Air-to-water heat pump underfloor heating in Ireland: 35°C design & SEAI grants
An air-to-water heat pump reaches its highest seasonal efficiency (SCOP 3.8 to 4.3.
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Flow temperature vs heat pump efficiency (SCOP)
| Heat emitter type | Design flow temperature (at -3°C outside) | Typical heat pump SCOP | Cost per kWh of heat (at 34.4c/kWh elec) |
|---|---|---|---|
| Wet underfloor heating in 50mm liquid screed (150mm pipe centres) | 32°C–35°C | 4.0 – 4.3 | 8.0c – 8.6c / kWh |
| Low-profile 15mm–18mm retrofit overlay (100mm–125mm centres) | 35°C–38°C | 3.7 – 4.0 | 8.6c – 9.3c / kWh |
| Oversized low-temperature K2/K3 radiators | 42°C–45°C | 3.2 – 3.5 | 9.8c – 10.7c / kWh |
| Standard high-temperature radiators | 50°C–55°C | 2.6 – 2.9 | 11.9c – 13.2c / kWh |
Every 1°C reduction in your heat pump's water flow temperature improves compressor efficiency by roughly 2.5% to 3%. Because a heated floor has 8 to 10 times the surface area of wall radiators, it keeps your rooms at 20°C–21°C while circulating lukewarm 35°C water.
2026 SEAI Heat Pump & Central Heating grant breakdown
| SEAI Grant Component (Pre-2021 Homes) | House Grant | Apartment Grant | Key Technical Requirement |
|---|---|---|---|
| Air-to-Water / Ground-Source Heat Pump Unit | €6,500 | €4,500 | Home must achieve Post-Works Heat Loss Indicator (HLI) ≤ 2.0 W/K/m² |
| Renewable Heat Bonus (replacing fossil boiler) | €4,000 | €4,000 | Applies when removing an existing oil, gas or solid-fuel central heating system |
| Central Heating Grant (Underfloor Heating / Radiators) | Up to €2,000 | Up to €1,000 | Covers installing new wet underfloor heating or low-temp heat pump radiators |
| Boiler Scrappage Bonus | Up to €2,000 | Up to €2,000 | Subject to SEAI scheme eligibility and prior approval before works start |
| Maximum Combined SEAI Heat Pump Package | Up to €14,500 | Up to €11,500 | Plus €200 towards independent Technical Assessment & €50 post-works BER |
Important: Underfloor Heating Installation Ireland (underfloorheatinginstallation.ie) is an independent installation introduction service, not SEAI or a government agency. All SEAI grant applications must be approved before works commence on site.
Commissioning a heat pump manifold: avoiding short-cycling
- No manifold blending valve on direct heat pump feeds: Unlike a 70°C gas boiler, a heat pump operates on weather compensation and sends 28°C–38°C water straight into the manifold. Fitting an unnecessary thermostatic mixing valve restricts flow rate and forces the heat pump to run hotter than needed.
- Minimum open loop volume & micro-zoning care: Over-zoning every tiny hallway with closed actuators can starve an inverter heat pump of minimum water flow (typically 12–18 L/min) during defrost and low-load cycles. Installers leave the main open-plan kitchen/living zone actuator-free (controlled directly by the heat pump's weather-compensation curve) or fit a small 50L–100L buffer vessel / volumiser.
- Pipe spacing for 35°C design: Living areas are clipped at 150mm centres (delivering ~55–65 W/m² at 35°C flow), while bathrooms and glazed sunrooms are tightened to 100mm centres so no supplementary towel radiator is needed for space heating.
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Underfloor heating installation guides & specifications
Detailed Irish engineering & cost guides covering pipe spacing, manifold balancing, 15mm retrofit overlays, 35°C heat pump commissioning and floor coverings.
More technical resources: Liquid screed for underfloor heating · All guides & calculator hub · Technical trade glossary (A–Z) · Frequently asked questions · All 26 counties covered
Heat pump UFH: common questions
What HLI score does my house need for the SEAI heat pump and underfloor heating grant?
SEAI requires a Heat Loss Indicator (HLI) of 2.0 W/K/m² or lower (or up to 2.3 W/K/m² if additional fabric upgrades such as attic or wall insulation are completed first, as verified by an SEAI Registered Technical Advisor).
Should I turn heat pump underfloor heating off at night?
No — on a well-insulated heated slab fed by an air-to-water heat pump, turning the heating completely off overnight forces the compressor to work hard at maximum output in the coldest morning air. Instead, use a gentle 1.5°C–2°C night setback (or slight night-rate pre-charge of the slab between 02:00 and 05:00 on a smart EV/night tariff) while letting weather compensation hold a steady temperature.
Can an air-to-water heat pump feed underfloor heating downstairs and radiators upstairs?
Yes — either by sizing upstairs K2/K3 radiators to run at the same 38°C–42°C flow temperature as the floor (simplest and highest SCOP), or by using a two-zone kit where upstairs radiators receive 45°C–48°C water and the downstairs manifold blends down to 35°C.
Underfloor heating installation across all 26 counties
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