A heat pump is a flagship renewable energy product alongside photovoltaics, using renewable energy sources to heat our homes. In political plans, their expansion is a crucial part of the so-called energy transformation. However, there are some "buts" - from a technical standpoint, it is an undeniable fact that heat pumps perform best under certain conditions, such as in new buildings with low heat loss, ideally with underfloor heating. It is no secret that only a portion of buildings meet these conditions. However, it is noticeable that heat pumps are also being installed in old houses. How does this work? Or maybe you are considering installing a heat pump in an old house yourself? If so, we invite you to read on.
✅ What You Need to Know About Installing a Heat Pump in an Old House.
Old houses often have significant heat losses - this is key to our considerations. This poses a serious problem when planning to replace a traditional boiler with a heat pump. Studies indicate that a heat pump can operate relatively efficiently (in terms of electricity consumption) in buildings with low heat losses.
✅ The efficiency of a heat pump drops significantly when heat losses exceed 60 W/m² (similar figures such as 65 W/m² or 70 W/m² are also common). In such situations, to maintain an appropriate temperature in the house, it is necessary to install a higher power pump, which involves high costs both in terms of purchase and operation. This is the first cost. Therefore, for old houses with significant heat losses, choosing a heat pump may prove inefficient and essentially uneconomical. But the necessity of installing a slightly larger or more expensive heat pump does not solve the issue and does not close the problem.
To assess the functionality of a heat pump in old buildings, the COP (Coefficient Of Performance) is crucial, which determines the ratio of the device's heating power to the electrical power needed for its operation.
Old houses usually have high-temperature heating systems with traditional radiators instead of underfloor heating. Heat pumps are most efficient and economical when working with low-temperature systems, such as underfloor heating. Modernizing by replacing radiators with new ones can involve high costs and not guarantee the expected thermal comfort, as new radiators - so-called panel radiators - also work best in buildings with low heat losses. Therefore, before deciding to replace the existing heat source with a heat pump, it is important to determine the heat losses in the house. This will allow for a proper assessment of the viability of such an investment.
✅ Why do heat pumps operate most economically with underfloor heating rather than radiators?
Many heat pump sellers will argue that it is possible to effectively heat a home with radiators using a heat pump. This is especially true if the heating system has been modernized and includes new types of radiators that do not contain a large volume of water, etc. Unfortunately, a radiator system will never be as economical (in terms of energy consumption) as underfloor heating. The operating system of a heat pump fits perfectly with underfloor heating.
A heat pump produces heat by extracting it from the air (for simplicity, we focus on the most popular type of heat pump, the air-to-water heat pump). Using special refrigerants, such as R32, R410a, R290, etc., it absorbs heat from the air, and it does this even at low temperatures. This is possible because the refrigerant has an unusual property in nature: the ability to absorb heat from the environment, even when it is warmer than the surroundings. This explains why heat pumps can operate in sub-zero temperatures outdoors.
However, this method only provides part of the heat needed to warm the house. To increase the refrigerant temperature, the heat pump starts compressing this gas. During compression, the temperature rises, allowing us to obtain a medium heated to 40°C or 60°C, depending on the heating system's needs.
Here’s the obvious point:
If the compressor is the main component of the heat pump consuming electricity at this stage, it must work much harder and longer with radiators to reach the higher temperature. Therefore, with radiators, there is always greater energy consumption, and the heat pump will not achieve its maximum efficiency, which it could reach with a low-temperature underfloor heating system.
(For those curious, heat pumps also have heaters, which are the most energy-intensive components, but we are omitting them for now as they are not involved in this stage of heat production. They start working only when it is so cold outside that the heat pump cannot extract heat from the air).
✅ We do not need to mention that operating at higher temperatures statistically wears out the components of the heat pump faster and increases the likelihood of malfunctions.
Many people living in old houses with traditional boilers, where the heating system is supplied at a temperature of 60°C, often achieve quite high indoor temperatures, such as 23-24°C, when the boiler is running. This is standard with coal or wood heating, after which, when the heating is off, the house cools down again. These individuals are convinced that 23°C (for example) is the necessary temperature for thermal comfort, below which it feels cold.
Meanwhile, people living in new homes with underfloor heating often maintain a temperature of 20-21°C and find it sufficient. They consider 23-24°C too high, with the exception of the bathroom, where a higher temperature is indeed desirable. How is this possible?
✅ In the case of underfloor heating, warmth starts from the feet. This is the most important part of the body for thermal comfort, where our thermal receptors are located. Because of this, with a warm floor, we do not need such a high room temperature to feel warm. It’s also worth noting that living in a lower temperature is healthier, especially in the bedroom. However, this article focuses on the economic aspect - it’s clear that heating a building to 21°C is cheaper than heating it to 24°C. Hence, a heat pump in an old house without underfloor heating will not be an economical heating solution.
✅ How to Prepare for Installing a Heat Pump in an Old House
Of course, installing a heat pump in an old house is ultimately possible and could be a solution. A heat pump in an old house can make sense, but only with a well-executed thermal modernization, involving a proper upgrade of the heating system and reducing heat losses. Naturally, an old house made from different materials than those we use today, with different insulation than modern buildings, will likely always have higher heat losses than a new building. However, heat loss is not just about the walls but mainly about windows, the roof, insulation (or lack thereof), and the quality of the heating system.
✅ Be aware that thermal modernization of an old house for a heat pump can be so expensive that you may ultimately decide that a high-temperature heat source (boiler) is a more feasible solution for now.
The first step of thermal modernization is an energy audit - an assessment of heat losses conducted by a specialist. Although it costs between 300 and 1000 €, it’s worth doing because the specialist will identify the points that need improvement, allowing you to proceed sensibly according to a plan, especially if you cannot do everything at once due to costs. This is usually the case, as it is difficult to replace the entire heating system, windows, doors, and possibly the roof in one year.
The most common tasks that need to be performed are:
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Let’s be honest - not always. It is only worthwhile if the decision to install a heat pump in an old house is accompanied by a decision for thorough thermal modernization, requiring substantial investment. Often, the amount of these investments makes it difficult to consider it ultimately profitable.
On the other hand, it is important to remember the maintenance-free operation of a heat pump, which is certainly significant if elderly people live in the building. However, without comprehensive preparation for the installation of a heat pump in an old house, you may end up facing exorbitant bills.
The worst situation occurs during severe frosts - heat pumps typically operate efficiently down to about -15°C. Beyond that, to produce heat, they need to activate electric heaters. Now imagine electric heaters with a power of 8-9 kW running almost continuously for three weeks.
✅ There are known cases where people who installed heat pumps in old houses suddenly discovered that during frosts around -20°C, the heat pump consumed 120-150 kWh per day, and the house was still not warm. We do not expect these individuals were informed by the sellers before installation that installing a heat pump in an old house without thorough thermal modernization could result in such an outcome.
I will allow myself to express a perhaps controversial opinion - subsidies for heat pumps in some old houses we have seen seem to be an enormous waste, if not a scam. Spending vast amounts of taxpayer money on subsidies for heat pumps in old houses is irrational and illogical - there should be clear restrictions on subsidizing heat pumps in certain buildings. However, we do not condemn people receiving subsidies for heat pumps in old houses. As long as it is a systemic solution, using it is normal. But systemically - it has nothing to do with rational spending of money.
Installing a heat pump in an old heating system can sometimes be necessary. Occasionally, modernization of the system is not possible. If we need to install a heat pump in an old system that operates in an open circuit, we certainly cannot simply connect it to a modern heating device like a heat pump. The same applies to a gas boiler.
In such situations, the heat pump must be separated from the old system using a plate heat exchanger. However, it must be a carefully selected heat exchanger suitable for the heat pump, as an undersized exchanger will result in an underheated house and the heat pump will not operate optimally, leading to increased energy consumption.
This approach creates two systems: one open heating system with an overflow tank, which can potentially be upgraded to a more modern and efficient system in the future, and a second closed-loop system in which the heat pump will operate.
This can be an excellent solution. It is a very good idea to have a backup heat source, such as an old boiler or a fireplace. This is a great alternative for severe winters because, to be clear, during mild winters when the temperature is around 0°C, there is no need to worry about high bills, making a heat pump for an old house a viable idea. The problem arises during extremely cold winters. In such cases, the boiler can be used to get through the harshest cold spells, after which heating can be switched back to the heat pump.
However, this solution has its issues. If you are using subsidies for the heat pump, one of the conditions is often the scrapping of the previous heat source. Even if it is relatively new and functional, it must be disposed of, supposedly for ecological reasons. Often, another condition for obtaining funding is having only one heat source, the heat pump, which is an impractical and short-sighted approach.
An additional heat source, even if it operates in an open system like a coal boiler or a fireplace, will not be problematic since it can be easily connected to the open system using the mentioned device, a plate heat exchanger.
Which plate exchanger for a heat pump?
Refrigerant heat exchanger - characteristics, principles of operation, structure
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