Plate Heat Exchanger or Storage Tank Coil?

✅ Preparing domestic hot water with a heat pump – the two most popular heat transfer solutions

Domestic hot water (DHW) is one of the most important elements of any heating system and the foundation of a building's comfort. In the case of heat pumps, the way it is prepared is especially important, because these devices work most efficiently at relatively low temperatures - which means the device transferring heat to the DHW must be extremely efficient (since it is harder to heat DHW with a heat pump operating at 50 degrees Celsius than with an old-type boiler at, say, 70 degrees).

In practice, we most often encounter two solutions: a DHW cylinder with a built-in coil and a system using an external plate heat exchanger.

Both solutions have their advantages, but they differ in their operating principle, efficiency, and possible applications. The choice of the right solution should depend not only on price, but above all on the parameters of the heat pump (or other heat source), the number of users, and the demand for hot water.

DHW cylinder with a coil – a classic and simple solution

A cylinder with a built-in coil is one of the most commonly used ways of preparing domestic hot water. In this case, the heating medium from the heat pump flows through a spiral pipe located inside the tank and transfers thermal energy to the water stored in the cylinder. A coil is a heat exchanger - essentially, a specific sub-type of one.

The biggest advantage of a coil in a cylinder is its simplicity. The installation requires fewer components, and doesn't need an additional exchanger or an additional control system. This is a convenient solution, often used in ready-made heat pump sets designed for single-family homes. You simply connect the supply circuit to the cylinder's outlet, to which the coil is already attached.

Limitations of a coil when working with a heat pump - or with low-temperature supply

Despite the popularity of this solution - which has essentially been, and probably still is, the standard - a coil has certain limitations. The most important limitations of coils are the heat exchange surface area and the heat transfer coefficient. To effectively transfer energy from the heat pump to the domestic water, the coil must have a sufficiently large surface area.

Heat pumps generally operate at lower temperatures than gas or oil boilers, which is why they require larger exchange surfaces. If the coil is too small, the DHW heating process can take a long time, and the device may operate less efficiently.

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A coil's surface area is not the same as the plate surface area of a plate exchanger

The heat transfer coefficient of a plate heat exchanger is significantly higher than that of a coil. This means that a coil with a surface area of 2 square meters will still exchange heat much less effectively than a plate exchanger with a plate surface area of 2 square meters. In fact, the heat transfer coefficient in plate exchangers is significantly higher - so much so that a plate exchanger with a surface area of 1 square meter will often, in practice, be more effective than a coil with a surface area of 3 or 3.5 square meters.

Why is that?

The answer becomes clear when analyzing how both devices work. A plate heat exchanger is a package of steel plates - with usually about 2mm of space between them. Water flowing through a plate exchanger has to be divided into thin, two-millimeter ,,slices", which are in contact with a hot steel plate on both sides. Obviously, under these conditions it heats up faster than in a cylinder with a coil, where the coil has to heat large, compact blocks of water.

☑️✅ Worth remembering - when replacing a coil with a plate exchanger, we don't compare the surface areas of both devices. These are two different types of devices and the comparison isn't compatible - a plate exchanger with a smaller surface area can match or even exceed the efficiency of even a large coil surface area.

Plate heat exchanger – higher efficiency and faster DHW heating

An alternative solution is the use of a plate heat exchanger. In this system, energy from the heat pump circuit is transferred to the domestic water through thin steel plates with a very large heat transfer area.

Thanks to their special construction, plate exchangers achieve very high efficiency with relatively small dimensions. They allow large amounts of energy to be transferred quickly, which is especially beneficial in installations requiring large amounts of hot water. So the device has huge advantages - and efficiency isn't the only thing that matters here. It also has certain drawbacks compared to coils.

A plate exchanger is a very small device - in the boiler room, that's an advantage

The advantage of a plate exchanger is its compactness. Modern boiler rooms don't have much space - they're designed to take up as little room as possible, so that space can instead be used for living areas. This matters, because a plate exchanger isn't a cylinder - it's a small flow-through device, weighing only a few kilograms. This makes it suitable for installation in many boiler rooms. It's rare for a situation to arise where you have to give it up simply because there's no room for it.

Why can a plate exchanger be more beneficial for a heat pump than coils?

Coils have always worked great, until the early 21st century, when transformations in heating technology and heating system design accelerated more broadly. Today the trend is low-temperature heating - not only when it comes to low-temperature underfloor heating based on manifolds or plate radiators - but also in the case of DHW. Modern heating devices - gas boilers, pellet boilers, or heat pumps - consume less energy when operating on so-called low supply temperatures. However, this requires greater efficiency from heat exchangers (including coils - since they are heat exchangers too).

This doesn't mean that coils won't work with a heat pump. They simply often require cylinders with much larger coils than were used in the 1990s, when the standard supply temperature was 60-70 degrees Celsius. In many cases, people installing heat pumps in an older house - which already had a cylinder with a coil - later discover that the same coil no longer provides hot water as effectively once the heat source has been replaced by a heat pump.

Pairing a heat pump with a plate exchanger can bring several significant benefits. Above all, the device can prepare DHW faster, because the exchanger provides very effective energy transfer.

This matters especially in larger houses, multi-family buildings, service facilities, or anywhere there is high demand for hot water. In such cases, a standard coil may prove insufficient.

A more efficient heat exchanger is better for the heat pump and for its compressor

A more efficient heat exchanger is also more beneficial for the heat pump itself - for its economics, but also for its longevity and reliability. With a heat exchanger that's too weak - let's assume it could be a coil (but equally well an improperly sized undersized plate exchanger) - the heat pump switches off more often, and then also switches back on more often. This is a factor that significantly increases power consumption - but also, over time, the wear on the compressor, which is the heart of the heat pump system, and whose failure is very troublesome.

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Does a plate exchanger have any drawbacks?

A plate exchanger, however, isn't a solution completely free of drawbacks. It requires additional installation components, such as a circulation pump, valves - basically everything that installing an additional circuit requires. This means a more complex system than with a classic cylinder with a coil. Someone has to install it - and that has to be paid for. With a coil, you get a cylinder with a ready-made system, and the installation itself is much simpler.

You also need to keep the cost in mind - a plate exchanger, if it's to be selected for low-temperature sources, should be chosen with an appropriate margin, i.e. a reserve of heat exchange surface area. That makes it a real cost in practice.

You should also remember the importance of properly sizing the exchanger. Unfortunately, there are many cases where plumbers don't know how to select this type of device, and as a result choose models that don't work well - for example, because they're too small. Connection mistakes with plate exchangers made by industry professionals are an almost widespread occurrence.

They later claim that the plate exchanger itself, as a device, is inefficient, or that a plate exchanger always shows a large heat loss between the supply side and the receiving side. These are obvious untruths resulting from sizing errors - but also from installation errors. Although this isn't a typical flaw of the device itself, it's worth mentioning, because many people have run into this problem - and ease of connection definitely speaks in favor of the coil.

DHW hygiene – storing water or instantaneous heating?

An important aspect of choosing a solution is also the issue of hygiene. In a classic cylinder, a large amount of water is stored for an extended period, which is why it's necessary to maintain appropriate temperatures and periodically protect the installation against the growth of bacteria, such as Legionella.

A system with a plate exchanger can work with a cylinder (of course it can!), but it can also operate in a more instantaneous way, heating water only when it's needed. This reduces water storage time and can increase comfort and safety of use. This solution is particularly valued, as it allows you to get rid of the cylinder, which takes up space.

Plate exchanger or coil – which solution to choose?

It can't be said unequivocally that one solution is always better than the other. For a small single-family home with moderate DHW consumption, a good-quality cylinder with an appropriately large coil can be a completely sufficient solution.

However, if high performance, fast DHW preparation, and making the most of the heat pump's capabilities matter to us, a plate heat exchanger will often be a more advanced and effective solution.

The final choice should depend on the parameters of the specific installation, the power of the heat pump, the number of users, and the expected comfort of hot water use. In many modern installations, it's precisely the plate exchanger that allows the heat pump's full potential to be used.

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Plate Heat Exchanger NORDIC Ba-32-40 1" 230kW

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