A heat transfer apparatus comprising a masonry heater with a firebox including a wall, and a coil pipe extending into the firebox of the masonry heater through a first aperture in the wall of the firebox and exiting through a second aperture in the wall of the firebox, the coil pipe winding back and forth in the firebox and being disposed so as to be exposed to fire in the firebox, wherein a heat transfer liquid enters the coil pipe from a supply side of the coil pipe at the first aperture and the heat transfer liquid exits the coil pipe from a return side of the coil pipe at the second aperture; a first sensor configured to detect a temperature of the heat transfer liquid in a liquid return path, on the return side of the coil pipe; a circulation pump configured to transfer the heat transfer liquid from the return path to an output of the heat transfer apparatus when the circulation pump is circulating the liquid, and stopping transfer of the heat transfer liquid to the output of the heat transfer apparatus when the circulation pump is not circulating the heat transfer liquid; and a controller configured to control whether the circulation pump circulates the heat transfer liquid based on a temperature detected by the first sensor; and characterized by a T-junction disposed on the outside of the masonry heater on the return side of the coil pipe; and a sensor sheath housing the first sensor, wherein the sensor sheath is inserted into a first port of the T-junction and extends from the first port of the T-junction through the T-junction into the coil pipe via a third port of the T-junction, the sensor sheath extending through the wall of the masonry heater into the firebox such that the first sensor is located within the firebox in the liquid return path and such that when the circulation pump is operating, liquid flows from the coil pipe inside the masonry heater around the sensor sheath and out of a second port of the T-junction toward an external heating device. a first sensor configured to detect a temperature of the heat transfer liquid in a liquid return path, on the return side of the coil pipe; a circulation pump configured to transfer the heat transfer liquid from the return path to an output of the heat transfer apparatus when the circulation pump is circulating the liquid, and stopping transfer of the heat transfer liquid to the output of the heat transfer apparatus when the circulation pump is not circulating the heat transfer liquid; and a controller configured to control whether the circulation pump circulates the heat transfer liquid based on a temperature detected by the first sensor; and characterized by a T-junction disposed on the outside of the masonry heater on the return side of the coil pipe; and a sensor sheath housing the first sensor, wherein the sensor sheath is inserted into a first port of the T-junction and extends from the first port of the T-junction through the T-junction into the coil pipe via a third port of the T-junction, the sensor sheath extending through the wall of the masonry heater into the firebox such that the first sensor is located within the firebox in the liquid return path and such that when the circulation pump is operating, liquid flows from the coil pipe inside the masonry heater around the sensor sheath and out of a second port of the T-junction toward an external heating device.