Are you considering installing a heat pump in a building with an old central heating system? How can you protect a heat pump from the damaging effects of an open system? Learn how heat pumps are practically isolated from old installations using plate heat exchangers.
This primarily concerns open systems, which is often the case when connecting a heat pump in an old house. Naturally, it is assumed that a complete renovation of the heating system—such as converting it to a closed system—is currently not feasible and has to wait. There can be many reasons for this, with high renovation costs often being the primary factor. Heat pumps generally cannot operate in open systems as they are considered too delicate (work sensors and temperature sensors may not withstand it), and manufacturers require installation in a closed system to maintain the warranty.
Heat pumps are not recommended for use with old heating systems, as these often have high-temperature requirements (e.g., 60-75°C), which do not align with the optimal working conditions of heat pumps. Heat pumps work most efficiently at lower temperatures (30-48°C), typical for modern low-temperature systems such as underfloor heating. Moreover, old systems may have poorly insulated pipes and radiators with a high water capacity, which increases heat loss and reduces the heat pump’s efficiency, leading to higher operational costs. However, this does not mean that people don’t install heat pumps in old houses.
✅ If you decide to go with such a solution, being aware of these limitations, it is worth dividing your central heating system into two parts: an open system for the radiators and a shorter circuit—a closed system—for the heat pump. In this case, a plate heat exchanger will connect both parts. This will be discussed further in the article.
There are various reasons, one of which is the maintenance-free operation of heat pumps. Higher costs and greater electricity consumption generated by heat pumps are issues primarily during winter, but less so during transitional seasons like autumn or spring. When outdoor temperatures are above freezing, heat pumps can operate maintenance-free and relatively economically, even in old heating systems. Additionally, having an open system has its advantages. It allows for the retention of another heat source, typically an older type, which is more suited to harsh winters. This could include a coal boiler, a wood boiler, or even a fireplace. In such cases, a hybrid heating system is possible—a heat pump with a backup boiler for particularly cold winters.
Other reasons why people install heat pumps in old heating systems include government subsidies, such as those from programs like Clean Air. Regardless of the merit or drawbacks of these programs, the installation of a heat pump can sometimes be almost free of charge, making it "economical" even in older buildings.
A plate heat exchanger is a small device that operates in a flow-through, counter-current manner. It has two channels, allowing it to transfer liquid through two systems—for example, a closed system (where the heat pump operates) and an open system, which can accommodate the building’s old installation. The plate heat exchanger functions to enable these two liquids to exchange thermal energy—in simpler terms, it allows the warmer medium to transfer heat to the cooler medium. This makes it possible to transfer heat generated by the heat pump to the old open system, and the key point here is that this happens without the liquids mixing. Thus, the two systems are connected, yet they remain separate.
The plate heat exchanger itself is a relatively compact device, weighing from a few to several kilograms—at least when talking about models used for connecting split or monoblock heat pumps in typical single-family homes.
To understand how a heat pump is separated from an old installation, simply take a look at the diagram below. A small closed-loop system needs to be created, typically a short circuit, in which the heat pump can operate safely. This closed system should be connected to the old heating system via a plate heat exchanger. The plate heat exchanger takes a central role in this setup, although it is merely an accessory—as its name suggests, it does not generate heat but simply facilitates the transfer of temperature between the two systems.
The heat pump connection diagram is simplified and includes only the basic elements necessary to understand the concept of separating the heat pump from the old open system, where the plate heat exchanger acts as a link. Everything to the right of the plate heat exchanger operates in an open system, while everything to the left functions in a closed system with the heat pump (1) as its core.
The simplified diagram omits the possibility of maintaining an additional heat source in the open system, which should operate with an open system and an open expansion vessel (e.g., a fireplace or a coal stove). Similarly, in the closed system alongside the heat pump, an additional heat source can be connected, but it must be one that requires a closed system, such as a condensing boiler.
Split heat pumps are almost entirely installed inside the building, apart from the outdoor unit. This means there is no need to fill their system with glycol, which is commonly used for monoblock heat pumps exposed to freezing conditions. In this case, the only reason for using a plate heat exchanger to separate a SPLIT heat pump is the potentially poor condition of the old system and the inability to install a SPLIT heat pump in an open system (this should be avoided, also due to warranty recommendations).
The situation may be different for monoblock heat pumps if their circuit is filled with glycol. More on this below.
Monoblock heat pumps differ slightly from split units. A part of the device is located outdoors, which is an advantage for those with limited indoor space. However, in regions with frequent power outages, there is a risk of freezing. During such outages, the pump may be unable to perform defrosting, which could result in freezing and damage to components. For this reason, some people opt to create a shorter circuit for the heat pump, even when connecting it to a new house with a safe and efficient closed system. This circuit is filled with antifreeze (glycol) or, more commonly, a glycol-water solution, which reduces the risk of freezing.
The glycol circuit of a monoblock heat pump should therefore be separated from the home’s central heating system, regardless of whether the house relies on an old or new heating installation. This represents a different technical motivation compared to split heat pumps, which are usually separated using a plate heat exchanger to protect against impurities in open systems.
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