Radiators Hot but Hot Water Cylinder Cold: System and Regular Boilers

How does a cylinder system decide to heat the water?

With a system or regular (heat-only) boiler, hot water is stored in a cylinder, usually in an airing cupboard. Boiler water never reaches your taps. It runs through a copper coil inside the cylinder, called the primary coil, and warms the stored water from the inside.

Three things have to line up before that happens. The programmer must have its hot water channel on. The cylinder thermostat, the dial strapped to the side of the tank, must read the water as below its setting. Then a motorised valve opens the route from the boiler to the coil, and a switch inside that valve tells the boiler and pump to run. The heating has its own call through the room thermostat, so it can work perfectly while any one of those hot water links has failed.

Most pumped systems use one of two layouts. An S-plan has two separate two-port zone valves, one for heating and one for hot water. A Y-plan has a single three-port mid-position valve that can send heat to the cylinder, the radiators or both. Some older regular boilers heat the cylinder by gravity circulation with no valve at all, and those are the systems most prone to air in the coil.

If there is no cylinder in the house and hot water comes straight from the boiler when you open a tap, you have a combi, and none of this applies. The combi version of this fault, usually a diverter valve, flow sensor or scaled plate heat exchanger, is covered with a tap test in heating on but no hot water.

What can I check myself tonight?

Work through these in order. None of them involve opening the boiler or touching wiring.

• Programmer: make sure the hot water channel is on, set to constant, or inside a timed period that covers now. Press boost or advance if it has one. After a power cut, many digital programmers lose the clock or revert to off.
• Cylinder thermostat: check the dial has not been knocked to its lowest setting. Around 60°C is normal. Turn it up and back slowly and listen for a click; no click at any setting can mean the thermostat has failed.
• The coil pipe: with hot water called for and the boiler running, carefully feel the pipe where it enters the coil on the side of the cylinder. If the pipes at the boiler are hot and this one stays cold, boiler water is not getting through.
• The zone valve: find the small box on a brass valve body near the cylinder or boiler. If its manual lever is latched in the open position, the valve is being held open by hand, which is a clue someone has already worked around a fault.
• Immersion heater: if you have one, switch it on to heat the cylinder tonight while the fault is found. It runs on electricity and does not depend on the boiler.

Do not open the programmer, wiring centre or valve head. They run at mains voltage. Never remove the boiler casing. If you smell gas, open windows, turn off the meter valve if it is safe to, keep clear of switches, and call the National Gas Emergency Service on 0800 111 999 from outside.

Is it the zone valve or the three-port valve?

On an S-plan system, the hot water two-port valve is the most common single cause of radiators hot and cylinder cold. The actuator head holds a small synchronous motor that drives the valve open against a spring. When the motor burns out, or the gears strip, the valve stays shut to the cylinder while the heating valve carries on normally. A motor that hums but never turns, or a head that feels cold when it should be powered, points to the actuator. Honeywell and Drayton are two common makes, and on most of them the head lifts off the valve body without draining the system.

The valve can also open but fail to tell the boiler. Each two-port head has an end switch that closes once the valve is fully open and sends power to the boiler and pump. If that switch fails, the valve opens, nothing fires, and the cylinder stays cold while the heating valve works.

A Y-plan three-port valve behaves differently. With no power to it, the spring holds it open to hot water only, so a dead motor usually leaves you with hot water and no heating, not the other way round. Hot radiators with a cold cylinder on a Y-plan more often mean a spindle seized in the heating position, or the valve being told the cylinder is already satisfied. That signal comes from the cylinder thermostat, which is why the thermostat and the wiring are checked next.

Cylinder thermostat, pump overrun and wiring faults

The cylinder thermostat is a simple switch. If it fails in the satisfied position, or its sensing face has slipped away from the copper, it never calls for heat. It must sit flat against the bare cylinder; a thermostat strapped over thick foam can read the water as hot long before it is. An engineer checks it with a meter rather than by guesswork.

Pump overrun is a feature on many system boilers. When the burner stops, the pump keeps running for a few minutes to carry heat out of the boiler. That needs an open route for the water. If the hot water valve closes as soon as the cylinder is satisfied and there is no bypass or permanent live to keep the pump going, the boiler can overheat and lock out. The tell-tale is a boiler that works on heating but shows an overheat fault or cuts out on hot-water-only calls, often in summer when the heating is off.

Wiring faults are common after a new programmer, room thermostat, smart control or boiler has been fitted. If a hot water call is wired to the wrong terminal in the wiring centre, the valve may open with nothing telling the boiler to fire, or the boiler may fire with the valve shut. A system that was fine until someone changed the controls usually has a wiring fault, not a failed part.

Could air in the cylinder coil be the problem?

Yes, especially straight after the system has been drained, power flushed or had a radiator or valve changed. Air rises to the highest point it can reach, and on many installations that is the cylinder coil or the pipework above it. The valve opens and the boiler fires, but water will not circulate through the airlocked coil, so the flow pipe gets hot close to the valve and the return from the cylinder stays cold. You may hear gurgling from the airing cupboard.

Gravity-fed hot water on older regular boilers is the most prone, because there is no pump to push the air through. On a pumped system, an engineer bleeds the air from the coil through an automatic or manual air vent, runs the pump on hot water only, and on a sealed system tops the pressure back up afterwards. If the airlock keeps coming back, air is being drawn in somewhere, often through a pump set too high on an open-vented system, which is a fault in its own right.

If none of these explains it, the guides to hot water faults cover faults in the cylinder itself, and immersion on but still no hot water covers the case where the backup fails too.

What the engineer does, and typical costs

The engineer starts at the controls, not the boiler. With hot water called for, they test for mains voltage at the hot water valve, check whether the actuator moves the valve, and check the end switch closes to fire the boiler. They test the cylinder thermostat and the programmer output with a meter, and read the wiring centre against the system layout. If everything electrical is working, they check the coil for air or a blockage.

Our no hot water repair visit starts with the £60 call-out at any hour, and the repair price is agreed with you on site before any work starts. Gas work is carried out by Gas Safe registered engineers. The full rate card is on our pricing page. For a sense of scale, these are typical UK market ranges, parts and labour together.

• Zone valve actuator head replaced: typically about £100 to £250
• Whole motorised valve body replaced: costs more than a head alone, because the system has to be drained and refilled
• Cylinder thermostat replaced: typically about £110 to £200
• Programmer or wiring centre fault: priced on the unit fitted and the time the fault takes to trace
• Coil or system bled of air: usually one of the smaller jobs