What Heat Pump Output Do I Need
What Heat Pump Output Do I Need
The question "what output heat pump should I buy" is probably the most common question anyone considering switching to a heat pump asks themselves. At first glance the answer seems simple - just look at the house's floor area and multiply by some number. In practice it isn't that simple, and an incorrect output estimate is exactly one of the most common causes of owner dissatisfaction with a newly installed heat pump. An undersized unit can't hold the temperature during frost; an oversized one, conversely, keeps switching on and off, which shortens its lifespan and increases electricity consumption. In this article we'll go step by step through how a heat pump's output is correctly calculated, what factors affect it, and we'll also show a specific calculation example for a real house.
Why "house area times some number" isn't enough
Many people rely on a simple rule of thumb like "1 kW per 10 m²" when estimating output. This rule can work approximately in some cases, but it's misleading because it completely ignores the most important factor - the building's heat loss. Two houses with the same 120 m² area can have completely different output needs if one was built in the 1980s without additional insulation and the other is a modern new build with a thick layer of insulation, triple glazing, and heat recovery ventilation. The output difference between them can be as much as sixfold.
The real, technically correct approach to choosing a heat pump's output is based on calculating the building's heat loss (in technical terminology, a heat loss calculation according to standard STN EN 12831), to which domestic hot water (DHW) preparation and an appropriate margin are then added. Let's break this process down step by step.
What house heat loss is and why it's the basis of the whole calculation
Heat loss is the amount of heat a house loses to its surroundings per unit of time, at a given difference between indoor and outdoor temperature. It's expressed in kilowatts (kW), and it's exactly this output that the house's heating system must continuously deliver to maintain the desired indoor temperature (usually 20-21 °C) even on the coldest day of the year.
Heat loss depends on several key factors:
- Area and volume of the building - a bigger house logically has a bigger loss, but it isn't a linear relationship, since the building's shape also matters (a compact cube loses less heat than a rambling house with many corners and recesses).
- Quality of insulation of the envelope, roof, and floor - this is by far the most significant factor. The difference between an uninsulated house and a well-insulated house of the same area can represent a 3 to 6-fold difference in heat loss.
- Type and condition of windows - old wooden windows with single or double glazing have several times worse thermal insulation properties than modern plastic or wooden windows with good triple glazing.
- Ventilation - air infiltration through leaks as well as controlled ventilation (especially without heat recovery) represent a significant, often underestimated component of heat loss.
- The design outdoor temperature for the location - Slovakia isn't climatically homogeneous. While Bratislava and southern Slovakia use a design outdoor temperature of around -11 to -12 °C, central Slovakia commonly uses -15 °C, and the mountainous areas of Orava, Kysuce, or Liptov use as low as -18 °C. The lower the design temperature, the higher the required heat loss coverage and therefore the unit's output.
An exact heat loss calculation is a fairly complex process that accounts for the heat transfer coefficients of individual structures (walls, roof, floor, windows, doors), their areas and orientation, and is usually carried out by a designer or an experienced installer using specialized software. For a rough estimate, however, we can also use simplified specific heat loss values per square metre of floor area, which are commonly used in professional practice.
Indicative table - how many kW a house needs by insulation and area
The table below shows indicative heat loss (and therefore the required heat source output) for various insulation categories and house sizes. This is a simplified estimate suitable for initial orientation - for a final decision, it's always advisable to have an exact calculation done.
| House type / insulation | Specific loss (W/m²) | 80 m² | 120 m² | 150 m² | 200 m² |
|---|---|---|---|---|---|
| Older uninsulated house (before 1980) | 120 W/m² | 9.6 kW | 14.4 kW | 18.0 kW | 24.0 kW |
| Partially insulated house (built approx. 1980-2000) | 85 W/m² | 6.8 kW | 10.2 kW | 12.75 kW | 17.0 kW |
| House insulated to today's standards (new build after 2015) | 50 W/m² | 4.0 kW | 6.0 kW | 7.5 kW | 10.0 kW |
| Low-energy to passive house | 20 W/m² | 1.6 kW | 2.4 kW | 3.0 kW | 4.0 kW |
The table shows why the topic of heat pump output is so sensitive - a house of the same size can need 2.4 kW or 14.4 kW depending on its condition. That's why any general recommendation like "an 8 kW pump is enough for 120 m²" is essentially worthless without knowing the condition of the house.
To make it clearer, let's also show these four values for a 120 m² house graphically:
Domestic hot water (DHW) preparation - don't forget to add it
The building's heat loss only covers heating, but most households also need to heat water from the heat pump for showering, bathing, and cooking. DHW preparation is usually handled via a tank heater (boiler), which acts as a buffer - the heat pump heats it gradually even outside peak demand, so the output doesn't need to be sized for instantaneous peak hot water demand (for example several people showering at once), but rather for average daily need.
For a typical 4-person household, an average additional output of roughly 1 to 1.5 kW is commonly assumed for DHW preparation (spread over a sufficiently large tank, typically 200-300 litres), so the needed water supply can be heated over the course of the day. For larger households, or if a jacuzzi or frequent bathing is planned, this value can increase.
Margin and safety factor
Even after an exact heat loss calculation and adding DHW, it's advisable in practice to add a margin of roughly 10 to 15% to the resulting figure. This margin covers:
- inaccuracies in the calculation (estimating heat transfer coefficients for older buildings without exact documentation is always only approximate),
- possible future changes - for example an extension, glazing in a terrace, converting a room from unheated to heated,
- extreme weather situations that exceed the statistical design temperature,
- faster recovery after a possible heating interruption (for example after a longer absence in winter).
However, the margin shouldn't be disproportionately high - oversizing brings its own problems, which we discuss in a separate section below.
Calculation procedure step by step
Let's now summarize the whole process into a clear diagram that anyone preparing to choose a heat pump with their supplier or designer can use:
Why the "at +7 °C" rating plate output isn't enough
This is one of the most common sources of error when choosing a heat pump. Manufacturers most often state the rated (nominal) output in catalogues measured at an outdoor temperature of +7 °C and heating water at 35 °C (A7/W35 conditions). This number looks nicely high, but it says nothing about what output the unit will actually deliver exactly when you need it most - during frost around -10 to -15 °C.
It's a physical property of air-water heat pumps that both their output and efficiency (COP) fall as the outdoor temperature drops, because the unit has to draw heat from increasingly colder and "leaner" air. While at +7 °C the pump might have an output of, say, 9 kW, at -12 °C it might be only 6 kW - a drop of a third. That's exactly why, when choosing, it's essential to look at the output curve (a graph of output versus outdoor temperature) at the design temperature of your location, not at a single marketing number from the catalogue.
The bivalent point - a key concept to understand
It's precisely because of this drop in output at low temperatures that the concept of the bivalent point is used when designing a system. It's the outdoor temperature below which the heat pump alone can no longer cover the house's full current heat loss, and a supplementary (bivalent) heat source has to be brought into operation - most often an electric heating element built directly into the heat pump, or an existing boiler in a hybrid solution.
Let's illustrate this with a specific example using the figures given above. Imagine a house with a heat loss of 7.5 kW at a design temperature of -12 °C (corresponding to the "house insulated to today's standards" category with a 150 m² area from our table). We chose a heat pump whose output is 9 kW at +7 °C, but which drops to 6 kW at -12 °C.
How these two curves - the house's heat demand and the pump's output - develop as temperature falls, and where they intersect, is shown in the following graph:
The graph shows that at higher outdoor temperatures (on the right), the heat pump's output is significantly higher than the house's current demand - the pump therefore only works at part of its output and runs in an economical mode most of the time. As temperature drops, heat demand rises while the pump's output falls, until around -6 to -8 °C the two curves intersect - that's the bivalent point. Below this temperature, the pump alone can no longer keep up, and a supplementary source (most often an electric heating element) briefly comes into operation to make up the missing difference - in our example, at -12 °C this is a difference of roughly 1.5 kW (7.5 kW demand minus 6 kW pump output).
This isn't a design flaw, but common and economically sensible practice - so-called bivalent (not monovalent) operation. The reason is simple: temperatures below the bivalent point occur only a few hours a year over the heating season (in Slovakia, generally on the order of a few dozen to a hundred hours a year), so it would be uneconomical and a poor investment to buy a substantially bigger and more expensive heat pump just to cover these brief peaks. The supplementary source thus makes up the missing output only during a small part of the year, while the heat pump itself covers the vast majority of the heating season (typically 90-98% of annual heat demand) in a highly efficient mode.
Monovalent operation - when it makes sense
In monovalent operation, the heat pump is designed to cover 100% of the heat loss even at the lowest design temperature without any supplementary source. This requires a substantially bigger (and more expensive) unit output, which is unused for most of the year. Monovalent operation is worthwhile especially in well-insulated houses with low heat loss, where even a smaller pump can cover the peak, or where the investor, for whatever reason, doesn't want any supplementary source in the system (for example due to fully electrifying the household without a gas or other connection).
What happens if you choose the output incorrectly
An incorrect output estimate shows up in two different ways depending on which direction the supplier got it wrong.
Undersized heat pump
If the unit's output is too low relative to the house's real heat loss, it shows up especially during the coldest days of the year - the desired indoor temperature won't be achievable, the supplementary (bivalent) heat source will run far more often and longer than planned, which significantly increases electricity consumption (an electric heating element has a COP of 1, i.e. it's several times less efficient than the heat pump itself). In the worst case, the house simply can't be adequately heated during frost.
Oversized heat pump
A less obvious, but equally real problem is the opposite extreme - the unit's output is unnecessarily high relative to the house's needs. The heat pump then runs at only a fraction of its output for most of the heating season, leading to so-called cycling - the compressor frequently switching on and off at short intervals instead of running smoothly for longer stretches. Cycling has several negative consequences: it increases mechanical wear on the compressor and shortens its lifespan, reduces the unit's real (not catalogue) efficiency, causes bigger swings in room temperature, and last but not least, means an unnecessarily higher upfront investment in a bigger, more expensive unit than was needed.
The overview below summarizes the consequences of both extremes compared with a correctly designed system:
A specific calculation example for a real house
To make the whole process clear, let's go through it with one specific, simplified example.
Given: A family house with a floor area of 150 m², built in 2018 to current insulation standards (corresponding to the "house insulated to today's standards" category from the table above), located in an area with a design outdoor temperature of -12 °C, home to a 4-person family.
Step 1 - heat loss: According to the table, at a specific loss of 50 W/m² and an area of 150 m², the heat loss comes to 7.5 kW.
Step 2 - DHW preparation: For a 4-person household we add an average output of 1.2 kW (combined with a sufficiently large tank, e.g. 250 litres). Total: 7.5 + 1.2 = 8.7 kW.
Step 3 - margin: We add a 12% margin: 8.7 × 1.12 = 9.74 kW, so rounded, we're looking for a unit with an output of around 9.5 to 10 kW at the design temperature of -12 °C.
Step 4 - comparison with the output curve: When choosing a specific model, it's important to look at the output declared at -12 °C (or the nearest lower value in the manufacturer's catalogue table), not the rating plate output at +7 °C, which can be as much as a third higher.
Step 5 - type of operation: With this output design, the system will work in a slightly bivalent mode with a bivalent point somewhere around -6 to -9 °C, which is a common and economically advantageous solution for this location - the supplementary source (built-in electric heating element) only activates during a relatively short part of the year.
This same process can be applied to any house - you just need to know, or have calculated, the real heat loss, add DHW and an appropriate margin, and compare the resulting figure with the specific unit's output curve at the design temperature of your location.
Specifics by type of heating system
Besides the output itself, it's important to also consider what heating system the heat pump will be connected to. Heat pumps achieve the highest efficiency with low-temperature systems (underfloor, wall heating) with a heating water temperature of around 30-40 °C. When connected to older radiator systems designed for higher temperatures (60-70 °C), it may be necessary either to increase the radiator surface area (replacing with larger or low-temperature radiators), or to accept that the pump will have to work at a higher outlet temperature, which reduces its COP and usually requires a slightly higher installed output to compensate for the lower efficiency at that temperature spread. We cover this topic in more detail in a separate article on combining a heat pump with underfloor heating or radiators.
Why have the calculation done by a professional
The values and process given in this article serve to help you understand the principle and for initial orientation before deciding. A real, binding heat loss calculation (a so-called heat loss calculation or energy assessment) should always be carried out before buying a heat pump by an experienced designer or installation company who knows the exact dimensions, orientation, construction makeup, and condition of your specific house. Simplified tables of specific loss per m² are only average values for a given category - your specific house can deviate from the average in either direction, for example due to large glazed areas facing north, leaky old windows, or, conversely, a good-quality subsequent renovation.
A serious heat pump supplier should always ask for at least basic information about the house (year of construction, thickness and type of insulation, window type, area, ceiling height, location) before offering a specific output, and ideally also carry out or request a heat loss calculation. If someone offers you a specific heat pump model and output without asking these questions, based purely on the house's floor area over the phone, it's wise to be cautious.
Summary
The correct heat pump output can't be estimated just from the house's floor area - the key is to know the building's real heat loss, which depends mainly on the quality of insulation and the design outdoor temperature of the location. To the heat loss, add the output for hot water preparation and an appropriate 10-15% margin; compare the resulting figure with the specific unit's output curve at the lowest design temperature (not with the rating plate output at +7 °C); and finally decide whether the system will operate monovalently or bivalently with a supplementary source below the bivalent point. Both undersizing and oversizing bring real operational problems, so it's worth investing time (and ideally also money in an energy assessment) in a correct calculation before buying the unit.
Frequently Asked Questions
Is the indicative table by house area enough, or do I need an exact calculation done?
The indicative table is a good starting point for an initial idea and comparing offers, but before a binding purchase of a specific unit, it's always advisable to have an exact heat loss calculation done tailored to your house. Differences between the average house in a given category and your specific house can be tens of percent, especially for atypical building shapes, large glazed areas, or incomplete insulation.
Is it better to choose a bigger output "just to be safe"?
No, oversizing brings its own problems - frequent cycling (switching on and off) of the compressor, which reduces the unit's real efficiency and lifespan, plus an unnecessarily higher upfront investment. A correctly sized system runs in long, smooth stretches and is more efficient and quieter.
What does it mean that a heat pump covers "90-98% of heat demand" even though it's smaller below the bivalent point than would theoretically be needed?
This is the difference between peak output and year-round energy demand. Extremely low temperatures below the bivalent point occur only a few hours a year during the heating season, so even though the supplementary source makes up the missing output at those moments, in terms of total annual energy consumption it's a small share. The heat pump therefore works alone in a highly efficient mode for most of the heating season.
How do I know the design outdoor temperature for my location?
Slovakia is divided into several climate zones in terms of design temperatures - roughly from around -11 to -12 °C in the south and lower-lying areas (for example around Bratislava), through -15 °C in central Slovakia, up to -18 °C in the mountainous areas of Orava, Kysuce, and Liptov. A designer or installation company can confirm the exact value for your municipality.
Does the type of heating system (radiators vs. underfloor heating) affect the required heat pump output?
It doesn't directly affect the size of the house's heat loss, but it does affect the efficiency with which the heat pump covers that loss. Underfloor and wall heating work with a lower heating water temperature (around 30-40 °C), at which heat pumps have a higher COP. Older radiator systems designed for higher temperatures (60-70 °C) may require either modification (larger radiators) or the pump operating at lower efficiency, which in practice is addressed with a slightly higher installed output.
What if the output turns out to have been chosen incorrectly only after installation?
With a mild mismatch, adjusting parameters can partly help (for example the weather compensation curve, the activation temperature of the supplementary source); with more significant undersizing or oversizing, correction is more complicated and often means needing to add to or replace the unit - which is why it's worth spending enough time on a thorough calculation before buying.
Related topics
- How to choose a heat pump
- Heat pump air-water vs. ground-water
- Heat pump with underfloor heating or radiators
- Heat pump installation - what you need to know
You can find the full range in the main category Heat pumps.
Have a question about heat pump output?
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