Abstract: A fireplace boiler for heating liquids is disclosed which is adapted to be fitted into a fireplace cavity and combined with a pre-existing domestic heating system. The boiler includes a grate made up of a plurality of heating tubes connected to headers through which a liquid to be heated flows. At one end of the grate is located a vertical sampling tube having an orifice therein through which the liquid flows between the headers. The flow of liquid in the sampling tube is reversible with the direction of flow being dependent upon whether the fluid in the boiler is being circulated through the heating system. The reversible direction of flow enables a temperature sensor located in the sampling tube to accurately sense the temperature of the fluid in the boiler at all times.