This study reports a systematic techno-economic assessment of the cost-optimal transmission for air as the heat transfer fluid at temperatures of up to 1200 °C, at scales of 1 to 1000 MW and at distances of up to 10 km. It employs a steady state heat transfer analysis, with energy balances, to assess the effect of scale, temperature and insulation on the heat losses and efficiency of the thermal transmission system, following by a techno-economic assessment. The sensitivity of the Levelised Cost of Heat, LCOHtr, to variations in thermal scale, operating temperature, distance, refractory and insulation thickness, ratio of the thickness of refractory and insulation, cost of any supplementary heat and lifetime is reported. The results show that LCOHtr decreases with an increase in thermal scale, as expected. The role of insulation is much more complex, since increasing the thickness of thermal barrier increases both cost and efficiency of the transmission, requiring an economic optimum to be determined for each of the various conditions assessed. Parameters are also coupled because a higher cost of supplied energy/ heat justifies the use of more insulation material. For a large GW scale thermal system, we estimate that it is possible to achieve a minimum overall LCOHtr,min of 0.16–0.36 USD/GJ/km of the length of the thermal transmission system, excluding additional location-specific costs such as land-access and local construction costs. These estimated costs are sufficiently attractive to justify ongoing development of systems to transport renewable heat to industry from sources such as concentrated solar thermal energy. •A thermal transmission for air up to 1200 °C, 1GW and 10 km.•Efficiency and LCOH of the transmission were reported for cost-optimal situations.•The cost-optimised efficiency is strongly dependent on the cost of energy.•Cost-optimised efficiency can be above 98% for thermal transmission system >100 MW.•The cost of GW scale thermal transmission <1% of the cost of energy being transmitted.