Heating Works but No Hot Water? What's Wrong and How It's Fixed

Updated July 2026 · Emergency Plumber Chelsea

Start by working out which kind of system you have, because the causes are different. A combi boiler heats water on demand as you open a tap, and there is no hot water cylinder in the airing cupboard. A system or regular boiler heats a stored cylinder of hot water, usually on a timer, and a regular (heat-only) boiler also has a cold water tank in the loft. If you are not sure, look for a large insulated copper or white steel cylinder in a cupboard. If there is one, skip to the cylinder section further down.

Next, narrow the symptom down. Run the hot tap in the kitchen and in the bathroom. If only one tap runs cold, the fault is probably that tap or a local mixing valve, not the boiler. If every hot tap runs cold, it is the system. On a combi, open a hot tap fully and watch the boiler display or listen for it: most combis show a tap or flame symbol and you will hear the fan and burner start within a few seconds. If nothing happens when the tap opens, the boiler is not detecting the demand. If it fires but the water only ever gets lukewarm, that points to a different set of causes. Our guide to hot water that is not hot enough covers the lukewarm case in detail.

On a combi, check the simple settings first. Many combis have a separate hot water temperature dial or button, and it is easy to knock it to minimum or switch hot water off entirely, especially on models with a summer and winter mode or an eco setting. Turn the hot water temperature up, make sure hot water is enabled, and try the tap again. If the boiler shows a fault code, note it before you do anything else; Worcester, Vaillant, Baxi and Ideal combis all have codes for hot water sensor faults, and our guides to Worcester, Ideal and Baxi fault codes explain the common ones. If you have a Worcester Bosch, our guide to a Worcester boiler with no hot water covers that make, and our guide to Vokera boiler problems does the same for Vokera combis.

Low flow can stop a combi firing for hot water. A combi only lights for hot water once enough water is moving through it, typically a couple of litres a minute, which it senses through a flow sensor. If the tap is only opened a little, or the flow is choked by a blocked tap aerator, a partly closed isolation valve under the boiler, or low mains pressure in the street, the boiler may never see enough flow to start. Open the tap fully and check that the cold mains flow is normal. If the cold is weak too, the problem is the supply, and our guide to low water pressure explains the likely causes.

A stuck diverter valve is the most common reason a combi gives heating but no hot water. Inside the boiler, the diverter valve sends the heated water either round the radiators or through the plate heat exchanger that heats your tap water. When it sticks, or its motor fails, it stays in the heating position and the taps never get the heat. A tell-tale sign is that the radiators get warm when you run a hot tap, even with the heating switched off. The valve can sometimes be freed up, but usually the actuator, the internal cartridge or the whole valve is replaced. It is inside the boiler casing, so it is a job for a Gas Safe engineer.

A failed flow sensor or scaled plate heat exchanger is the next suspect. The flow sensor is a small turbine or switch that tells the boiler you have opened a tap. Debris and limescale can jam it, and London's hard water makes that more likely here than in most of the country. The plate heat exchanger is where the heat passes into the tap water, and when it scales up the hot water becomes weak, fluctuates or cuts out while the heating carries on as normal. An engineer can clean or replace the flow sensor and descale or replace the plate heat exchanger, and a scale reducer on the cold feed helps stop it happening again. Our guide to hard water and limescale explains why Chelsea boilers are affected.

If you have a hot water cylinder, check the programmer first. Most system and regular boilers have separate heating and hot water channels, and after a power cut or a clock change the hot water channel is often off or set to the wrong times. Press the hot water boost or advance button, or set hot water to constant for an hour. If the cylinder heats, the fault is the timer or its settings. Then check the cylinder thermostat, a small dial strapped to the side of the cylinder, usually set somewhere around 60°C. If it has been turned down or has failed, the cylinder will not call for heat.

The motorised zone valve is the usual culprit on a cylinder system. It is a valve on the pipework near the cylinder or boiler with a small grey or white actuator head on top. When hot water is called for, the head opens the valve and a switch inside tells the boiler to fire. If the motor or switch fails, the valve stays shut to the cylinder and all the heat goes to the radiators. Many heads have a manual lever; if moving it to open brings hot water back, the head has failed and needs replacing. On a mid-position valve serving both heating and hot water, a failure can leave you with heating only. The head is a quick swap for an engineer, but it is wired into the controls, so it is not a DIY job.

If the hot tap runs dry rather than cold, the problem is different. On a gravity-fed system the cylinder is fed from a cold water tank in the loft, and if the float valve on that tank sticks, the tank empties and the hot taps slow to a trickle or stop. That is not a heating fault at all; our guide to no water coming out of the hot tap walks through it. If you have an immersion heater, switching it on is a good way to get hot water while you wait for a repair, and a good test: if the immersion heats the cylinder, the cylinder is fine and the fault is on the boiler side. If the immersion will not heat it either, see our guide to an immersion heater not working.

Know what not to do. Do not take the casing off a gas boiler to get at the diverter valve or sensors; everything behind it is Gas Safe work. Do not keep resetting a boiler that is showing a hot water fault, and do not turn the heating flow temperature right up to force heat through, because that does not reach the taps and wastes gas. If you smell gas at any point, leave the property and call the National Gas Emergency Service on 0800 111 999.

If your heating works but you still have no hot water after these checks, call Emergency Plumber Chelsea on 020 3002 6667. Our boiler repair service uses Gas Safe registered engineers who replace diverter valves, flow sensors, plate heat exchangers, zone valves and cylinder thermostats across SW3, SW10, Knightsbridge, Sloane Square, South Kensington and Fulham. If the fault is the other way round, with hot water but cold radiators, see our guide to no heating but hot water. There is no call-out fee on jobs over 3 hours.

Quick tips

• Test every hot tap: one cold tap is a tap fault, all of them is the system.
• On a combi, open the hot tap fully and listen for the boiler firing.
• Radiators warming up when you run a hot tap points to a stuck diverter valve.
• With a cylinder, press hot water boost before anything else.
• An immersion heater gives you hot water while you wait, and shows whether the cylinder is at fault.

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