Ask Tim: What should my set point be for my cooling tower water temperature?

“What is the best temperature for my water condenser?” I have encountered this question a number of times during my first weeks at Stromquist & Company.

It is common knowledge that lowering the cooling tower sump temperature set point increases a chillers efficiency. As we lower the cooling tower sump temperature the condenser becomes sub cooled.

Sub cooling can increase the chiller’s efficiency about 1% per degree of sub cooling. Increased chiller efficiency means energy savings.

It is important to remember that there is a limit to this type of energy savings.

For example, on Trane chillers efficiency improvement bottoms out at around a 75 degree set point.
Lowering the set point past 75 degrees results in an increase in power consumption from the tower fan and pumps without improving the chiller efficiency.

It is important to maintain accurate temperature settings with all chillers. Since my background is Trane, I will speak specifically about set points when using a Trane Chiller.

If the temperature is too low, within the 60 degree range, you risk the oil carrying over into the evaporator. If the temperature is too high, above 75 degrees, the chiller is consuming excess energy and increasing your monthly energy costs.

The correct temperature range for Trane Chillers is between 72 and 75 degrees. Trane chillers that operate within these dimensions are maximizing energy savings and minimizing the risk of oil carrying over into the evaporator. In conclusion, save on energy costs and the environment and check your chiller set points.

Using a good remote bulb temperature controller like the Johnson System 450 or the Honeywell T775 is another important thing to consider. If your temperature controller is not accurate or does not control properly then any possible energy savings will be negated. So make sure you have a quality remote bulb temperature control.

I will be blogging at so send your questions about chillers, motors, and variable frequency drives to me. My email address is for your answers at Ask Tim.

Also, check out my chiller classes.

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7 Responses

  1. HVAC system designers got excited when affordable Variable Speed Drives (VSD) became available. They remembered that horsepower varied with the cube root of flow (fan/pump laws #3). So at half flow, 1/8 horsepower was required. But at closer examination, this was only true for a static distribution (one with no valves or dampers that restricted flow in order to maintain control) systems. To approach the savings predicted by law #3, applying this to a VAV AHU fan required a control system that tried to keep the VAV box dampers as open as possible; i.e. the fan pressure should be reset down in order to keep the most demanding box damper full open. If all boxes throttled their dampers to less-than-open positions, energy would be wasted.

    Throughout HVAC systems, controls typically throttle valves and dampers to control flow, and the throttling effect chews up horsepower. With DDC systems and network communications of today’s control systems, the better fan controllers can monitor the box damper positions and adjust the fan pressure setpoint to minimize this wasted horsepower.

    Looking around large HVAC systems, there are few loads that can approach efficiencies suggested by law #3, and one is the cooling tower fan. In my opinion, this is a perfect place for a VSD. As condenser water temperature varies, the fan speed can be modulated with no losses through modulating valves or dampers. And with PI control, temperature can be maintained at relatively constant setpoints. Compare this to using an on-off (or 2-stage) cooling tower thermostat that cycles the fan(s). To minimize fan short cycling, significant water temperature differentials (3 to 4 degrees?) between on and off setpoints are required, so the chiller efficiency and output continuously varies (minimally). Using VSDs on tower fans make lots of sense. If towers must operate in freezing outside air periods, at minimum fan speeds (dictated by the VSD specifications), a 2-position (on-off) algorithm may be invoked to cycle the fan on and off (but at the minimum speed) until the load rises. Typical Honeywell and Johnson remote bulb controllers will probably not have network communications and may not have PI control capability. A central DDC system may be necessary.

  2. ddc system
    for sensing temperature & drive for cooling tower fan. complete details
    optimum use for cooling tower fan.

  3. I’m having an on going problem with my tower water system. It used to be within a few degrees of the set point. Now it’s what ever it wants it seems.For example this week alone with temp out side 20 degrees it ranged from 39 to 65 degrees. The set point is 65. In the summer if its 90 degrees out side the tower is about 80-85 degrees. My boss says it’s ambient temperature. Isn’t one of the reasons for a tower system is to control your water temp with in reason so you can have consistent molding. I really have no idea from day to day of what I really have & it gets tiring to hear it all depends on what the outside temp is.
    Thank You I hope you have an answer I can live with. BILL

  4. I like this !
    Coolint Towers should have inteligent setpoint based on chillers limits , cooling tower performance and outside WetBulb temperature variations for every minute on each day.
    We have implemented this on DDC controller and we save a lot of ennergy for our customers.
    Another bennefit working with our control system is that we will have less fouling on the condenser tubes due the lower water temperatures.
    Savings depend on the tipe of chiller for example VSD chillers will save average 2% for every 1°F lower temp. on the cooling tower.
    Normally the lowest cooling water temp starts on 60°F but there are limitations for machine models and also the moment the machine is running , for example at start-up we recomend work with minimum of 25°C for some screw chillers.

  5. Referencial de expectativas a fim de que desenvolvimento da competência leitora e também escritora no interior de ciclo II do ensino fundamental.

  6. I have dual compressor chiller. The chiller produce at 40 deg f. And 8000 gpm Chill water. How do I sizing the cooling tower?

  7. Great post and great advices! Love the yellow top on you! As always, love your style!

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