do.
- Error codes that will not clear after a reset, which point to an internal fault that needs diagnosing properly
- Water leaking from the boiler or pipework. Turn the water off at the stopcock first.
- Banging, whistling or gurgling from the boiler, which indicates a problem inside it
- Basic checks have not worked
- The boiler is fifteen years old or more and failing regularly. At that age replacement can be better value than another repair.
For the boiler faults on this list, you need a Gas Safe registered engineer and a proper boiler repair rather than more resets. A leak that is doing damage now is a job for an emergency plumber.
Preventing it
Regular maintenance prevents most hot water problems.
- Annual boiler service, which catches most of this before it strands you. Ours is from £105.
- Check pressure monthly, and top up if it drops below 1 bar
- Bleed radiators yearly
- Insulate the condensate pipe before winter
- Test the hot water before the cold weather arrives rather than during it
- Keep the boiler manual somewhere you can find it, and know your basic controls
- Deal with small problems quickly. An occasional lockout or a small drip is far cheaper to sort out now than as a breakdown in January.
Frequently asked
Why do I have heating but no hot water?
On a combi that is usually a diverter valve fault. On a system boiler with a cylinder it is more likely a motorised valve or a programmer problem. The boiler itself is working, it is just not sending heat where you want it. Either way it needs an engineer.
How long should it take for hot water to come back?
A combi heats on demand, so effectively instantly once it is working. A system boiler needs thirty to sixty minutes to heat a full cylinder. If you have just reset or repressurised, give it five to ten minutes before testing.
Can I shower if the boiler has failed?
If you have an electric shower fed straight off the mains, yes, it works independently of the boiler. A mixer shower blends hot and cold, so it will not work properly without hot water.
Is no hot water an emergency?
It is urgent rather than dangerous, and it depends who is in the house. With young children, elderly residents or anyone with a health condition, especially in winter, we treat it as a priority.
How much does it cost to fix no hot water?
It depends entirely on the cause. Topping up the pressure costs nothing if you do it yourself, and a thermostat or sensor is a relatively modest repair. A failed heat exchanger, or a boiler old enough to need replacing, costs considerably more. Either way you should get a clear price before any work starts.