Realistic £/kW band
£1,200 to £2,300 per kW installed for a school retrofit, mid-band £1,700/kW. Includes wet-system upgrade (radiators, pipework), buffer vessel, controls, and electrical works.
kgCO2e and £/tCO2e
Replacing 620,000 kWh/yr of gas (typical 3,000m² primary) displaces ~85 tonnes CO2e/yr. Over 20 years that is 1,700 tonnes. A £700k ASHP scheme scores £412/tCO2e: under the £510 ceiling but with limited headroom, so fabric works are what pull it clear.
Does it need a wet-system upgrade?
Almost always yes. Schools sized for 70-80°C flow need to step down to 45-55°C for an ASHP. That means larger radiators (or fan-coil), possibly new pipework. Salix funds the wet-system upgrade as enabling works [DESNZ].
Phase 3/4 examples
ASHP-led schemes in the Phase 4 funded-project list [DESNZ] tend to cluster around £280 to £320/tCO2e where fabric was included; without fabric they push toward the £510 ceiling and are filtered out if they breach it.
See: fabric-first, GSHP, estimator, case awards, £510 ceiling.