A construction unit (1, 2) forms part of a gas heater and includes a valve (9) which is actuatable by way of a stem (10). The stem (10) includes positive-fit device. The stem (10) is controllable from the outside via a tool which can be brought into engagement with the positive fit device. A detent engagement device (18, 27) holds the tool (21) in engagement with the stem (10), so that the valve is controllable without the aid of outside tools.