Variable Speed Pumping
Manual balancing will not work with direct return circuited, variable speed systems. As the two-way valves close, the header pressures decrease. This pressure reduction causes the problems described below.
At 100 percent flow, the balancing valves absorb increasing amounts of head the closer the coil is to the pump. The balancing valve at Terminal A is set at a pressure drop of 77 feet to provide an equal balance.
The second diagram shows the same system at 50 percent flow with all ATC’s modulated equally, except Terminal A which is wide open. At half flow, the distribution losses are 25 percent. This reduces the pump head to 50 feet and the differential pressure across Terminal A is reduced to 48 feet.
The wide open ATC valve at Terminal A is calling for full flow, but there is a 31 percent flow deficit. The reason is that this terminal was balanced with 102 feet of differential pressure for full flow and now only has 48 feet.
With AutoFlow regulators installed, the 100 percent flow conditions are exactly the same as a manually balanced system. Also, the drop caused by the balancing valve in each air handler is the same for both manual and automatic devices.
However, the situation differs at 50 percent flow when the ATC valve at Terminal A opens calling for full flow. Because the ATC and the coil require pressure drops of 15 feet and ten feet, only 23 feet remain (48 – 15 – 10 = 23).
The AutoFlow regulator automatically adjusts to the 23 foot drop to prevent overflow.
The AutoFlow regulator has adjusted from 77 feet to 23 feet differential pressure, yet still maintains the same flow rate. When the variable speed pump head pressure decreased, it adjusted automatically. This dynamic valve fits the needs of the variable speed balancing perfectly!
The diagrams to the right represent a ten story building with identical air handlers on each floor. Each air handler coil has a ten foot drop and the ATC valve has a 15 foot drop at design flow. The distribution pipe loss is four feet between floors on the supply and the return. The variable speed sensor is placed at the top floor and is set for 30 feet to provide full flow to that air handler at all times.