This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
It quite frequently happens that the kitchen boiler does not have sufficient capacity for the entire house, and it is not desirable to use a larger boiler on account of the limited space in the kitchen. In such cases a second boiler may be connected with the laundry stove if one is provided, and the water pipes from both boilers be connected together at some point so that they may both discharge hot water into the same general supply.
Stopcocks should be placed in the pipe connections as shown, so that either boiler may be shut off for repairs without interfering with the operation of the other. Waste cocks should always be used for this purpose, so that when closed there will be a connection between the boiler and the atmosphere. This will prevent damage to the boiler in case those in charge should forget to open the cocks when starting up a fire in the stove with which the boiler is connected.