No hot water but the heating still works

Which system do you have? That decides the likely cause

Before anything else, find out where your hot water comes from. A combi heats water on demand as it passes through the boiler, so there is no cylinder in the airing cupboard or loft. A system or regular boiler heats a stored cylinder, usually a Megaflo or another unvented cylinder, or an older vented tank fed from the loft.

Ideal Heating draws the same line: on a heat-only or system boiler with working radiators and a cold cylinder, the problem is more often in the system controls than in the boiler; on a combi, suspicion falls on the boiler itself. Worcester Bosch wiring documentation shows why — an S-plan uses two two-port zone valves, a Y-plan a single three-port valve, and each decides whether the hot water from the boiler goes to the radiators or to the cylinder coil. One valve failing takes out one circuit and leaves the other working perfectly.

If your system has a cylinder, it is worth reading our guide on combi, system and regular boilers so you know which components you are talking about when you call.

What you can check yourself in five minutes

Six checks cover everything a householder can safely do when the hot taps run cold but the radiators still warm up. None of them involves opening the boiler casing, and none of them is gas work.

• Hot water temperature setting. On a combi this is often a separate dial or menu from the heating control. Ideal Heating points out that a setting below 30°C feels distinctly cold at the tap.
• Programmer or timer. Heating and hot water usually have separate time channels, so hot water can be switched off while the heating still runs. Check the clock and date too — a timer stuck on the wrong time will heat at the wrong hours.
• Cylinder thermostat. If you have a hot water cylinder, the strap-on thermostat may have been knocked down. Check that it is not set low.
• Boiler pressure. The gauge should normally read about 1 to 1.5 bar when cold. Outside that range the boiler may shut down on safety grounds. See our guide to low boiler pressure.
• Reset button. If the boiler is showing a lockout, one reset is reasonable. If it locks out again, stop and call an engineer — the lockout is doing its job.
• Fault code. Note it down exactly. Codes are specific to each manufacturer, and an unclear code is a reason to call rather than to experiment.

One thing to note rather than adjust: Baxi says the service valve on the cold mains feed under a combi should be fully open, with the handle in line with the pipe. Tell the engineer what you can see. Our wider checklist for a boiler that has gone quiet is in what to check before you call an engineer.

On a combi: the diverter valve is the usual suspect

Inside a combi, a diverter valve decides where the heat goes. Open a tap and the valve should swing over to send heat to the domestic hot water side; close the tap and it returns to the radiators. Viessmann describes the classic symptom of a valve stuck in the heating position: the heating works, the taps stay cold.

Vaillant notes that failed diverter valves are a common fault, particularly on older appliances, and that the remedy is cleaning or replacement by a qualified engineer. The valve sits inside the boiler casing, so this is not a job for the householder. The same applies to the plate heat exchanger, the part that passes heat from the boiler water to your tap water — if it silts up or scales up, only hot water suffers, because the radiators never pass through it.

On a cylinder system: valves, thermostats and the immersion heater

With a stored cylinder, the boiler may be heating the radiators happily while nothing reaches the cylinder coil. A motorised zone valve that has seized, a failed cylinder thermostat or a wiring centre fault will all produce exactly that pattern. Replacing a zone valve or an actuator head is engineer work.

Many cylinders also have an immersion heater. Baxi compares it to a kettle: electricity heats an element in the cylinder until the thermostat is satisfied. If the boiler circuit to your cylinder has failed, switching the immersion on gets you through the evening while you wait for a repair. It is a stopgap, not a fix, and it uses electricity rather than gas.

Unvented cylinders also carry a thermal cut-out with a reset stud as protection against overheating. Heatrae Sadia's own installation manual is blunt about it: the cut-out must never be bypassed, and if it keeps tripping the thermostat needs checking or replacing. If yours has tripped more than once, leave it alone and get the cylinder looked at. Cylinders and immersion heaters need routine servicing like anything else — see our page on Megaflo and unvented systems.

Lukewarm or short-lived hot water is a different fault

Cold taps and lukewarm taps point in different directions. If the water is warm but never properly hot, or the flow has dropped off over the years, limescale is a strong candidate in hard water areas such as London and the Home Counties. Viessmann explains how scale builds up in pipework and heat exchangers and chokes the flow. Ideal Heating describes kettling — a boiling or rumbling noise from the boiler — as another sign that scale has taken hold.

Hot water that runs warm for a minute and then goes cold on a cylinder system usually means the cylinder simply has not been reheated, which sends you back to the programmer, the zone valve and the cylinder thermostat.

Safe hot water temperatures: hot enough, not scalding

Do not be tempted to turn a cylinder down to lukewarm to save energy. HSE guidance for hot and cold water systems is that water should be stored at at least 60°C and distributed so that outlets reach 50°C within one minute. Legionella bacteria do not survive above 60°C and are dormant below 20°C.

At the tap, the risk reverses. HSE puts the scalding risk above 44°C and recommends thermostatic mixing valves limiting baths and showers to a maximum of 44°C where high storage temperatures are run for legionella control. That matters most in homes with young children, older people or disabled residents.

Who is allowed to work on it

FM Power is Gas Safe registered and works across London and the surrounding counties, seven days a week including evenings and weekends. If you have lost hot water completely and the checks above have not brought it back, our emergency boiler repair service covers the urgent cases.

When you would rather someone diagnosed it properly than kept guessing, our boiler service and repairs page explains how a visit works and how you get a written fixed price before anything is touched.

Sources

• Viessmann – Boiler working but no hot water
• Vaillant – How to fix a boiler diverter valve
• Ideal Heating – No hot water? Here's what to do
• Baxi – What to check if you have no hot water
• Baxi – Can a hot water cylinder work with an immersion heater?
• Worcester Bosch – Greenstar wiring centre (S-plan and Y-plan)
• Worcester Bosch – Resetting your boiler when it locks out
• Viessmann – How to prevent limescale in boilers
• Ideal Heating – What is kettling in a boiler?
• Heatrae Sadia – PremierPlus unvented cylinder installation manual
• HSE – Hot and cold water systems
• HSE – Hot and cold water systems: things to consider
• legislation.gov.uk – Gas Safety (Installation and Use) Regulations 1998, reg. 3
• legislation.gov.uk – Gas Safety (Installation and Use) (Amendment) Regulations 2018
• HSE – Check an engineer: are they Gas Safe registered?