Balancing Options for Systems with Variable Speed Pump Control

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By Chad Edmondson

Last week we learned that ASHRAE defines a balanced system as one that delivers a flow sufficient to achieve a 97% heat transfer at each coil. This week will look at why we balance and outline the strategies that are available to us.

Let’s start with a quick review of key ASHRAE 90.1 requirements.

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First of all, we know we have to do it. ASHRAE makes that pretty clear in the above 90.1 Standard. Why does ASHRAE care? Because balancing saves energy!

Not only do we have to balance, we must also provide a written balance report to the owner or representative for the HVAC system. This requirement applies to any building with a conditioned space of greater than 5000 sq. ft., which includes most any non-residential building.

Second, ASHRAE tells us we must proportionally balance and thus be able to reduce the speed of the pump as much as we can and still meet design flow conditions. In other words, we must trim the pump impeller or be able to reduce the speed of the pump so that we have just enough flow to deliver 100% of flow at the critical zone with the valve wide open:

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Finally, we must provide the owner with the suggested setpoints for the system, including pressure in air duct, hot and chilled water temperature setpoints, Delta T’s, and, most relevant to our current discussion, control head setpoint.

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Balancing Options

What are our options for balancing? On this point, ASHRAE is flexible. We may use any of the following components to balance, as long as the outcome is a verifiably balanced system:

  • Calibrated balancing valves (i.e. circuit setters)

  • Automatic system-powered flow control (flow limiting) valves

  • Standard ball or butterfly valves with pressure gauges or test plugs for measuring pressure drop across the coil or heat exchanger so that appropriate flows can be determined based on manufacture’s performance data

  • Pressure-independent control valves

  • Automatic control valves

Ball/butterfly valves and automatic control valves aren’t really relevant options in today’s sophisticated systems, so we will limit our upcoming exploration of control methods to the remaining three. Stay tuned as we test each under specific scenarios to show how they can and can’t be used to meet ASHRAE balancing standards and how each balance type changes the control area of the system.