What Fan Coil Capacity Do I Need for My Room
What fan coil capacity do you actually need for your room?
This is probably the most important question you need to ask before choosing a specific fan coil model. Experience from practice speaks clearly: an undersized fan coil will not reach the desired temperature even at full capacity, while an oversized one unnecessarily switches between heating and cooling cycles, creates uncomfortable air currents, and consumes more energy than necessary. Correct sizing is therefore key – and it's not as difficult as it may seem at first glance, if you know what to look for.
In this article, we will show you how to calculate the capacity of a fan coil, what influences it, what approximate values are for different types of rooms, and how to avoid the most common mistakes when choosing. If you are also interested in what type of fan coil to choose (built-in vs. wall-mounted, number of pipes, etc.), see also the related articles How to choose a fan coil – capacity, type of mounting and number of pipes and Built-in vs. wall-mounted fan coils – which solution is suitable for my project.
What exactly does the capacity of a fan coil express?
A fan coil is a device that transfers heat (or cooling) from water to air in a room using a heat exchanger and a fan. Its capacity is given in kilowatts (kW) and expresses how much thermal energy the device can deliver or remove from a space per unit of time.
An important detail that many overlook: a fan coil usually has two different capacities – heating and cooling. These values are NOT identical. A typical fan coil may have a heating capacity of 3.5 kW and a cooling capacity of 2.8 kW at the same dimensions. Why? Because the physical processes of heat delivery and heat removal are not symmetrical – they depend on the temperature of the incoming water, air humidity, room temperature, and other factors.
The capacity of a fan coil also varies according to the speed of the fan. Manufacturers specify the capacity at maximum speed (high), medium (med), and low (low). For long-term comfortable operation, the value at medium speed is usually used, as maximum speed generates noise that is unpleasant during continuous operation.
Room heat loss – the basis of every calculation
The capacity of a fan coil must cover the heat loss of the room – that is, the amount of heat the room loses per hour at a given temperature difference between the indoor and outdoor environment. An accurate calculation of heat loss according to the standard STN EN 12831 is done by a designer, but for approximate sizing, you can use the so-called approximate method.
Approximate method of heat loss calculation
The basic formula is simple:
Q [W] = V [m³] × ΔT [K] × k
where:
- V = room volume (m³) = area × ceiling height
- ΔT = difference between the desired indoor and outdoor design temperature (in °K, which is numerically the same as °C)
- k = heat loss coefficient depending on the quality of the building insulation
Values of the coefficient k according to the type of building:
| Type of building / insulation | Coefficient k | Typical example |
|---|---|---|
| Old building without insulation | 0.9 – 1.2 | Panel building before reconstruction |
| Standard building, partial insulation | 0.6 – 0.9 | House from the 90s, new windows |
| Well-insulated building (modern construction) | 0.4 – 0.6 | New building with EPS 150mm |
| Low-energy / passive building | 0.2 – 0.4 | Passive house, certified |
Practical calculation example
Imagine an office room with dimensions 6 × 4 meters, ceiling height 2.7 m, in a modern office building from 2010 with average insulation. The design outdoor temperature in Slovakia is standardly −15 °C, the desired indoor temperature is 21 °C.
- Room volume: 6 × 4 × 2.7 = 64.8 m³
- ΔT: 21 − (−15) = 36 K
- Coefficient k: 0.7 (average insulation)
- Heat loss: 64.8 × 36 × 0.7 = 1 633 W ≈ 1.6 kW
For this room, you therefore need a fan coil with a heating capacity of at least 1.6 kW. With a reserve (usually 20–30 %), you will choose a fan coil with a capacity of around 2.0 kW. This reserve is important – real conditions differ from calculated ones, the room sometimes cools down faster when the corridor is open, more people bring in humidity, etc.
What affects the required fan coil capacity?
The room volume and heat loss are just the basics. In practice, there is a whole range of factors that change the result, and an experienced designer or salesperson will never omit them.
1. Room orientation and solar gains
Rooms facing south and west have significant solar gains in summer – the sun heats up through the windows and increases the thermal load that the fan coil must handle during cooling. Conversely, a room facing north has smaller solar gains but loses more heat in winter. For cooling capacity, southern rooms with large glazed areas are calculated with an increase in capacity coefficient of up to 20–30 %.
2. Number of people and heat load from equipment
Each person produces about 80–120 W of heat at rest (up to 200 W during physical work). In an office room with 10 employees and their computers, the human and electronic load can easily add 1,500 – 2,000 W. This is a decisive factor for commercial and administrative spaces. For cooling, you therefore need a significantly higher capacity than for heating alone.
3. Inlet water temperature and system type
Fancoils operate most efficiently at certain inlet water temperatures. Manufacturers state capacity under standard conditions, for example:
- Heating: inlet water 45°C / outlet water 40°C / room temperature 20°C
- Cooling: inlet water 7°C / outlet water 12°C / room temperature 27°C, relative humidity 50%
If your system operates at different temperatures (e.g. a heat pump with low-temperature heating at 35–40°C), the actual fan coil capacity will be lower than stated in the catalog. This is a very common mistake in designing systems with heat pumps – the fan coil seems undersized, but the problem is the lower water temperature.
4. Ceiling height and type of space
Ceiling height significantly affects the volume of air that needs to be heated or cooled. Standard residential rooms have ceilings of 2.4–2.7 m. Commercial premises, restaurants or production halls may have ceilings of 3.5–5 m, which dramatically increases the required capacity. In addition, with large ceiling heights, there is a problem with stratification – heat rises and the room at ground level remains cold.
5. Thermal bridges and construction defects
Corner rooms, corner apartments, spaces above garages or under un-insulated roofs have significantly higher heat loss coefficients. Such rooms must be calculated with a higher k coefficient – sometimes as much as 30–50 % higher than would correspond to a standard insulated building.
Approximate capacity tables for typical rooms
The following tables serve for quick orientation in practice. They are based on experience and are intended as a first estimate – not as a substitute for a design calculation. They calculate with a design outdoor temperature of −15 °C, indoor temperature of 20–22 °C, standard insulation and ceiling height of 2.6 m.
Heating capacity – residential spaces
| Room type | Area | Old building | New building |
|---|---|---|---|
| Children's room | 12–15 m² | 1.2–1.6 kW | 0.7–1.0 kW |
| Bedroom | 15–20 m² | 1.5–2.2 kW | 0.9–1.4 kW |
| Living room | 25–35 m² | 2.5–4.0 kW | 1.5–2.5 kW |
| Kitchen with dining area | 20–28 m² | 2.0–3.0 kW | 1.2–1.8 kW |
| Corridor, entrance hall | 8–12 m² | 0.8–1.2 kW | 0.4–0.8 kW |
Capacity for commercial and administrative spaces
| Space type | Area | Heating | Cooling |
|---|---|---|---|
| Office (2–4 people) | 20–30 m² | 1.5–2.5 kW | 2.0–3.5 kW |
| Open space office | 60–100 m² | 4.0–7.0 kW | 6.0–12.0 kW |
| Meeting room / classroom | 30–50 m² | 2.0–3.5 kW | 4.0–8.0 kW |
| Hotel room | 20–30 m² | 1.5–2.5 kW | 1.8–3.0 kW |
| Restaurant, café | 80–150 m² | 5.0–10.0 kW | 8.0–18.0 kW |
| Shop, retail store | 50–100 m² | 3.5–7.0 kW | 5.0–10.0 kW |
Note that in commercial spaces, the cooling capacity is significantly higher than the heating capacity. This is due to internal heat gains from people, computers, lighting, and kitchen appliances.
How to work with fan coil technical data sheets – reading performance tables
Every serious fan coil has a technical data sheet with a performance table under various conditions. Learning how to read it is a skill that will save you from many problems.
A typical heating performance table looks like this (example for one model):
| Water inlet temperature [°C] | Room air temperature [°C] | LOW capacity [kW] | MED capacity [kW] | HIGH capacity [kW] |
|---|---|---|---|---|
| 50 | 20 | 1.9 | 2.7 | 3.4 |
| 45 | 20 | 1.6 | 2.3 | 2.9 |
| 40 | 20 | 1.2 | 1.8 | 2.2 |
| 35 | 20 | 0.8 | 1.2 | 1.5 |
From the table it is clear that the same fan coil at a water temperature of 35 °C (typical for a heat pump) achieves only 1.5 kW at HIGH speed – whereas at 50 °C it achieves 3.4 kW. The difference is dramatic – more than double. Therefore, when designing systems with heat pumps, it is essential to always work with values under low-temperature conditions, otherwise you can easily make a mistake by a factor of 2.
One fan coil or multiple – how to cover a larger space?
If you have a space with a larger area or a complex shape (e.g. L-shape, open-plan space with a kitchen), you basically have two options:
- One more powerful fan coil – simpler, cheaper to install, but air distribution may be uneven and comfort may be poor in remote corners of the space.
- Two or more fan coils of lower capacity – better air distribution, more even temperature, quieter operation (each runs at a lower speed), but higher cost and more complex installation and hydraulics.
In practice, for residential spaces up to 30 m², one fan coil is sufficient. For spaces of 30–60 m², it depends on the shape – in an open rectangular space, one powerful fan coil may be sufficient, while in an L-shape or a room with alcoves and a segmented ceiling, two smaller units are usually a better choice.
For commercial and hotel spaces, individual fan coils are typically designed for each zone (room), which allows for separate regulation according to actual needs and occupancy. In open-plan layouts (shops, restaurants), multiple fan coils are placed symmetrically to ensure even distribution.
Noise – a hidden parameter related to performance
Many people consider only performance and price, but in practice, fan coil noise is equally important – especially in bedrooms, hotel rooms, living rooms, and offices. Fan coil noise mainly depends on the fan speed and is directly related to how much you have oversized the fan coil.
If you choose a fan coil with performance just at the edge of the requirement, it will have to operate at full speed (HIGH), which generates significantly higher noise – typically 42–48 dB(A). On the contrary, with an oversized fan coil (performance 30–40 % higher than the strict minimum), the device usually operates at medium or low speed and the noise drops to 28–35 dB(A), which is already a comfortable level.
For bedrooms and hotel rooms, it is therefore recommended to dimension the fan coil so that at medium speed (MED) it covers the full thermal/cooling demand. This practically means a performance 20–40 % higher than the calculated loss. This is called a "quiet" design of a fan coil system.
Modern DC INVERTER fan coils with smooth speed control elegantly solve this issue – read more about it in the article Regulation of fan coils – from a simple switch to smooth DC INVERTER regulation.
Specific situations from practice – examples of customer orders
Case 1: Corner apartment in a panel building
The customer had a corner apartment in a panel building from 1978, third floor. Living room 28 m², two external walls, large window oriented to the northwest. No window replacement and no insulation. Calculated heat loss: 3.6 kW. We selected a fan coil with a heating capacity of 4.5 kW (at water temperature 50 °C), which after installation operates at medium speed even at -10 °C outside. At extreme cold below -15 °C, it needs maximum speed.
Case 2: Modern office with large windows
Office in a new building, southern façade, glazing 60 % of the wall area, 35 m², 8 workstations. Heat loss for heating: only 1.8 kW (new building). But thermal load for cooling: 8 people × 100 W = 800 W, 8 computers × 150 W = 1 200 W, lighting 200 W, solar gains through the south 2 000 W. Total cooling load: 5.2 kW. In this case, the cooling capacity is three times higher than the heating capacity – a common situation in modern offices.
Case 3: Hotel project, 24 rooms
Four-pipe fan coil system in a new boutique hotel, each room 22–26 m². For guest comfort, quietness was key – maximum 32 dB(A) at night. We selected fan coils with a heating capacity of 2.5 kW and cooling capacity of 2.1 kW for rooms of 22 m², which at medium speed covered the thermal losses with a reserve. At night, the thermostat limits the maximum speed to LOW, achieving 28 dB(A). A complete overview of connection options can be found in the article Two-pipe and four-pipe fan coil connections – the difference and when to use which.
Case 4: Reconstruction of a family house, heat pump
House from 1985, partially insulated, fan coils as a replacement for radiators in combination with an air-to-water heat pump. Design water temperature 40/35 °C. Calculated loss for the living room 35 m²: 3.2 kW. According to the catalog, a fan coil with a "declared" capacity of 3.0 kW at 45 °C – but at 40 °C, the actual capacity is only 2.4 kW. Result: the fan coil was insufficient. We recommended a model with a capacity of 4.5 kW at 45 °C, which at 40 °C provides real 3.6 kW with sufficient reserve.
When and how to include a reserve in the calculation?
Reserve is often talked about, but rarely explained how to work with it specifically. Here are practical rules:
- +20 % reserve: New buildings, well-insulated buildings with a precise project, average orientation, low-temperature heat pump → always choose a fan coil one step higher
- +25–30 % reserve: Older house without major reconstruction, corner location, large windows, southwest orientation, heating with large fluctuations (e.g., wood stove with accumulator)
- +35–40 % reserve (for cooling): Commercial spaces with high occupancy, kitchens, server rooms, spaces with intense lighting → thermal load is hard to predict and can be higher than it seems
- No reserve: Only for spaces with constant thermal load, precise project documentation and verified real values (e.g., during reconstruction of an existing system with measured values)
Accessories and installation details that affect the real performance
The performance of a fan coil depends not only on the device itself, but also on how it is installed and what accessories are used. In practice, we have seen cases where incorrect installation reduced the real performance by 15–25 % compared to the catalog.
With built-in fancoils, the performance depends on proper air intake. If the intake grid is too small, too dirty, or the air has no free access, the fan operates "against resistance" and the performance drops. For proper suction function, for example, the Intake Kit for IVAR.SLI DC 400 is used, which ensures optimal air intake into the device. Similarly, the Intake Grid for IVAR.SLI DC 600 is dimensioned to match the flow capacity of the respective model.
For projects where the fancoil is installed in a drywall ceiling or in a masonry housing, the Installation Box for Air Conditioners and Fancoils is used. A properly dimensioned box ensures that the fancoil has sufficient air circulation around it and does not operate in a "stuffy" space, which would reduce its performance and lifespan.
For devices where air quality is important (e.g., healthcare facilities, allergy-sensitive rooms, or spaces with vulnerable individuals), the UVC Lamp for IVAR.SL, SLI 400 – L=475mm is available. This lamp eliminates microorganisms on the heat exchanger and improves the hygienic quality of the air without affecting the thermal performance of the device.
In order for the fancoil to maintain performance at an optimal level depending on actual demand, it requires quality control. The Control for IVAR.SL, SLS – Built-in, 4-speed, 230V allows smooth adjustment of performance according to the current room temperature, thus saving energy and extending the device's lifespan.
More about the correct installation and commissioning of a fancoil can be found in the article "Fancoil Installation – Procedure, Connection and Commissioning," which also discusses hydraulic connection and flow setting.
Most Common Mistakes When Selecting Fancoil Capacity
After years of working with fancoil systems, we have identified several recurring mistakes that customers and even some installers make:
- Selecting based on m² without considering insulation: A blanket calculation of "50 W per m²" applies only to a certain type of building. For an old house, it could be 120 W/m², for a passive house 20 W/m². Thermal losses must always be considered.
- Ignoring the supply water temperature: The most common mistake when combining with a heat pump. The catalog performance is at 45 °C, but the heat pump operates at 35–40 °C.
- Neglecting cooling capacity in commercial spaces: Cooling capacity is crucial in offices and retail spaces, yet it is often underestimated by customers.
- Selecting based on the lowest price: A cheaper fancoil with lower performance will operate at maximum speed, will be noisy, and will have a shorter lifespan.
- Ignoring installation conditions: A fancoil in a too tight housing without proper air intake loses performance.
- Not considering the number of people and equipment: The actual load in an office can be double the theoretical thermal loss.
Practical Steps – How to Proceed When Selecting Capacity
Let's summarize the procedure into simple steps that everyone should follow,
Do you have a question on this topic?
Not sure how to decide or dealing with a specific situation in your home? Write to us – we are happy to help.