Undersizing refers to the use of a heat exchanger with lower capacity than required by the installation. Put simply, it's the use of a heat exchanger that is inefficient to the extent that it cannot transfer the appropriate amount of heat from the heating side, i.e., from the heating circuit to the installation side (where heat receivers are located, such as radiators or underfloor heating).
Is this phenomenon dangerous for the installation? Not always, but sometimes it can be.
Underheated house or apartment
Let's assume we have a heating boiler (or other heat source) with a power of 20 kW (as calculated by the designer to meet the demand for our house or apartment). However, we have installed a heat exchanger that can realistically only transfer 15 kW to the installation side. The conclusion is simple: the required 20 kW will not be transferred to the house, which means it will be underheated. This will be most noticeable during severe winter.
It's worth noting here not to select a heat exchanger based solely on its nominal power (usually calculated for high temperatures and flows, whereas popular coal or solid fuel boilers typically operate at lower temperatures). The heat exchanger should be calculated to be able to transfer the required power under specific conditions for a specific boiler, in our example, it was 20 kW.
❌ The fact that a heat exchanger has a nominal power of 50 kW often means that it can achieve 50 kW when supplied with high temperature, achievable in industrail conditions (e.g., 120°C), but it absolutely cannot achieve 50 kW of power when supplied with a coal boiler operating at 50-55°C.
✅ It's best to consult a specialist or heat exchanger vendor for proper selection.
Smaller heat exchangers tend to clog more frequently and easily. A clogged heat exchanger is effectively blocked, preventing or significantly impeding the operation of the heating system. Such a heat exchanger is not always easy to clean effectively and often leads to the need to purchase a new one. In the case of a properly sized heat exchanger, clogging or scaling may also occur, but experience shows that this happens after many years, whereas with a too small heat exchanger, it often occurs after 1-2 seasons.
This is a dangerous phenomenon. In the case of a too small heat exchanger, the water on the boiler side remains constantly heated. Since the heat exchanger is too inefficient to absorb its temperature and transfer it to the radiators' side, hot water returns to the boiler. Fortunately, most boilers nowadays shut down in such situations, but with older boilers that lack such a mechanism, there is a risk of damaging the installation. However, water boiling in the heating or fireplace system occurs when the heat exchanger is significantly undersized. If it is undersized within a small range, it is often possible to avoid this.
In this post, we focus on the phenomenon of undersizing, but as an additional piece of information - it's worth noting that the selection of a plate heat exchanger should take into account the surface area of the plates in relation to the boiler's power.
✅ For NORDIC TEC heat exchangers, it looks as follows - in the case of central heating boilers. For a 10kW boiler operating at a standard of 50-60 degrees Celsius, approximately 0.3m2 of plate surface area is recommended. However, this ratio can be smaller, for example, 0.25m2 per 10kW boiler if the heating system is relatively new and located in a newly constructed building. NOTE - the diagram and calculator do not apply to the selection of plate heat exchangers for connecting heat pumps.
The most common cause of undersizing heat exchangers is basing the decision solely on price. It is worth comparing the price of the heat exchanger with the price of the boiler, fireplace, installation costs, etc. Then it is worth considering whether it makes sense to risk the installation for savings ranging from 10-15 to over a 25-30 €, as often such a price difference allows for the purchase of a heat exchanger with the correct power margin.
The second common cause is basing the decision on the nominal power of the heat exchanger. Just because the nominal power of the heat exchanger is 110 kW does not mean that it can achieve 110 kW in an installation with a coal boiler operating at relatively low temperatures (around 50-55 degrees Celsius). The heat exchanger achieves nominal power under industrial conditions, where supply temperatures and flows are significantly higher. In the case of a coal furnace, it achieves only about 50-60 kW depending on the flow rate, slightly more in the case of an eco-pea furnace, which usually has slightly higher temperature performance, etc. Ignorance of these rules, often by installers, often leads to the practical problem of undersizing, although we absolutely excluded it. The heat exchanger should be selected not based on nominal power but on the achievable power for a given heat source.
✅ In case of selection issues - it is best to contact our selection service.
Oversizing the heat exchanger - what is it for?
What is the production process of brazed plate heat exchangers?
How to choose a heat exchanger for a boiler?
How to properly connect a Nordic Tec plate heat exchanger? Can I reverse the flow directions?
History of plate heat exchangers
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