Abstract: A catalytic heater includes a catalyst, for the combustion of a gaseous hydrocarbon in air, supported on a catalyst support. Fuel diffuses into the catalyst support from one (proximal) side. Air diffuses into the catalyst support from the other (distal) side. The catalytic combustion of the fuel heats the catalyst support. The resulting infrared radiation from the distal side of the catalyst support is directed at an object to be heated, for example the fluid chamber of a boiler. A modular boiler includes several catalytic heaters, with the distal sides of their catalyst supports facing a chamber wherethrough a fluid such as water is conducted and heated by the infrared radiation from the heaters.