No hot water
What has failed depends entirely on how your house makes hot water, and there are three arrangements in use. Identifying yours takes one look in the airing cupboard.
What it usually means
British homes heat water in three quite different ways, and the fault lists barely overlap. Reading advice written for one arrangement while you own another is the reason this problem so often gets diagnosed wrongly and expensively, so the first job is not diagnosis at all — it is working out which of the three you have.
If there is no cylinder anywhere in the house, you have a combi, and your hot water is made in the two seconds between opening the tap and it arriving. Nothing is stored, so nothing can have gone cold; what can fail is the sensing of the tap being opened, the valve that redirects heat away from the radiators, or the small plate heat exchanger that hands that heat to your tap water.
If there is an insulated cylinder, the boiler is not making your hot water directly at all. It is pumping hot water through a coil inside that cylinder on a schedule, and a separate chain of parts decides whether that happens: the programmer's hot water channel, a cylinder thermostat strapped to the side under the insulation, and a motorised valve that has to swing across to send heat that way. Every one of those can fail with a boiler that is in perfect health and radiators that are perfectly warm.
And if the cylinder has a thick cable running into a boss in its side, it can also be heated electrically by an immersion heater, either as the main method or as the backup nobody has tested in five years. On a mains-pressure unvented cylinder there is a further pair: a thermal cut-out that latches off and needs resetting by hand, and the incoming mains pressure the whole thing depends on.
The cylinder itself is a useful instrument here. Feel it from top to bottom through the insulation. Warm at the top and cold lower down means something has put a little heat in and stopped — typically an immersion, or a coil heating only briefly. Cold from top to bottom means no heat has reached it at all today, and that is a controls or valve question rather than a boiler one.
How to narrow it down
Work through these in order.
Is there a hot water cylinder in the house?
This is the question that decides everything else, so answer it before anything else. Look in the airing cupboard, the loft and any boxed-in space upstairs for a tall insulated tank. No cylinder means a combi and a short list of three parts. A cylinder means a much longer list, most of which is not inside the boiler.
Is the heating still working normally?
If the radiators are hot and the water is not, the burner, the gas and the pump are all fine, and the fault is in whatever routes heat towards your water rather than your radiators. On a combi that is the diverter valve or the plate exchanger; with a cylinder it is far more often the thermostat or the motorised valve.
Is the cylinder warm anywhere?
Run a hand down it from the top. Uniformly cold means nothing has heated it. Warm in the top third only usually means either an immersion heater has been on, or the coil ran briefly and stopped. Hot at the top and cold at the bottom after a full heating cycle can also mean the coil is scaled or partly blocked.
Does the boiler fire up when you open a hot tap?
On a combi it should start within a couple of seconds of the tap being opened reasonably wide. If it does, the boiler knows what you want and the fault is downstream of that. If nothing happens at all, it is not being told, and the flow sensor is the first suspect rather than anything expensive.
Is there a switch or timer anywhere marked immersion, or water heating?
Often in the airing cupboard or on the wall outside it, sometimes with a neon. If your cylinder is heated electrically, that switch and the thermostat behind the cylinder’s access cover are the whole story, and a boiler engineer is the wrong trade to call for it.
Has the programmer lost its settings?
A programmer that lost its battery backup during a power cut sits there looking entirely normal while calling for nothing. Check that the time and date are right, that the hot water channel is on rather than off, and that nobody has left it on a summer schedule with hot water disabled.
Is the water scalding rather than cold?
That is the opposite fault and it is worth acting on rather than living with. A cylinder thermostat that has failed high, or an immersion whose stat has stuck closed, will keep putting heat in past the temperature it was set to, and stored water hot enough to scald reaches every tap in the house.
Codes commonly shown with this
- A7Worcester BoschThe hot water temperature sensor has failed. Heating usually carries on as normal while the hot water runs cold, scalding, or swings between the two.Get it checked
- L1IdealThe boiler is not seeing enough water flow through it to run safely, so it has stopped. Usually a pump, an air lock, or a blocked filter.Get it checked
- FDIdealNo water flow has been detected. The pump is the prime suspect and this will become an overheat if it is left.Get it checked
- 103AristonThe appliance is reporting insufficient circulation — water is not moving through it fast enough. Worth acting on before it becomes an overheat.Get it checked
- E125PottertonCirculation fault (primary circuit) on every Potterton code display — the water did not warm as expected after lighting, or overshot twice. Pump, air or sludge; an engineer job after one reset.Get it checked
When it is urgent
Take care
What it costs
Typical repair cost
£0–£550
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Get an engineer to look at this
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.