Air-to-Water Heat Pump: Selection and Sizing Step by Step
, By NeueEnergie24, 9 min reading time
How do you find the right air-to-water heat pump? We explain heating load, flow temperature, storage, noise and installation location, and provide a checklist for your conversation with the installer.
In brief: An air-to-water heat pump works efficiently when it matches the actual heating load of the building and the required flow temperatures. What matters most is a proper heating load calculation, hydraulic balancing, flow temperatures that are as low as possible, and a sensibly chosen installation location. This article walks you through selection and sizing step by step and shows which questions you should clarify before buying. Please note: it refers to the situation in Germany, including German consumer advice, associations, standards and grid operators.
How an air-to-water heat pump works
According to the Verbraucherzentrale (the German consumer advice organisation), a fan draws in outside air and passes it over an evaporator, where a refrigerant absorbs heat. The refrigerant is compressed, releases its heat to the heating water and is then expanded again. The Verbraucherzentrale describes air-to-water systems as the most flexible variant. They are comparatively simple to install because no groundworks or drilling are needed, but they lose efficiency at very low outdoor temperatures.
Step 1: Check the building and heating system
As a rule of thumb, the Verbraucherzentrale considers a building suitable if the flow temperature of the heating system stays below 55 degrees Celsius all year round, if possible. Underfloor, wall or ceiling heating is considered ideal because it works with low temperatures. Radiators that are too small can be replaced with larger ones or with low-temperature radiators. Good thermal insulation improves efficiency; the Verbraucherzentrale recommends considering an energy-efficiency renovation before replacing the heating system, so that the new heating system matches the changed heat demand.
- What flow temperature do your radiators need on the coldest days?
- Are there rooms with underfloor or other surface heating?
- Has insulation or have windows been upgraded, or is that planned?
- What has been your consumption of fuel or electricity so far?
Step 2: Calculate the heating load
The heating load describes how much heat output a building needs under design conditions. For the room-by-room calculation, professionals in Germany apply the method of DIN EN 12831-1. The Bundesverband Wärmepumpe (BWP, the German Heat Pump Association) offers a heating load calculator based on the simplified method of the standard and makes clear that this rough calculation does not replace a detailed heating load calculation in line with the generally recognised rules of technology. So do not estimate the heating load from past consumption alone, but have a qualified professional calculate it.
Why this matters: the Verbraucherzentrale points out that a system that is too small will not heat sufficiently, and a system that is oversized will wear out faster.
Step 3: Choose output and design
The heating load is the basis for choosing the heating output. Hot water preparation and the design outdoor temperature are also factored in. Sizing also includes defining the bivalence point. From this outdoor temperature downward, the heating load is covered jointly by the heat pump and an electric backup heater. In modern units with inverter technology, the output can be adjusted continuously. According to the Verbraucherzentrale, this saves energy and runs more quietly.
| Criterion | What to look for |
|---|---|
| Heating output | Matched to the calculated heating load, neither significantly too small nor too large |
| Flow temperature | As low as possible; rule of thumb: below 55 degrees Celsius all year round |
| Power supply | Single-phase (230 V) or three-phase (400 V), depending on the unit and the house connection |
| Controls and connectivity | Interfaces, for example for photovoltaics and smart home (see our article Combining a heat pump and photovoltaics) |
| Refrigerant | According to the Verbraucherzentrale, some refrigerants have a high global warming potential; substances with a low potential are often natural, such as propane |
| Noise | Night or whisper mode, installation location (see below) |
Step 4: Plan storage and hydraulics
According to the Verbraucherzentrale, a buffer tank is not always necessary, but it is indispensable in hybrid heating systems. Intermediate storage causes losses; with inverter units, the heating pipework itself can be sufficient as storage. A hot water cylinder is normally used for domestic hot water. After installation, hydraulic balancing is important so that all heating circuits receive the correct flow. You will find suitable components in our categories Buffer tanks, Hot water cylinders and in the Building services section.
Step 5: Installation location and noise
The Verbraucherzentrale names a few principles for the installation location: air-source heat pumps can produce noise, newer units are quieter and often have a night or whisper mode. You should keep sufficient distance from bedrooms and neighbouring buildings. Lawns absorb sound, while corners between walls amplify it. The prevailing wind direction should not blow onto the air outlet. Possible noise protection measures include a vibration-damping base, a sound-insulating cover, noise barriers or hedges. According to the Verbraucherzentrale, no permit is required for air-source heat pumps in Germany; nevertheless, please clarify local rules on distances and noise with your installer or the municipality before ordering.
Step 6: Registration and operation
According to the Verbraucherzentrale, operation must be registered with the grid operator; a separate meter measures the heating electricity. Special heat pump tariffs can be cheaper; in return, interruptible tariffs are somewhat cheaper still. The seasonal performance factor (SPF, German: Jahresarbeitszahl, JAZ) serves as the efficiency indicator: it is the heat delivered divided by the electricity consumed over one year. The Verbraucherzentrale regards an SPF of 3 or more as efficient; air-source heat pumps usually reach values of 3 to 4. If the figure is conspicuously low, discuss it with your installer.
The right order for existing buildings
For existing buildings, the following order has proven its worth: first look at the heat demand and the insulation, then have the heating load calculated, then check radiators and surface heating for their flow temperatures, and only after that specify the heat pump together with the storage tanks. The Verbraucherzentrale recommends getting advice on the building's energy balance and the required heating output before commissioning a heating contractor. This helps you avoid a system that turns out too large after a later renovation. Also ask contractors to disclose their heating load assumptions in the quotation; this makes it easier to compare several quotations, which the Verbraucherzentrale also advises.
Cooling and hot water
According to the Verbraucherzentrale, reversible air-to-water heat pumps can actively cool by reversing the circuit. For cooling, the heat pump controller must provide dew point monitoring, and with underfloor heating the surface temperature must not fall below 20 degrees Celsius in cooling mode. If you want to cool, you should take this into account early on, when selecting the unit and planning the hydraulics.
Typical sources of error
- Radiators that are too small and force high flow temperatures.
- Inaccurate planning, for example an estimated rather than calculated heating load.
- A system that is not set up properly: the Verbraucherzentrale points out that the default settings after installation are often not optimal and should be readjusted.
- Missing hydraulic balancing after renovation work.
Quality seals and efficiency labels
According to the Verbraucherzentrale, the EHPA quality label stands for certain technical requirements, a 24-hour service and ten years of spare part availability. The EU heating label is only of limited help, according to the Verbraucherzentrale, since comparable units all reach the best efficiency class; information on power consumption and noise level can nevertheless be useful. Therefore compare units using the data sheets and their suitability for your building.
Which units can you find with us?
In the category Heat pumps you will find air-to-water units, storage tanks and further components from various manufacturers. We group Viessmann system components under Viessmann. Which unit suits your heating load depends on your building; for your selection, please gather your heating load, flow temperature and house connection details and talk to us.
Checklist for the conversation with the installer
- Was the heating load calculated room by room and the sizing documented?
- What flow temperature is planned for radiators and surface heating?
- Is hydraulic balancing included?
- Where will the outdoor unit be placed, and how was noise taken into account?
- What meter and tariff requirements does the grid operator impose?
- How will the hot water cylinder and, if applicable, the buffer tank be sized?
Frequently asked questions
Is previous fuel consumption enough for sizing?
It can serve as a reference point, but it does not replace a heating load calculation. The BWP stresses that a rough calculation does not replace a detailed heating load calculation in line with the generally recognised rules of technology.
Does a heat pump work with radiators?
Yes, as long as the flow temperature stays low enough. The Verbraucherzentrale gives a rule of thumb of below 55 degrees Celsius; radiators that are too small can be replaced.
Do I need a permit?
According to the Verbraucherzentrale, not for air-source heat pumps; for ground or groundwater use, the lower water authority (Untere Wasserbehörde) must be involved. You register operation with the grid operator.
Does a heat pump become less efficient in winter?
Yes. The Verbraucherzentrale points out that air-to-water heat pumps lose efficiency at very low outdoor temperatures. Good sizing takes this into account.
Sources
- Verbraucherzentrale: Wärmepumpe: Alles, was Sie wissen müssen (as of 14 April 2026)
- Verbraucherzentrale: Wärmepumpe im Eigenheim: Welche Voraussetzungen muss mein Haus erfüllen? (as of 25 March 2026)
- Verbraucherzentrale: Technik und Ausstattung: Was gehört alles zu einer Wärmepumpenanlage? (as of 5 January 2026)
- Verbraucherzentrale: Welche Arten von Wärmepumpen gibt es und wie funktionieren sie? (as of 23 February 2026)
- Bundesverband Wärmepumpe: Heizlastrechner
Would you like to discuss the selection of suitable components? We are happy to advise you by email: kontakt@energie24.shop. The heating load calculation and the planning of the system are carried out by your installer.