Cylinder Heating Coil Fault: Signs and How It Is Tested

What is the heating coil in my hot water cylinder?

On an indirect cylinder, the boiler does not heat the stored water directly. It sends hot water around a closed loop of copper tube, called the heating coil, which sits inside the cylinder. Heat passes through the coil wall into the stored water, and the water in the coil goes back to the boiler as the return. The stored water and the boiler water are meant never to mix. The flow to the coil is switched by a two-port valve on most systems.

That makes the coil the part that fails in two quite different ways. A coil that stops passing heat leaves you with a cold cylinder while the boiler runs happily. A coil that develops a hole lets the two waters cross, and the signs then appear in the heating system as pressure loss or a tank that keeps filling. A direct cylinder, which is heated only by an immersion element, has no coil, so this page does not apply to it.

How does a failing coil present?

The symptoms split into the ones that suggest the coil has stopped transferring heat and the ones that suggest it is leaking. Keep the two lists apart, because they have different causes and different repairs.

• Heats slowly, or reaches only a lukewarm temperature while the boiler fires: the coil is scaled or sludged on the water side, restricted, or airlocked
• Cylinder stays cold while the heating works: the heat is not reaching the coil, which is usually the valve, pump or controls rather than the coil
• Boiler pressure falling on a sealed system, or a feed and expansion tank in the loft that keeps topping up: a coil pinhole losing primary water into the cylinder
• Hot water tasting or smelling different, or looking dark: heating water crossing into the stored side, which is a sign of a coil leak and not a cosmetic issue
• Hot water temperature that swings or the cylinder that overheats: usually a control fault, but worth including in the test below

How do I test the flow and return at the cylinder?

The test is the quickest way to tell a coil problem from a controls problem, and it needs only a clip-on pipe thermometer. Do not use bare hands to judge pipe temperature, because the primary pipes can be hot enough to scald. Make sure the boiler is calling for hot water: set the programmer to hot water on, turn the cylinder thermostat to its normal setting, and give it a few minutes.

Find the two pipes that connect the coil to the heating system. They are normally on the side or top of the cylinder, close together, and one of them passes through a motorised valve or a pump. Take a reading on the flow, which is the pipe carrying heat to the coil, and on the return, which is the pipe leaving the coil. Read them against each other, and then read the cylinder temperature.

Three results account for most coil complaints. Flow hot and return clearly cooler, with the cylinder warming up, means the coil is working and the cylinder is just heating slowly. Flow hot and return nearly as hot, with the cylinder staying cool, means heat is going in but not coming out into the stored water, which points the engineer at the coil. Flow cold or only warm means heat is not reaching the cylinder at all, so the fault is upstream: a stuck two-port valve, a pump, the programmer or the boiler.

These readings are a guide, and the engineer reads them with the system running and the temperatures taken properly. They do not replace the diagnosis, but they save a wasted visit on the wrong part. If the cylinder stays cold and the heating is fine, heating on but no hot water from the cylinder takes you through the controls in order.

Can a coil leak be confirmed, and is it dangerous?

A coil leak is not a dangerous fault in itself, but it causes problems that need dealing with. The heating system loses water, and the boiler may lock out on low pressure on a sealed system. The inhibitor in the heating circuit, which is there to protect the radiators and boiler, can end up in the stored water. If the hot water goes to bath, kitchen and drinking taps, that is a reason to stop using it until the cause is found. For advice on drinking water, follow your water company guidance.

Confirming a leak is an engineer test. On a vented system the engineer watches the feed and expansion tank and the cylinder levels, which is covered in overflow dripping on a vented cylinder. On a sealed system they isolate the coil, pressurise it and watch the gauge, then compare against the rest of the heating circuit. A topped-up system that keeps losing pressure with no visible leak and no radiator or pipe fault is a strong pointer.

Do not keep topping up a heating system that loses pressure without finding out why. Fresh water brings in oxygen that corrodes radiators and the boiler, and the loss can hide a coil leak that is getting worse.

Can a coil be repaired, or does the cylinder need replacing?

A scaled or blocked coil can sometimes be treated with a chemical flush or by clearing an airlock, which is worth trying first where the coil is sound. A leaking coil cannot be patched with any lasting result, because it is sealed inside the shell of the cylinder, and in practice the cylinder is replaced.

That is where the repair or replace decision becomes real. If the coil has failed but the cylinder is otherwise sound and the replacement is straightforward, a like-for-like cylinder is the usual answer. If the system has other issues, such as old controls, a failed valve or a cylinder that is too small, those can be dealt with at the same visit. Cylinder repair or replace sets out the decision rule, and the hot water cylinder fault repair service covers what is done where a repair is the right answer.

Heat pump cylinders have much larger coils because the heat pump works at a lower water temperature. A fault there is a different diagnosis, and heat pump cylinder guide explains why.