What Is a Heat Exchanger? Boilers, Combis and Cylinders

The basic idea in plain terms

Heat always flows from something hotter to something cooler. A heat exchanger simply puts two streams of water, or gas and water, either side of a thin metal wall and lets that happen as efficiently as possible. The hot side gives up its heat, the cooler side picks it up, and the two never touch.

That separation matters. The water in your radiators is dark, full of inhibitor and never meant to come out of a tap. The water you wash with has to stay clean. A heat exchanger is what lets one boiler heat both without contaminating your drinking water.

The main heat exchanger inside your boiler

Every gas boiler has a primary heat exchanger. The burner fires into it, and the water of the heating circuit flows through its passages and picks up the heat. From there the pump sends that water to the radiators, to the cylinder coil, or in a combi to the plate heat exchanger.

In a modern condensing boiler the main heat exchanger is large and made with narrow waterways, usually in stainless steel or an aluminium alloy, so it can pull enough heat out of the flue gases to make the water vapour in them condense.

Because those modern waterways are narrow, they are sensitive to sludge. A system that has never been cleaned or fitted with a magnetic filter tends to deposit its debris in the boiler heat exchanger, and our article on how long should a boiler last explains why that is one of the main things that ends a boiler early.

The plate heat exchanger in a combi

A combi boiler has a second, smaller heat exchanger that a system or regular boiler does not. When you open a hot tap, a diverter valve sends the hot heating water through a block of thin stacked plates. The cold mains water runs through the alternate gaps between those plates, picks up the heat, and comes out at the tap. Our guide on how does a combi boiler work goes through that sequence step by step.

The plates are close together to give a large surface in a small space, which is exactly why they scale up in hard water. London water is hard across most of the city, and the plate heat exchanger is usually the first place it shows. Hot water that starts hot and then fades, or swings between hot and cool while the heating works perfectly, is a typical sign. Our page on a blocked plate heat exchanger covers the symptoms and whether descaling or replacement makes more sense.

The coil inside a hot water cylinder

If you have a boiler and a hot water cylinder, the cylinder almost certainly contains a heat exchanger in the form of a coil of pipe near the bottom. Hot water from the boiler flows through the coil and back, warming the stored water around it. This is what makes a cylinder indirect. A direct cylinder has no coil and is heated only by electric immersion heaters, which our guide to direct vs indirect hot water cylinders explains.

Some cylinders have two coils. A twin coil cylinder uses the lower coil for solar thermal panels or a heat pump and the upper one for a boiler. Cylinders made for heat pumps have a much larger coil, because a heat pump delivers water at a lower temperature and needs more surface area to heat the cylinder in a sensible time. Our article on the heat pump hot water cylinder goes into that difference.

Coils do not block the way a plate heat exchanger can, but they do scale on the outside where they sit in the stored tap water. The result is slow recovery: the boiler runs for longer and longer to bring the cylinder up to temperature, and the hot water runs out sooner than it used to.

Other places you will find one

Several other bits of kit around the home rely on a heat exchanger too.

• Thermal stores and Pulsacoil type cylinders store hot boiler water and heat the mains water as it passes through a coil or plate heat exchanger on its way to the tap. Our guide to the Gledhill Pulsacoil explains how that works in practice.
• Heat pumps use a heat exchanger to pass heat from their refrigerant into the water that goes to the radiators and cylinder.

Signs a heat exchanger is struggling

Heat exchangers rarely fail without warning. The signs usually build over weeks or months, and they differ depending on which one is affected.

• Combi hot water that is lukewarm, fades after a minute or surges hot and cold, while the radiators are fine. This points to the plate heat exchanger.
• A cylinder that takes far longer to heat than it used to, or runs out of hot water quickly. Often a scaled coil, though a sticking valve or thermostat can look the same.
• A boiler that rumbles, bangs or sounds like a kettle when the burner is on. This is kettling, usually caused by scale or sludge restricting flow through the main heat exchanger.
• Water inside the boiler casing or a pressure gauge that keeps dropping. A leaking main heat exchanger is possible, although valves and seals are more common causes.

If you ever suspect damage to the heat exchanger inside a gas appliance, if you see sooting or scorch marks around the boiler, or if a carbon monoxide alarm sounds, turn the boiler off and do not use it until a Gas Safe registered engineer has checked it. Our guide on a carbon monoxide alarm going off covers what to do straight away.

What causes the damage

Two things account for most heat exchanger trouble. On the tap water side it is limescale, which forms faster the hotter the water gets and the harder the supply. London water is hard, which is why our guide on hard water and hot water cylinders in London is relevant to almost every home we visit.

On the heating side it is sludge: black iron oxide from the inside of radiators and pipes, which builds up when the system water has no inhibitor, when there is no magnetic filter, or when a small leak means fresh oxygen rich water keeps getting added. It collects wherever the flow slows down or the passages narrow, and the boiler heat exchanger is the narrowest point of the whole circuit.

Corrosion and age account for the rest. After many years of heating and cooling, a heat exchanger can eventually split or weep at a joint.

Repair, clean or replace?

A scaled plate heat exchanger in a combi is a fairly common replacement and is often worth doing on a boiler that is otherwise sound. It is gas appliance work, because the case has to come off a room sealed boiler, so it needs a Gas Safe registered engineer.

A main boiler heat exchanger is a different matter. It is one of the most expensive parts in the boiler and fitting one is a long job, so on an older boiler it can make more sense to put the money towards a replacement. An honest engineer will tell you which side of that line your boiler sits on.

A cylinder coil cannot usually be replaced on its own. If a coil has scaled badly or started to leak, the realistic options are to live with slower recovery, to rely more on the immersion heater, or to replace the cylinder. Our guide on signs your cylinder needs replacing helps with that decision, and our pricing page shows how we charge for the diagnosis before any of it.

Getting the right part looked at

The hard part is rarely the repair. It is working out which heat exchanger, if any, is really at fault, because a stuck diverter valve, a failing cylinder thermostat or a sticking motorised valve can produce almost identical symptoms. Replacing a heat exchanger that was never the problem is an expensive mistake.

If your hot water has gone lukewarm, your cylinder is slow to heat or your boiler has started to sound unhappy, call us on 020 4652 5111 or send an enquiry. Tell us the boiler make, whether you have a cylinder, and when the problem started. We have 20 engineers across London who deal with boilers, combis and unvented cylinders every day, and our rates are on the pricing page, agreed before anyone attends.