Heat is taken from your air outside the house employing an external heat pump unit. This is transferred into your own home to provide your heating. The ECOMOD 290HT choice of monobloc air source heat pumps come in three chassis sizes and five outputs: 15kW, 18kW, 27kW, 40kW and 50kW. https://purethermal.co.uk/media-news/q4-launch-lithium-large-capacity-r290-ashp-from-pure-thermal-and-enerblue/