This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
205. In the plans shown in Figs. 73 and 74, the flow and return mains are connected only by the radiator branches, and there is no way of maintaining a flow of water through them when the radiators are shut off. This arrangement of mains is called an open circuit.
206. When all the radiators, except one or two, are shut off, the amount of circulation is likely to be too small to keep the water in the mains at a proper working temperature. Then, when the other radiators are opened for use, considerable time must be spent in waiting for the whole system to heat up to the desired degree.
This slowness may be obviated by keeping up a good circulation through the mains at all times, regardless of the radiators, by connecting the flow and return mains by a pipe k, as shown in Fig. 76. This arrangement is called a closed circuit. The connection should be large enough to pass as much water as the whole number of radiators when in full operation. As long as a proper fire is maintained in the boiler, an active circulation will go on in the mains, and the water will be always at the maximum temperature, so that any or all of the radiators may be promptly supplied with hot water as soon as the valves are opened.
The closed circuit is desirable for all situations where the simple circuit, Fig. 72, is not used, and is adapted to high buildings as well as low ones. It is superior to all others in long, low buildings, of one or two stories, where the mains must extend a long distance horizontally, as in cases like Fig. 77.
There is quite a difference of opinion regarding the true meaning of the terms "open" and "closed" circuits. The terms and the meanings thereof, as adopted by us, may seem contrary to common sense; but they coincide with the meanings of the same words when applied to electrical or other professions, and, therefore, will help to prevent confusion or misunderstanding.