Air-to-Water Fan Coil Units: FAQs

Air-to-water fan coil units are becoming an increasingly popular solution for low-carbon heating and cooling, particularly when paired with air source heat pumps in residential and commercial buildings. Offering fast response times, flexible zoning, and reduced refrigerant within occupied spaces, air-to-water fan coil units provide an efficient alternative to traditional radiators and underfloor heating.

In this FAQ guide, Oceanair answers the most common questions around fan coil systems - covering how they work, efficiency, controls, compliance, and key fan coil system design and specification considerations - to help you determine whether they’re right for your project.

1. What is an air-to-water fan coil unit?

An air-to-water fan coil unit (FCU) uses heated or chilled water to heat or cool a space. The water is typically supplied by an air source heat pump, boiler, or chiller. A fan then draws room air across a water coil and distributes the newly conditioned air into the space. Hence the name “fan coil”.

Fan coils are available in a range of formats, including wall-mounted, floor-standing, ducted, and cassette units, allowing systems to be tailored to different building layouts and design requirements. This variety makes it easier to achieve the right balance of comfort, appearance, noise levels, and installation flexibility, whether the priority is visible room units, concealed solutions, or even air distribution. Having multiple fan coil options also helps ensure systems are correctly specified for each space, improving performance, efficiency, and overall user comfort.

2. How is an air-to-water fan coil different from an air-to-air system?

Air-to-water fan coil systems use heated or chilled water, rather than refrigerant, to provide heating and cooling, allowing a single central heat pump to serve multiple indoor units without running refrigerant pipework throughout the building. Because the heating and cooling is delivered by water, installers do not need to hold an F-Gas certification to install or maintain the fan coil units, while controls remain very similar to air-to-air systems. This approach makes zoning simpler, reduces refrigerant within occupied spaces, and is particularly advantageous in larger buildings or where flammable refrigerants are a consideration, helping projects meet UK safety and environmental requirements more easily.

3. Are air-to-water fan coils suitable for heat pump systems?

Yes. Heat pump compatible fan coils are well suited to low-temperature operation, particularly when sized correctly. Many current fan coil units (FCUs) are designed to deliver effective heating with flow temperatures as low as 35 - 45 °C.

4. Can air-to-water fan coils provide both heating and cooling?

Yes. When connected to a reversible heat pump or chiller, air-to-water fan coils can provide year-round heating and cooling, although most domestic air source heat pumps only enable cooling above certain outdoor temperatures set by the manufacturer's software, which cannot be adjusted.  However, for most projects this is not an issue, but if you require cooling in the winter, then you should speak with our specialists today, to help you find the best products for your project. 

To achieve optimal performance and user experience, we typically design systems from the outset of specification to allow automatic heating and cooling changeover, otherwise homeowners will be required to manually switch modes during operation. Fan coil systems can be configured as 2-pipe or 4-pipe, with 4-pipe systems offering faster changeover and, in some cases, simultaneous heating and cooling, depending on the overall system design and heat pump capacity. Because priorities such as hot water production, cooling demand, and heating vary between projects, we strongly recommend contacting Oceanair early for a detailed fan coil specification and system design breakdown, ensuring the correct products and controls are selected for your application.

5. What building types are air-to-water fan coils commonly used in?

Air-to-water fan coils are widely used in:

• Offices and commercial buildings 
• Hotels and apartments
• Schools and universities
• Healthcare environments 
• Fan coils for retrofit projects

They are very popular in both new build and refurbishment projects due to their versatility, and ability to deliver effective heating and cooling, more efficiently than traditional radiators and underfloor heating.

6. Are air-to-water fan coils suitable for retrofit projects?

Yes. Fan coils are frequently used as radiator replacement fan coils in retrofit applications, particularly where improved cooling or compatibility with heat pumps is required. Pipework sizing and electrical provisions need to be taken into consideration, which are calculated for you by Oceanair's specialised engineers.

7. How efficient are air-to-water fan coil systems?

System efficiency depends on the heat source, which controls are used, and operating temperatures. When paired with high-efficiency heat pumps and weather compensation, air-to-water fan coil systems can achieve excellent seasonal efficiencies and reduced carbon emissions. A win for both the environment and bank balances.

8. Are air-to-water fan coils noisy?

Modern fan coil units are built to run very quietly, using efficient fans that can slow down when full power isn’t needed, and sound-reducing materials inside the unit. Correct selection, sizing, and commissioning are essential to meet typical UK noise criteria for offices, hotels, and residential spaces.

Again, all of which are calculated and accounted for by Oceanair’s specialists, making the process much simpler for you.

9. Do air-to-water fan coils require regular maintenance?

Yes. Routine maintenance typically includes:

• Filter cleaning or replacement
• Coil inspection and cleaning
• Condensate tray and drain checks
• Fan and control inspection

Maintenance requirements are generally straightforward and familiar to HVAC service engineers.

10. How are air-to-water fan coils controlled?

Fan coils can be controlled via:

• Local wall thermostats
• BMS integration for fan coil systems 
• Room controllers with fan speed and valve control
• Smart or networked HVAC controls

Choosing the correct control system plays a key role in comfort, efficiency, and running costs and we are more than happy to advise on the best strategy for your needs and equipment.

11. Do air-to-water fan coils need condensate drainage?

Yes, when operating in cooling mode. Any fan coil providing cooling will produce condensate and must be connected to a suitable gravity or pumped drain system in line with best practice.

12. Can fan coils be used with underfloor heating?

Yes. Hybrid systems are often used, where underfloor heating keeps the building gently warm, and fan coil units are used to quickly heat or cool rooms when temperatures change, such as in spaces that get a lot of sun through windows.

13. Are air-to-water fan coils compliant with UK building regulations?

When correctly designed, fan coil systems can support Part L compliant heating systems, low-carbon strategies, and reduced refrigerant volume objectives. Final compliance depends on overall system design and controls.

14. What are the main advantages of fan coils?

There are many advantages to using fan coils. Some of the key benefits include:

• Compatibility with low-carbon heat pumps
• Reduced refrigerant in occupied spaces
• Fast heating and cooling response
• Lower running costs compared to radiators
• Flexible zoning and control
• Suitability for new build and retrofit

15. What should be considered when specifying air-to-water fan coils?

When assessing potential projects, Oceanair’s engineers consider and calculate the following:

• Heat pump flow temperatures
• Noise criteria
• Available pipework routes
• Condensate management
• Controls integration
• Maintenance access

These calculations, and more, are critical to achieving expected performance, and Oceanair’s experience across a wide range of projects helps ensure systems are specified correctly to meet - and often exceed - performance requirements.

Speak to our HVAC specialists to ensure your system is designed correctly from the outset.