A heat exchanging unit exchanges heat between a fluid to be heated and exhaust gas. The heat exchanging unit includes a heat exchange portion, a header portion, and a flow changing portion. The heat exchange portion includes a heat exchange pipe in the interior of which the fluid to be heated flows. The header portion is connected to the heat exchange pipe, the header portion allowing the fluid to be heated to flow from the header portion to the heat exchange pipe or from the heat exchange pipe to the header portion. The flow changing portion changes the state of flow of the exhaust gas introduced into the heat exchange portion.