A room heating system, comprising: a thermal distribution medium piping formed to circulate a thermal distribution fluid; and an indoor unit, the indoor unit comprising: a heat exchanger, and a portion of the thermal distribution medium piping and the heater; characterized in that the heater is a backup heater comprising: a heating element; a housing for the heating element; a thermal distribution fluid inlet into the housing and a thermal distribution fluid outlet from the housing; the housing for containing the thermal distribution fluid as it follows a flow path from the inlet to the outlet and over the heating element; at least one safety feature connection including a passage through the housing; and a safety feature (46, 48, 50) connected to the safety feature connection; wherein the housing comprises opposed ends with a side wall extending longitudinally therebetween and the heating element is mounted to one of the ends and extends towards the other end, and wherein at least one of the fluid inlet to the housing and the fluid outlet from the housing include a passage through the side wall, wherein the housing is mounted in a predetermined orientation, and wherein the safety feature is a gas release valve operable to release gas accumulated in the heater, wherein the safety feature is mounted at a top of the housing where gas can accumulate in use and the inlet and outlet are positioned below the safety feature, and wherein the predetermined orientation is so that the longitude of the housing is substantially horizontal and the safety feature is mounted to a longitudinal centre of the side wall, and wherein the heater is mounted at a top part of the indoor unit and the indoor unit is mounted with respect to said thermal distribution piping such that the heater forms an uppermost fluid distribution medium containing component connected by the thermal distribution piping of the system. a thermal distribution medium piping formed to circulate a thermal distribution fluid; and an indoor unit, the indoor unit comprising: a heat exchanger, and a portion of the thermal distribution medium piping and the heater; characterized in that the heater is a heat exchanger, and a portion of the thermal distribution medium piping and the heater; characterized in that the heater is a backup heater comprising: a heating element; a housing for the heating element; a thermal distribution fluid inlet into the housing and a thermal distribution fluid outlet from the housing; the housing for containing the thermal distribution fluid as it follows a flow path from the inlet to the outlet and over the heating element; at least one safety feature connection including a passage through the housing; and a safety feature (46, 48, 50) connected to the safety feature connection; wherein the housing comprises opposed ends with a side wall extending longitudinally therebetween and the heating element is mounted to one of the ends and extends towards the other end, and wherein at least one of the fluid inlet to the housing and the fluid outlet from the housing include a passage through the side wall, wherein the housing is mounted in a predetermined orientation, and wherein the safety feature is a gas release valve operable to release gas accumulated in the heater, wherein the safety feature is mounted at a top of the housing where gas can accumulate in use and the inlet and outlet are positioned below the safety feature, and wherein the predetermined orientation is so that the longitude of the housing is substantially horizontal and the safety feature is mounted to a longitudinal centre of the side wall, and wherein the heater is mounted at a top part of the indoor unit and the indoor unit is mounted with respect to said thermal distribution piping such that the heater forms an uppermost fluid distribution medium containing component connected by the thermal distribution piping of the system. a heating element; a housing for the heating element; a thermal distribution fluid inlet into the housing and a thermal distribution fluid outlet from the housing; the housing for containing the thermal distribution fluid as it follows a flow path from the inlet to the outlet and over the heating element; at least one safety feature connection including a passage through the housing; and a safety feature (46, 48, 50) connected to the safety feature connection; wherein the housing comprises opposed ends with a side wall extending longitudinally therebetween and the heating element is mounted to one of the ends and extends towards the other end, and wherein at least one of the fluid inlet to the housing and the fluid outlet from the housing include a passage through the side wall, wherein the housing is mounted in a predetermined orientation, and wherein the safety feature is a gas release valve operable to release gas accumulated in the heater, wherein the safety feature is mounted at a top of the housing where gas can accumulate in use and the inlet and outlet are positioned below the safety feature, and wherein the predetermined orientation is so that the longitude of the housing is substantially horizontal and the safety feature is mounted to a longitudinal centre of the side wall, and wherein the heater is mounted at a top part of the indoor unit and the indoor unit is mounted with respect to said thermal distribution piping such that the heater forms an uppermost fluid distribution medium containing component connected by the thermal distribution piping of the system.