Emitter Selection & Sizing for Low-Temperature Systems
Why emitter choice is the single most impactful design decision for heat pump efficiency — and how to get sizing right for every room.
Why Emitter Selection Matters
The coefficient of performance (COP) of a heat pump is directly related to the flow temperature it needs to deliver. Lower flow temperatures mean higher efficiency — and the emitter determines the minimum flow temperature required to heat each room adequately.
Output at Different Flow Temperatures
Heat output drops significantly at lower flow temperatures. Each 1°C reduction improves COP by approximately 2.5% — making correct emitter sizing the key to efficiency.
| Emitter | Output at 75°C | Output at 55°C | Output at 45°C | Output at 35°C |
|---|---|---|---|---|
| UFH (per m²) | N/A | 80W | 65W | 50W |
| Skirting board heating (per m) | 180W | 120W | 100W | 70W |
| Standard radiator (600×1000) | 1,200W | 720W | 480W | 280W |
| Double panel radiator (600×1400) | 2,100W | 1,260W | 840W | 490W |
| Fan convector (single unit) | 6,000W | 4,200W | 3,200W | 2,100W |
Sizing Emitters for Low-Temperature Operation
At lower flow temperatures, every emitter produces less heat output per unit. Correct sizing compensates for this by increasing emitter surface area — whether that means larger radiators, more underfloor heating zones, or longer runs of skirting board heating.
Calculate Room-by-Room Heat Loss
Start with an accurate heat loss calculation for each room using the FHS fabric standards (U-values of 0.18 for walls, 0.13 for roof, 1.2 for windows). Account for ventilation heat loss from MVHR. The tighter fabric of FHS homes means heat loss figures will be significantly lower than older buildings.
Determine Design Flow Temperature
Choose the lowest practical flow temperature for your heat pump system. For new-build FHS homes with good fabric, 35–40°C is achievable with properly sized emitters. Each degree lower improves annual efficiency by approximately 2.5%.
Apply Delta-T Correction Factors
Manufacturer output data is typically quoted at delta-T 50 (75°C flow / 65°C return). For heat pump systems at delta-T 20 (40°C flow / 35°C return), multiply the quoted output by approximately 0.30. This correction factor is why oversizing is essential.
Select Emitters Room by Room
Match emitter type and size to each room based on corrected output. Bedrooms may suit underfloor heating (slow response acceptable). Living rooms may benefit from skirting board heating or oversized radiators, which offer moderate response times at low flow temperatures. Bathrooms may benefit from a towel radiator plus supplementary UFH.
Design Approaches for FHS Compliance
Whole-House Single Emitter
Using one emitter type throughout simplifies installation, commissioning, and maintenance. Underfloor heating is the most common whole-house approach for new builds, offering the lowest flow temperatures and invisible installation.
Best for: New builds with screed floors, uniform room sizesHybrid Emitter Strategy
Combining emitter types room by room optimises both efficiency and comfort. UFH in bedrooms and hallways, skirting board heating or oversized radiators in living areas, fan convectors in high-demand utility rooms.
Best for: Projects prioritising both efficiency and rapid responseRoom-by-Room Optimisation
Each room gets the emitter best suited to its use pattern, heat loss characteristics, and occupant preferences. This approach maximises COP but requires more design time and installer coordination.
Best for: High-spec developments, bespoke homesCommon Emitter Sizing Mistakes
Using Gas Boiler Sizing Data
Emitter output data from manufacturers is typically quoted at delta-T 50 (gas boiler conditions). Using these figures without applying correction factors for heat pump flow temperatures results in severely undersized emitters and cold rooms.
Ignoring Response Time Requirements
Underfloor heating is highly efficient but has a 1–6 hour response time due to thermal mass. Heat pumps are typically run continuously to mitigate this, but in rooms where occupants need more responsive warm-up (e.g., bathrooms), a supplementary emitter such as skirting board heating or a towel radiator may be beneficial.
Oversizing the Heat Pump Instead of the Emitters
When rooms are cold, the instinct is to increase heat pump output — but this raises flow temperature and reduces COP. The correct approach is to increase emitter surface area so the same heat can be delivered at a lower flow temperature.
Neglecting Weather Compensation
Heat pumps should modulate flow temperature based on outdoor temperature. If emitters are only sized for the coldest design day, the system runs at unnecessarily high flow temperatures for most of the heating season. Size emitters for mild conditions and use weather compensation to optimise year-round.
Forgetting Furniture Obstructions
Radiators behind sofas lose significant output as airflow is restricted. Plan radiator positions considering realistic furniture layouts, not empty rooms. Skirting board heating is less affected by furniture as it runs at low level around the full room perimeter.
Emitter & Heating Questions
What heating systems are allowed under FHS?
The FHS does not prescribe specific heating systems but sets carbon reduction targets that effectively require low-carbon heat sources. Compliant options include air source heat pumps (ASHP), ground source heat pumps (GSHP), shared ground loop systems, heat networks (district heating), and potentially hydrogen-ready boilers in specific scenarios. The vast majority of FHS homes will use heat pumps as the primary heat source.
Do I need a heat pump for a new build?
In almost all cases, yes. The 75–80% carbon reduction target under FHS cannot realistically be achieved with combustion heating. Air source heat pumps are the most common choice for individual homes, while ground source heat pumps or heat networks suit larger developments. The specific heat pump type and capacity should be determined by a heating design engineer based on the calculated heat loss of the dwelling.
What size radiators do I need for a heat pump?
Radiators for heat pump systems typically need to be 1.5–2× larger than those specified for gas boiler systems. This is because heat pumps operate most efficiently at flow temperatures of 35–45°C (compared to 65–80°C for gas boilers), and the lower temperature differential means each radiator emits less heat per square metre. A heating engineer should perform room-by-room heat loss calculations and size emitters using the correct delta-T for the design flow temperature.
What are the best emitters for heat pumps?
The best emitter depends on the application. Underfloor heating offers the lowest flow temperatures (35–45°C) and maximises COP but has slow response times (1–6 hours) — heat pumps are typically run continuously to mitigate this. Skirting board heating offers a good balance — operating at 35–50°C with moderate response time while replacing conventional skirting boards, providing distributed heat around the room perimeter. Oversized radiators offer similar moderate response times at low flow temperatures, with the advantage of being familiar technology. Fan convectors deliver the highest output from compact units but produce some noise. Many FHS homes will use a combination of emitter types optimised room by room.
Can I use underfloor heating with a heat pump?
Yes — underfloor heating is one of the best emitter choices for heat pump systems. It operates at the lowest flow temperatures of any emitter type (35–45°C), which maximises the heat pump coefficient of performance (COP). The large surface area of the entire floor compensates for the low temperature. UFH is particularly effective in new-build FHS homes where it can be designed into the floor construction from the outset.
What is skirting board heating?
Skirting board heating is a low-profile heat emitter system that replaces conventional skirting boards around the perimeter of rooms. It uses hydronic (water-based) heat exchangers concealed within skirting-height casings, typically 150mm tall. The extended linear length provides substantial surface area for heat emission at low flow temperatures (35–50°C), making it well-suited to heat pump systems. Installation complexity is low as it follows existing skirting lines.