The evil operational effects of uncontrolled pump "interlocks" were initially discussed in a series of ASHRAE Journal Articles. The term pump interlocks is a reference to pumps operating in unforeseen paralleled and series combinations -- the often observable results being:

1. Return main operation at a higher pressure than supply.
2. Distribution main flow reversal.
3. Uncontrollable flow change in the using building circuits.

The referenced article series did not analytically describe why and how the troubles occurred -- principally because analytical methods had not yet been developed. It is the purpose of this discussion to analytically describe "interlock" troubles and to finally provide simple design and operational rules for solution of existing problems as well as prevention of future troubles. Application of the design rules may also lead to new pump energy conserving design approaches.