So far, our discussion has provided an abstract look at VPF system design and operation. Questions of a more practical nature are "Do they really work?" and if so, "How well?" For answers, we spoke to several engineers who have applied systems with variable primary flow. Two of them share their experiences below.

"We installed a variable-primary-flow system in a mall. During operation, we were surprised to find that the pumps never operated below 60 percent flow. We discovered that the anchor tenant had disconnected their valve controllers, were running the coils wide open, and were varying airflow to maintain a constant leaving-air temperature and humidity.

"Though we weren't allowed to change this setup, the good news is that we had already saved almost 80 percent of our pumping energy since the pumps were operating at about 60 percent of their design flow and at about 20 percent (0.6³ = 0.216) of their design power."

The second engineer recounts a similar story, again reminding us that the real world is less than perfect:

"We recently applied a VPF system in a hospital. We replaced the major air handlers and used two-way modulating valves, but the patient rooms are still served by fan-coils with three-way valves. The good news is that the fan-coils act as the bypass and keep us above the minimum chiller flow rates. The not-so-good news is that we've saved a lot of pumping energy, but not as much as we could."

Continue on to Implementing A VPF System or return to the Table of Contents.