Figure 28  General 10 – Top Up Tanks – Closed Loop Pressurized Recirculation System

Two similar views of a domestic heating system are shown. Both layouts are fully recirculating with no flow outlets from the system.

In the first layout a top up header tank is included to make up any leakage from the system. 

In the second layout the header tank and make-up pipe have been replaced by a single tank at node N1, which is defined to represent the same pressure as provided by the column of liquid in the make-up pipe and header tank in the first system. 

The elevation of the tank and the head of water in the tank provide a pressure reference within the system, relative to which the other node pressures are calculated. When a system is completely recirculating, such as a heating or cooling system, or a sealed heat recovery system, the user must provide a starting pressure reference to allow a solution to be calculated that gives the pressure at each node.

Note: Pipe Flow Expert will check that the pressure at each node is above 0.000 psi (absolute). Pressures below 0.000 psia are impossible to achieve.