Replacing a boiler heat exchanger: costs, symptoms and which part you need

Short answer: a boiler has two types of heat exchanger, and the difference in price is large. The plate heat exchanger makes your hot tap water and usually costs €200 to €290. The main heat exchanger is the heart of the boiler and, depending on the brand, costs €560 to well over €1,250. For a boiler older than twelve years, replacing the main heat exchanger is rarely worthwhile. Work on a heat exchanger in a gas boiler may only be carried out by a certified gas engineer.

Two heat exchangers, two very different stories

A combi boiler usually contains two. The primary or main heat exchanger sits above the burner and transfers the combustion heat to the heating water. It is made of aluminium or stainless steel, sits in the sealed part of the boiler and is by far the most expensive part. The plate heat exchanger, also called the domestic hot water or secondary heat exchanger, is mounted on the side and transfers the heat from the heating water to the tap water. It is a compact stack of metal plates that can be replaced separately.

Knowing which of the two has failed determines whether you are looking at a repair of a few hundred euros or at the question of whether the boiler still has a future.

How you can tell the plate heat exchanger has failed

The heating works fine, but the hot water no longer does. Typical signs: the tap only gets lukewarm, the temperature fluctuates while you shower, or the boiler briefly fires and switches off again as soon as you ask for hot water. In almost every case the heat exchanger is then clogged with limescale or dirt. In hard-water areas this happens faster, especially with boilers without water treatment.

Sometimes an engineer can still clean the heat exchanger chemically. If it is leaking, you will see the heating pressure rise when you run hot water: the tap water is then leaking into the heating system.

How you can tell the main heat exchanger has failed

This is almost always accompanied by faults. The boiler locks out on overheating or temperature difference, or water appears under the boiler that does not come from the condensate drain. With a leaking main heat exchanger the heating pressure keeps dropping without you finding a leak in the pipes. Soot formation or a persistently high flue gas temperature can also indicate that the heat exchanger is fouled or damaged on the inside.

Important: do not keep resetting with these complaints. If the safety device intervenes, there is a reason.

What does a heat exchanger cost?

The amounts below are part prices including VAT from our catalogue, excluding labour. The current price is always on the product page.

Part	Example	Price
Plate heat exchanger (domestic hot water)	Atag plate heat exchanger iCon1, manufacturer number S4851300	€ 208.95
Plate heat exchanger (domestic hot water)	Atag plate heat exchanger iCon2, S4851410	€ 205.95
Plate heat exchanger (domestic hot water)	ATAG plate heat exchanger 35 kW PF/ESHR, S4722600	€ 280.95
Main heat exchanger	Intergas heat exchanger large, 146617	€ 575.95
Main heat exchanger	Intergas heat exchanger HReco large, 147207	€ 564.95
Main heat exchanger	Vaillant heat exchanger VHR24-28, 065114	€ 694.95
Main heat exchanger	Nefit heat exchanger 24 kW, 73844	€ 1,258.95

Labour comes on top of that. Swapping a plate heat exchanger is usually about an hour's work; replacing a main heat exchanger means taking most of the boiler apart and takes half a day. So for a main heat exchanger, expect a total that comes fairly close to the cost of a new boiler.

Replace it or get a new boiler?

The rule of thumb we use ourselves: if the repair costs less than a third of the price of a new appliance and the boiler is younger than twelve years, repair it. With a main heat exchanger at €560 to €1,300 plus labour, an older boiler is usually above that threshold. Then take into account that a new boiler is more efficient, comes with a new warranty and puts the parts problem behind you for ten years.

A plate heat exchanger is a different matter. The cost stays limited, so even in a ten-year-old boiler replacing it is almost always the sensible choice. In fact, it is one of the few repairs that really makes a boiler perform as good as new on hot water again.

Can I do this myself?

No, not on a gas boiler. To replace a heat exchanger the boiler has to be opened and you get into the sealed combustion section. Work on gas appliances should only be carried out by a certified gas engineer. Feel free to order the part yourself, but have it fitted by a certified gas engineer; that is also the condition under which the manufacturer's warranty on the part applies.

What you can do yourself is limit the cause. Background on servicing and safe heating can be found at Milieu Centraal.

How to make a heat exchanger last longer

Three things destroy heat exchangers: limescale, dirty heating water and too little flow. Have the boiler serviced every year so that dirt is removed in time. Keep the water pressure at the right level, because a system that constantly needs topping up keeps taking in fresh, oxygen-rich water, which attacks the metal. And when renovating or fitting a new pump, always have the flow checked: a heat exchanger that gets too little water through it becomes far too hot in places.

Frequently asked questions

Hoe weet ik welke warmtewisselaar in mijn ketel zit?

Check the rating plate for the brand, type and output of the boiler. Manufacturers supply a specific heat exchanger for each series and output class; a block for a 24 kW appliance does not fit a 30 kW version. With the brand, type and the number on the old part, use our spare parts catalogue to find the right item.

Can a plate heat exchanger be cleaned instead of replaced?

Sometimes. An engineer can descale it chemically if the plates are still intact. If the heat exchanger is leaking or badly damaged, cleaning no longer helps. On boilers where the heat exchanger is cheap, engineers often go straight for replacement because the labour for cleaning costs almost as much.

How long does a heat exchanger last?

A main heat exchanger is designed to last the life of the boiler, fifteen years or more with good servicing. A plate heat exchanger gives out sooner, especially in hard-water areas; eight to twelve years is not unusual there.

Is a leaking heat exchanger covered by the warranty?

That depends on its age and whether the appliance has been serviced every year. Many manufacturers give a longer warranty on the main heat exchanger than on the rest of the boiler, but do require proof of annual servicing by a qualified installer. So keep your service records.