How Do You Control a Chiller?

York YMC2 CenterPar Chiller DrawingTo some controlling a chiller in a large central energy plant (CEP) may seem difficult, but in all honesty with today’s electronics, controlling a chiller can be one of the easiest tasks in a CEP. Let’s discuss some of the things you typically would control on a chiller and how it can be quick and easy to integrate into a distributed controls system.

Are we in ‘Control’?

When taking over control of a chiller now-a-days really means we need to ‘Enable’ it to run. The difference between controlling and enabling is that with a simple relay we can hit a set of contacts on a chiller’s internal controls or change a variable via a communication protocol and let it know the Building Automation System (BAS) is requesting it to run. To me controlling something means I am the definitive signal making the equipment turn on and run. In this case with a chiller, the internal controls need a bunch of other safeties, flow switches, pressures and temperatures to be at their required positions before the chiller will ever spin up. Therefore, we aren’t ‘controlling’ the chiller, we are merely giving it an ‘Enable’ signal.

Pumpage and Other Things That Flow

Now there are many different types of chillers out there that utilize different types of media for cooling such as water, refrigerant, air, gas even ice. Some of those same medias are used for cooling off the heat generated by the chilling process itself. Either way things have to flow before the chiller will run or continue to run.

Most chillers incorporate the ability to self control pumps, condenser fans and other necessary equipment on their own internal controls. This is advisable if those pieces of equipment need to be modulated based on sensors already incorporated to the chiller. An exception to this in most of the installations I have come across is water pumps like chilled and condenser water pumps. The reason for this, I believe, is so that the BAS can monitor run times and rotate pumps, especially in the case of pump failures.

Belimo BFV_F6_HD_SY Butterfly ValveOne more thing that needs to be controlled by someone, especially in larger, multi-unit systems are isolation valves. Isolation Valves are used to block flow through non-running chillers so you don’t have the not-so-cold return water flowing back to the central energy plant only to run through a ‘dead’ or non-running chiller and mix right back into the supply. These valves need to work with the pumps and preferably timed so as not to cause the pumps to dead head for long periods of time.

It’s All About the Chill

The goal of any chiller is to … well Chill! There is always a goal or set point for the discharge temperature of a chiller. This set point is sometimes hard set in an engineer’s Sequence of Operations or it can be set from the Controls System.

I definitely prefer the latter method because then the BAS can determine an efficient set point by comparing it to readings external to the chiller like outside ambient conditions, heat loads in the building, positions of valves in the building, etc. This can be done a number of different ways, but some basic ones are by using a common communication protocol and running LAN wire to the chiller’s controls or using a hardwired method. The limitation to the hardwired method is in the chiller’s controls. Some chiller manufacturers only allow a ‘reset’ to the set point entered in on the chiller panel. Meaning, the base set point is 42°F and based on a 0-10VDC or 4-20ma signal you can move that set point +/- 10°F (adj).

Keep Your Eyes on that Chiller

just like everyone talks about how an A/C unit is the biggest consumer of energy in a home… so is a chiller and CEP to a commercial building or facility. Small changes at the chiller or plant can create the biggest saving in energy, but if you don’t have sensors monitoring the various temperatures, amperage and pressures, you won’t know what those minute energy saving changes can be.

Where’s the Rest?

Multiple Chillers in a Central Energy Plant

If you are thinking I stopped short of explaining all the things to control on a chiller… sorry to disappoint you, but in most HVAC chiller applications that is all there is to control. If you Enable the chiller to run and possibly give it an adjustment to the discharge temperature, you are pretty much done with getting the chiller under control with a DDC Controls System. Of course this is generally speaking, but the central plant as a whole is the largest concentration of control points, just not in the chiller itself. We can discuss that in another post…



What points have you ‘controlled’ or monitored on your projects or what information do you have access to in your facility?

8 Responses to How Do You Control a Chiller?

  1. AFitz January 26, 2012 at 16:04

    Is it necessary to know how the individual pieces of the chiller when controlling it through a DDC system? What about the different types of chillers, are they “controlled” differently by the BAS or the same?

    • The Controls Freak January 26, 2012 at 18:11

      AFitz, thanks for the comment. I would get myself in trouble with alot of people if I said, “Nope! That’s all you need to know.”

      This article was written to put focus on more important things in a Central Energy Plant than just ‘how to control the chiller’. So, yes you need to know more about WHEN to enable and disable that chiller more than the HOW. Since as I wrote above… the HOW is easy. OFF.

      I am sure I will be posting up some articles and videos to help piece together the sequences of operation that tie together to make a Central Energy Plant (CEP) and chillers by extension… run.

  2. Blu Voltaire January 27, 2012 at 18:33

    Great post ! Thanks Abel

  3. Carol B. August 27, 2013 at 19:21

    I think it is important to know the chillers you are dealing with. We have multi chiller plants on campus where one chiller is better for certain load conditions, and automation can enable the right chiller for the particular load, We also have an ice tank as part of one chiller plant which we use to reduce power consumption when the cost of electricity is high. Making the more complex scenarios work is not without headaches, but the cost savings are worth it.

  4. Ian Willett August 30, 2013 at 08:17


    Smashing article, as a consultant, I wholeheartedly agree, give the chiller an enable / disable signal and then leave them alone from a controlling point of view. I have seen and worked on too many projects where people (designers and clients) think that they know how to control a chiller better than the manufacturers and then try to get the BMS specialist to to make the chillers work differently… recipe for disaster…. sure the BMS guys should know when to enable and disable from building data (temps / pressures), failures of other chillers, or electrical shedding loads, (the usual control stuff) or even taking data from the chillers via ModBUS / BACnet… but playing with the chillers leave well alone.

    My only other bug bear is that the chiller manufacturers on board controller are generally very slow in returning critical information via ModBUS and my thoughts are that the industry should really start standardising on a native BACnet IP solution.

    • Abel B Ramirez II August 30, 2013 at 11:29

      Very insightful info, Ian, especially coming from a consultant with your years of experience. Glad to see more diverse people and positions participating on TCF.

      I don’t forsee a native BACnet solution to chiller controls anytime soon, but almost all the larger manufacturers have some sort of OEM BACnet gateway or their own BACnet gateway add-on card. These usually will make things ‘talk’. Some are better than others, but the best ones are the ones you’ve already had to troubleshoot with the manufacturer so that on the next project you already know what to watch out for and what to tell the local service tech what files they need to upload/download to avoid problems. ;)

      Kinda strange when the Controls guy is telling the manufacturer’s chiller guy how to setup ‘his’ equipment.

  5. Haytham Ibrahim March 12, 2014 at 05:33

    Nice post and great website
    Thank you very much

  6. Haytham Ibrahim March 12, 2014 at 05:34

    Nice Post and great website
    Thank you very much!


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