Variable Primary Chilled Water Systems Part 6: What to Consider Before Choosing This Design Option

By Chad Edmondson

As we conclude this series, we want stress that when it comes to variable flow and chilled water systems, there is no “one approach fits all” solution. Like virtually every other aspect of mechanical HVAC design, the ideal solution is wholly dependent on the design of the specific building, the building purpose, and the requirements of the owner and the occupants.

That said, there are certain factors that provide solid clues as to whether variable primary design will yield favorable results. Here are the questions that engineers should consider before making that design decision:

Can the system flow be reduced by at least 30%? In other words, if the minimum required flow at any given time is 70% or more of peak design flow, then a variable primary system is unlikely to yield savings that will be worth the added complexity of control.

Can the building tolerate a modest change in water supply temperature? When staging up a variable primary system, there will always be a temporary increase in supply water temperatures. Typically this is not for a period greater than 10 or 15 minutes, but we have seen supply temperatures increase to as high as 50°F during this interim period. Obviously, if the leaving supply temperature is critical to the operation of the building, then this could be prohibitive.

Are the building operators well-trained and present? Variable primary systems are more complex and tend to be more successful when the building is supported by onsite mechanical staff who are familiar with the design and control of the chilled water plant.

Does building modeling demonstrate cost savings? Certainly, engineers will want to model and compare the operational cost of primary secondary, variable primary and possibly some combination of both before deciding.

Are the pumps and chillers equally sized? If this is a renovation project with existing equipment, it is important to consider how the current chillers and pumps are sized. Variable primary systems are much simpler to control when all chillers and primary pumps are uniformly sized.

Properly applied, variable primary chilled water design can be a financially strategic option for many types of facilities. But every situation should be carefully considered. As always, we are here to help!