Combine a heat pump and underfloor heating smartly

You are thinking about installing a heat pump. Maybe because your old gas or oil heating system will soon have to go. You quickly come across a topic that is mentioned again and again: underfloor heating. Many people say a heat pump will not work properly without it. But is that really true?

In older buildings, this often leads to uncertainty: Are my radiators sufficient for this? Do I have to install underfloor heating? Or can the existing heat distribution be adapted?

In this article, we explain how the technology behind it works, which requirements are decisive, and when installing underfloor heating is really worthwhile. We also show you what to pay attention to if you want to combine both.

How does a heat pump with underfloor heating work?

A heat pump uses energy from the environment, depending on the system from the outdoor air, the ground, or groundwater. However, over 90 percent of the heating heat pumps sold are air-source heat pumps, which use outdoor air as an environmental heat source. This environmental heat is absorbed and transported via a refrigerant circuit.

The compressor is decisive for the heating output: It increases the pressure of the refrigerant, causing its temperature to rise, similar to a refrigerator, only in the opposite direction. In a refrigerator, heat is removed from the inside and released outside into the kitchen; in a heating heat pump, heat is absorbed from the outdoor air and released into the interior rooms via the heating system.

For this to work efficiently, the so-called temperature lift (temperature difference between the environmental heat source and the heating temperature) must remain as low as possible. The smaller the difference between source temperature and heating water temperature, the less electricity the system needs. And this is exactly where underfloor heating comes into play.

Conventional radiators usually operate in a range that can be well above 55 degrees. As a result, the temperature lift compared with underfloor heating, where the heating temperature is rarely above 40 °C, is significantly greater and efficiency is correspondingly lower. This is a challenge for a heat pump, because the higher the required flow temperature, the worse the energy efficiency.

Underfloor heating systems, on the other hand, distribute heat across the floor over a large area. This means a flow temperature of 30 to 35 degrees is usually sufficient. This is a range in which heat pumps work much more efficiently.

A specific example: If the heating water has to be heated to 60 degrees, electricity consumption increases. At 35 degrees, the system works much more economically. That is why many experts recommend: Keep the flow temperature below 55 °C if possible.

Can a heat pump only be operated with underfloor heating?

Many people think a heat pump only works with underfloor heating. That is not true. A house can also be heated reliably with a heat pump using radiators, as long as all requirements are met.

Underfloor heating systems have clear advantages here: They distribute heat over a large area and therefore often manage with low temperatures. Radiators need significantly higher temperatures. This means the heat pump works less efficiently with radiators than with underfloor heating. Older models with a small heat exchanger surface area are less suitable in particular. This can become expensive in the long run, but it does not have to …

Whether your heating system is suitable depends not only on the type of heating surfaces, but also on the condition of the building. The better the thermal insulation, the lower the heat demand and the more likely radiators will also work with a heat pump.

In some cases, it is enough to replace individual radiators or replace them with larger models. So-called low-temperature radiators are also an option for improving efficiency. Despite low temperatures, they emit enough heat because their surface area is enlarged or heat emission is actively supported by small fans on the radiator, for example by small fans.

The year your house was built can be an important point of reference. Buildings after 1978 were equipped with better insulation and increasingly also with modern panel radiators.

It is different with older houses: Here, additional measures are sometimes usually needed, either for thermal insulation or for heating surfaces. But this can also be solved. What matters is to check the system properly before you get started.

Requirement: Is your current heating system sufficient for heat transfer?

Before you install a heat pump, you must have it checked whether your current heat transfer system (radiators) is suitable for it. The decisive factor is whether your house can cope with low flow temperatures. A rough guideline per square meter is not enough for this.

You absolutely need a room-by-room heating load calculation. It shows how much heat is required in each individual room so that it remains pleasantly warm even on cold days. This calculation takes into account factors such as window areas, the thermal insulation standard of the exterior components, and the use of the room.

Only with this data can it later be reliably assessed whether your radiator heat pumps can work together efficiently. With or an existing surface heating system, this is usually the case without additional measures.

A common mistake is to simply estimate the heating load roughly instead of calculating it. Example: Imagine two rooms with the same floor area, but with completely different conditions.

One room is on the ground floor, with three exterior walls and large windows; the other is under the well-insulated roof, surrounded by heated neighboring rooms. In the estimate, both are assigned the same heating output, even though in reality they differ extremely from each other. That is why an estimate is not sufficient.

Next step: Check heating surfaces

Once the heating load has been determined by calculation, the next step follows: checking the heating surfaces. This involves looking at whether the radiators in the respective rooms can deliver the required output in order to transfer the calculated amount of heat to the room. With underfloor heating systems, this is usually the case, since they already operate with low system temperatures. With radiators, this does not necessarily have to be the case.

If the heating output of a radiator is not sufficient, there are two options. Either the thermal insulation of the room must be improved, or the radiator must be replaced with one with a higher heat output. If this is checked and implemented in every room, the heat pump can operate at low temperatures. This noticeably reduces electricity consumption.

Together with the calculated radiator check, the valve settings of the individual radiators are also calculated. These are required for what is known as hydraulic balancing. In this process, the specialist company sets the valves according to the calculation so that every radiator receives the amount of heating water it actually needs. This setting ensures that all rooms receive exactly as much heat as they need.

Without this step, the system runs unevenly. Some rooms become sufficiently warm, others do not. The result: The entire system has to work at higher temperatures than actually necessary for the undersupplied rooms. And that makes the system less efficient. As an upper limit for an efficient system, the flow temperature should never exceed 55 °C even on the coldest days of the year.

For every radiator, it must be checked whether it can deliver the required heating output into the room at the standard outdoor temperature (here -10°C) with a maximum of 55°C.

Avoid mistakes: Be sure to check these 5 points before installation

Before you install a heat pump, you should clarify a few key points; otherwise, the system will not run as planned later or will cost more than it should.

• Flow temperatures above 55 °C? Then it can get expensive: Heat pumps work best with low temperatures. If the system often has to deliver more than 55 degrees, electricity consumption rises. Therefore, check beforehand whether your heating system can manage with a maximum of 55 °C or less even on the coldest days.
• Radiators unsuitable? Why you should pay attention to the surface area: Old sectional radiators or small compact radiators are rarely suitable for low temperatures. If the heating surface is too small, the radiator delivers too little output and the room does not get warm. Replacing it with larger or special low-temperature radiators often helps.
• No insulation under the screed? Heat loss threatens: Especially with retrofitted underfloor heating, you should pay attention to the structure. If the insulation layer under the screed is missing, part of the heat is lost downward, especially into unheated basement rooms. This can significantly reduce the efficiency of the overall system.
• No buffer storage tank? Problems with cycling are inevitable: Without a buffer storage tank, the heat pump can switch on and off too often when demand fluctuates. Frequent cycling puts strain on the technology and reduces efficiency. The storage tank compensates for load fluctuations and can bridge shutdown periods.

Radiators or underfloor heating: Realistically assess costs

If you want to convert your heating to a heat pump, you should not only keep an eye on the technology, but also on the costs. These depend heavily on what is already in place at your home and how much you need to adapt.

In new builds, the matter is usually clear. Here, planning is done from the start with low flow temperatures through surface heating. The technology is coordinated, and installation proceeds without major surprises.

In existing buildings, this is different. Especially if you still use conventional radiators or the energy standard is rather moderate. Then the question arises: Can the system stay as it is? Or does it need improvement?

Typical example: A single-family house from 1995, about 140 square meters, with panel radiators and average insulation. In many cases, the existing heating surfaces are sufficient or can be adapted with reasonable effort.

If a suitable heat pump is installed, some of the radiators are replaced, and hydraulic balancing is carried out, the total costs are often between 25,000 and 35,000 euros.

It is different if, instead of using the radiators, underfloor heating has to be additionally milled into the screed or other extensive renovation work is required. This is only possible in connection with a full interior renovation. Milling underfloor heating pipes into existing screed is only possible if the floor structure technically allows it.

In addition, the costs for retrofitted underfloor heating can make the measures overall extremely more expensive.

The German state supports switching to a heat pump, but the conditions change regularly. However, under the following links you will find all current subsidies available:

• Subsidy overview specifically for heat pumps in Germany
• Subsidy overview for residential building renovation (updated daily, German)
• Subsidy database with convenient search function (German)

Use underfloor heating or radiators efficiently: 6 ways

• Set the flow temperature correctly: The heating curve determines how warm the heating water should be depending on the outdoor temperature. If it is too high, the system wastes energy. Have the heating curve set properly once, or do it yourself and observe whether all rooms become evenly warm. You can find out how this works in the following video.

• Optimize heating circuits: If there is more than one heating circuit, they may be of unequal length and supplied with heating water unevenly. This can be balanced out through hydraulic balancing or adjustments to the valves. In new builds, this is part of the basic setting. In existing buildings, a recheck is worthwhile.
• Use smart-home integration: Smart thermostats or room controllers detect when you are at home and automatically adjust the temperatures. This reduces unnecessary running times without sacrificing comfort. Time programs can also be implemented more easily with them.
• Bleed and maintain regularly: Air in the system disrupts the flow and reduces heat emission. Once per heating season, you should check the system for air and bleed it if necessary. It also does no harm to look at the pressure in the system. If the pressure in the system is too low, water should be refilled.

How long does a heat pump take to heat up underfloor heating?

Underfloor heating systems react more slowly than conventional radiators. This is due to the structure: The heating pipes are embedded in the screed, which acts as a heat store. It takes some time for the floor to warm up and for the heat to become noticeable in the room. Depending on the structure, it can take between one and three hours.

How quickly the system responds depends, among other things, on the pipe spacing, the thickness of the screed, and the amount of water in the heating circuit. Systems with low volume or thin screed work somewhat faster.

The insulation beneath the heating coils also has an influence. If the necessary layer is missing there, part of the heat migrates downward. This extends the heat-up time and causes unnecessary losses. In older houses with an uninsulated floor structure, this can be a real disadvantage.

In new builds, this is different: There, insulation is standard, the heating circuits are properly planned, and the control system matches the inertia of the system. The only important thing is: Underfloor heating works continuously and not like a radiator with quick jumps. Anyone who takes this into account and does not constantly control the system manually gets a stable, even indoor climate.

Conclusion: How to plan your heat pump and underfloor heating correctly

If you want to install a heat pump, the interaction of the individual components matters. Underfloor heating is advantageous because it manages with low flow temperatures. At the same time, however, it is also a considerable effort and can actually only be retrofitted when the building is unoccupied.

Therefore, in most cases, people ultimately opt for the radiator solution after all. But before you get started, you should clarify the basics. Have the heating load determined, have the heating surfaces checked, and have the hydraulic balancing calculated.

If you are unsure, we will be happy to help you. With our heat pump check you get an initial assessment: What is feasible, where do you need to make improvements, and which technology suits your house?

Whether new build or renovation: With the right planning, the heat pump can be operated efficiently and reliably not only with underfloor heating, but also with radiators. You do not need a complete conversion and no luxury solution either, but a system that fits your building. If you are looking for additional low-cost thermal insulation measures, take a look at the following article. In it, you will find some quick and simple solutions that do not cost much but achieve a lot.

Image sources:

1. Featured image: Underfloor heating (Adobe Stock)
2. Heat pump with buffer storage tank (Photo: Werner Schäfer)
3. Panel radiator under window (Photo: Theresa Rundel)
4. Flow temperature (Photo: Adobe Stock / graphic editing: Carsten Herbert)
5. Milling in pipes (Photo: DML-Fußbodenheizung GmbH)
6. Screenshot: Excerpt from the Energiesparkommissar website