Abstract: A hot water central heating system with a conduit system conducting a water flow and comprising intercommunicating pipes which intersect in decks and substantially fills the cross-section of the combustion chamber of a boiler and offers a large flow resistance to the firing gases in order to lower the exhaust gas temperature. A pressure limiting valve disposed upstream of the circulating pump protects the hot water conduit system against excessive pressure resulting from the formation of vapor. Corrosion-resistant lining of the exhaust gas chimney by means of an internally enamelled pipe is recommended owing to the low exhaust gas temperature which can be reached. Fuel is continuously supplied to the burner, the fuel flow rate being modulated by a controller responsive to heat sensor means placed in the hot-water outlet of the boiler.