Summary Conventional heater is limited by the time and energy‐consuming design, showing high energy loss and causing a negative impact to the environment. A novel air heater integrated with microwave heating and activated carbon (AC) as thermal energy storage has been developed in this study to investigate the heat transfer efficiency in a helical coil. Microwave radiation provides volumetric heating to the AC, which acts as a microwave absorbent to absorb and convert microwave energy into heat energy for storage and subsequently heating the helical coil that in turn heats the air indirectly. The helical coil exerted centrifugal force and induced secondary flow to significantly enhance the heat transfer. Microwave radiation induced a high heating rate of 30°C/min to heat the AC. Low heat loss (6.12°C/min) was shown during the cooling down of AC, indicating high heat retention by the AC. Then, the convective heat transfer between helical coil and air with over a range of AC temperatures was examined. The highest overall heat transfer coefficient (U0) and coefficient of performance (COP) with various AC temperatures were recorded at 310.815 W/normalm2·°C and 3.958, respectively, at 50 L/min of air flow rate. Nusselt number, Reynolds number, and Dean number were increased with higher air flow rate. The experimental results showed good agreement with literature where convective heat transfer in the helical coil is more efficient at the high‐temperature region as well as when higher flow rate was applied. The use of microwave air heater shows a potentially high efficiency of energy‐saving with capacity to generate hot air at 100°C to 250°C in about 26 min after switching on the microwave oven.