Heat Pump Defrosting: Underfloor Heating vs Fan Coils
When an air source heat pump stops delivering heat to defrost its outdoor coil, what happens indoors depends on more than the outdoor unit. The heating terminal, its operating water temperature and how much heat the system stores all shape what occupants feel.
That matters when you specify a heat pump for a damp, cold climate. A fan coil unit and a hydronic radiant floor can be connected to the same heat pump and still react differently to a pause in heat supply. Nominal heating capacity alone will not show that.
A 2026 study in Building and Environment compared the two terminal types under controlled frosting conditions. The results point to a question contractors and buyers should ask at the design stage: what happens on the room side while the heat pump defrosts?
Why Defrosting Affects a Hydronic Heating System
Frost can form on an air source heat pump's outdoor coil in cold, humid weather. As it accumulates, the unit needs to clear the coil to maintain operation. Reverse-cycle defrost interrupts normal heating, and heat may be drawn from the circulating water to melt the frost.
Supply and return water temperatures can shift during defrost. Whether anyone notices indoors depends partly on the terminal and the rest of the hydraulic system.
A radiant floor has considerable thermal mass. A fan coil transfers heat to room air more directly and responds faster when water temperature changes. That speed helps room control, but it also makes the fan coil's behaviour during defrost worth checking. Actual results depend on fan controls, water volume, any thermal storage and the defrost sequence.
What Did the Terminal Comparison Find?
The researchers ran one heat pump in an environmental chamber, operating it separately with fan coils and radiant floors. Both modes were tested at an outdoor condition of 2°C dry-bulb and 1°C wet-bulb. There was no buffer tank, so the comparison excluded that additional heat store.
Across two complete frosting–defrosting cycles, the radiant-floor mode averaged a COP of 2.91, compared with 2.40 for the fan-coil mode: a 21.3% difference in this experimental system.
Defrosting caused supply and return water temperature fluctuations in both modes, with larger fluctuations in the fan-coil mode. The same rooms held steadier indoor temperatures when heated by the radiant floors; in fan-coil mode, they showed a larger indoor temperature response.
This does not establish that every radiant-floor installation will use 21.3% less electricity than a fan-coil installation. The study used one experimental heat pump, fixed operating conditions and two consecutive cycles. A real project has its own weather, controls, water volume, possible buffer storage and terminal sizing. The figure is a result from that test, not a seasonal efficiency rating or a MENRED product claim.
Does That Mean Radiant Floors Are Always the Better Choice?
No. The two terminals serve different project needs.
Hydronic underfloor heating suits spaces where steady heating and a suitable low-temperature water design are priorities. Its thermal mass can soften the immediate response to a brief interruption, but it also makes the floor slower to respond to a setpoint change.
Fan coils respond faster and can suit projects that need quicker room control or heating and cooling from the same terminal. Their performance depends on selection for the available water temperature and on how the fans operate during defrost.
Some projects use both. The designer then needs to account for different water-temperature and flow requirements, with suitable zoning, mixing and hydraulic arrangements where needed. Treat the heat pump, terminals and controls as one system.
What Should the Project Specification Say About Defrost?
For a heat pump paired with underfloor heating, fan coils or both, the proposal should make these decisions visible:
| Design item | Question to resolve |
|---|---|
| Heating terminals | Which rooms use radiant floors, fan coils or another terminal? |
| Design water temperature | What supply and return temperatures does each terminal require at the design heating load? |
| Terminal output | Can each terminal meet its room load at those water temperatures? |
| Water volume and storage | Is the available system volume sufficient for the selected heat pump's defrost requirements? Is a buffer tank or another arrangement specified? |
| Defrost operation | What happens to water circulation, fan-coil fans and room heat delivery during defrost? |
| Controls and handover | How will the installer check the defrost sequence and record room and water-temperature behaviour? |
A buffer tank can be part of a defrost strategy, but whether one is needed, its size and its connection arrangement depend on the selected heat pump's requirements and the hydraulic design. Floor thermal mass is likewise no substitute for checking the unit's minimum water volume and operating limits.
For buyers, these questions make quotations easier to compare. Two proposals can list the same heat pump capacity and still differ widely in terminal selection, design water temperature and defrost arrangements.
What This Means for MENRED Heat Pump Projects
The MENRED MGC DC inverter air source heat pump range covers 10–44 kW models for hydronic applications, including underfloor heating and fan-coil systems. The published chamber study did not test a MENRED unit, so its COP figures are not MENRED performance data. Model selection must use the relevant product data and the project's design conditions.
For a project in a damp, cold region, specify the heat pump together with its terminals. Ask for the building heating load, outdoor design conditions, required water temperatures, terminal schedule and proposed defrost heat source or storage arrangement. Those details tell you more about the complete system than a nominal kW figure alone.
Frequently Asked Questions
Why does an air source heat pump need to defrost?
Frost can form on the outdoor coil under certain cold and humid conditions. Clearing it allows the unit to continue normal heating operation. The defrost method and control sequence vary by model.
Will underfloor heating keep a room from cooling during defrost?
Its thermal mass can soften the immediate indoor response to a brief interruption, as the cited experiment showed. It cannot guarantee a stable temperature in every building. Heat loss, defrost duration, water volume and controls still matter.
Is the 21.3% COP difference a figure I can use for a project estimate?
No. It compares two terminal modes in one controlled experimental system over two frosting–defrosting cycles. Use performance data for the selected heat pump and a project-specific system design for estimates.
Can one heat pump serve both underfloor heating and fan coils?
Yes, if the selected model and hydraulic design support both. The designer should check each terminal's water-temperature and flow requirements, then specify suitable zones, mixing and controls.
Specify the Heat Pump and Terminals Together
If you are planning a hydronic heating project with radiant floors, fan coils or both, send MENRED the building load, location, design outdoor temperature, terminal types and target water temperatures. We can discuss a suitable MGC heat pump model and the system information needed to assess its integration.