Diversity in a System with Variable Speed Pump Control

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By Chad Edmondson

Comfort and efficiency are the ultimate goals of variable speed pump control. As technical as the discussions may become, it all comes back to comfort and efficiency. With that in mind, let’s review the concept of diversity, because managing system diversity enables us to maximize energy efficiency.

By now most of you probably understand that system diversity refers to the fluctuating loads that occur in most any commercial or institutional building or facility throughout a 24-hour period. Loads naturally vary for all of the following reasons:

  • The movement of people from one space to another throughout the day. Obviously, the people in a building or facility can’t be everywhere at once. (e.g. students migrate from classrooms, to the cafeteria, to the dorms, etc. throughout the day.)
  • The east, south, west, and north exposures of a building heat up at different times throughout the day. This impacts the “solar” load on a building’s cooling system.
  • And, to a lesser degree, the opening and closing of doors also has some impact on the heat loss and heat gain of the building.

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All of this creates diversity in the system, which means that peak loads do not (cannot) occur in every part of a facility all at once. A simple way to see if a system has diversity is to add the total flow of all of the coiling coils and compare that to your chiller plant design flow.  In the above example, the total connected load is 4000 tons, however the engineer has determined that the instantaneous maximum load at any given time is only 3000. This gives the building a diversity factor of .75 or 3000 ÷ 4000. (For an in-depth review of system diversity, see Hydronic Balancing Part 2: Making the Most of System Diversity).

If we know our peak block load, we can minimize chiller capacity and “move” cooling supply from one place to another as needed. Variable speed pumping helps increase energy efficiency in these diverse systems but there is a limit to how much we can slow the pump down while keeping the occupants comfortable.

How System Diversity Impacts Control Strategy

The diversity characteristics of a given system should be considered when choosing a control strategy for variable speed pumps in a hydronic heating or cooling system. Specifically, the amount of diversity and the time and location of loads will dictate whether curve control or area control is the best strategy.

Generally speaking, systems with a lot of diversity will experience more “misses” when pumps are controlled based on the control curve. When misses occur, comfort suffers, and plant operators have a tendency to increase the control head value on the pumps to keep occupants happy. Ultimately this solution costs the owner more money because it increases the horsepower of the pumps, causing them to consume more energy.

How much will a miss impact comfort? Again, it depends on where and when the loads occur. Some spaces may be able to tolerate the misses at certain times during the day. Other spaces may not.

Reset and Setback Considerations

Nighttime setback and chilled water reset could also cause problems and should be taken into consideration when deciding how to make the most of system diversity.

Think about a high school with nighttime setback. What happens when the system starts up in the morning and all zones call for heating or cooling at once? In our above example, this would mean that the system would suddenly require 4000 tons of cooling when the chiller plant can only provide 3000 tons. In this scenario, the coils closest to the chiller plant would get all the flow, while the more distant coils would be starved of flow.

A similar problem could occur in systems with chilled water reset.  When chilled water supply temperature is temporarily reset to a higher temperature during certain hours of the day, it can cause the system to flow more water than it was designed to flow.  

Neither of these are untenable problems, of course. Control systems can be designed to allocate flow to various zones in staged or sequential manner so that occupant comfort isn’t impacted. The point is, nighttime setback and chilled water reset are important considerations when applying variable speed pumping to take advantage of system diversity. In many cases, these efficiency strategies will conflict with a variable speed system that relies on a non-dynamic curve control strategy.

Next week we’ll take this all a bit further by exploring flow tolerance.