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The Cx Files

Chapter 3: When it Does Not Add Up

Jun 16, 2022

Quince Taylor CCP

Our Commissioning team performed forensic detective work on a chilled water (CHW) system. The CHW system measurements reflected a high volume of water leaving the CHW pump and too little chilled water at a large number of individual HVAC units. The water out from the pump must equal the water circulating in the system in a closed loop hydronic system. This does not add up…unless there is a major leak! So, where did the water go?

In addition to leaving the Testing, Adjusting, and Balancing (TAB) team unable to continue, left unattended the problem would lead to wasted energy and unstable room temperature control.

Let us look at the where and what followed by the how and why?


The new five level residence building consisted of three levels above grade containing 302 hotel rooms, with the two levels below grade, consisting of a full basement and full cellar. A core module, also five levels, tied the three arms of the building implemented in three project phases. Fan-coils equipped with a hydronic chilled water and heating hot water supplied HVAC to each room.


The mechanical contractor installed and started the hydronic (CHW pump, tanks, valves, etc.) and HVAC equipment. The TAB team measured the CHW pump performance. Everything was fine up to this point. The TAB project scope involved differential pressure readings of the CHW automatic flow limiting balancing valves (FLVs) on the 350 fan-coil units and 6 air-handling units. The CHW pump differential pressure (DP, ΔP) setpoint was set to reach a minimum of 2.0 ΔP at the FLV on the worst-case unit. This allows for efficient system operation and better room temperature control.

Let us go over to a technical detail for a moment. The CHW pump circulating the water ran at the nameplate motor amperage. However, several CHW FLVs installed on fan-coils measured less than the minimum required pressure drops of 2.0 ΔP mentioned earlier to confirm design waterflow.

The numbers did not add up: The CHW pump performance data reflected design waterflow and the fan-coil units indicated not enough waterflow.

How & Why

How and why did this happen? Troubleshooting should not be looking for a needle in the haystack. We started with a simple “could be/no way” list approach. This is a process where we cut the obvious and then progressively eliminate variables. This process allows the team to compare this system to one that is working properly.

No Way...
  1. Electrical issue? No way. This is mechanical in nature.
  2. System leak? A leak would show up with water puddling on the floor or flooding rooms. In addition, the make-up water meter would show water entering the system to make-up for water leaving the system.
  3. Could you add to the “No way” list?

Could it Be...
  1. A pump rotation issue? A pump spinning backwards may still show flow and positive pressure. The Cx team did not want to assume so we verified the pump rotation again.
  2. The hydrometer used to take the readings? The hydrometer used to measure the water pressure differential had a very recent calibration certification from the manufacturer. However, the TAB team used a different calibrated meter to eliminate a faulty meter from the equation.
  3. Inaccurate pump performance data? To verify the waterflow determined using the pump curve, the team used an ultrasonic strap on meter. The flow from the ultrasonic meter and the pump curve matched within 5%! In addition, the system’s flow transmitter was also within an acceptable percentage of the pump curve and ultrasonic meter.
  4. Piping distribution issues? Could be! Perhaps a bypass valve?
  5. Fan-coil component issues? Should we trust the FLV pressure measurement? What issues could result from a faulty fan-coil hydronic component?

We used the same forensic tool for the piping distribution and fan-coil troubleshooting: Differential static pressure profile.

The team prepared a system piping diagram based on the mechanical drawing piping detail. The readings were noted on the diagram to see the big picture. The testing started by measuring the differential pressure between the return and supply mains using pressure-temperature access ports. Some were installed at each drain as a temporary measure to record the differential pressure across the riser return and supply pipes. It added up!

The pressure drops across the cooling control valve and cooling coil both showed well above design waterflow on certain units and low on other units. We are getting warm. Why was the pressure drop across the FLV exceptionally low? What could cause a FLV to have above design waterflow and below design pressure drop? The question is how to go from “could be” to “this is it”?

An inspection of the FLVs revealed that the manufacturer placed the flow direction sticker in the wrong direction on a batch of FLVs, resulting in the FLV being installed backwards. Once reversed to the correct direction, the pressure drop reached the required 2.0 ΔP.

The head scratcher continued! Although certain FLVs were installed in the correct direction, the pressure drops on other FLVS indicated low pressure drop with above normal pressure drop across the cooling coil. We put the FLV under the Cx microscope by disassembling it. [See image below].

Hayes Fluid Controls

The rubber diaphragm is position sensitive and when installed backwards or even missing forces the valve wide open with flow or allows for full flow through the valve, hence low pressure drops although lots of water is moving through the FLV. The rubber diaphragm was removed prior to soldering but installed backwards or never reinstalled in some cases during the installation process. The install team corrected the issue by following the FLV Installation, Operation, and Maintenance (IOM) Manual install procedures to wrap valve body with a wet rag.

What can we take away from something that did not add up?

  1. Quality Assurance is a fundamental part of the construction process and that includes following the procedures outlined in the equipment and part IOM Manuals. The manufacturer detailed a solution to the valve soldering issue in the IOM manuals. On the other hand, the misplaced valve direction sticker was a Quality Control issue beyond the project.
  2. TAB is indispensable during the construction project. In addition to helping with energy conservation and temperature control, TAB is a troubleshooting and mechanical system health tool. As a result, our Cx team TAB report review occurs before the functional testing phase.
  3. Troubleshooting should not involve looking for a needle in a haystack. The Cx team which included the mechanical and TAB contractors dissected system to narrow down the issue using actual system measurements. Viewing the system pressures at each major connection in the system and across each fan-coil component allowed us to focus our efforts on the real cause in less time.

Once it all added up, the system was balanced, energy payback rebates were set in motion, and the system supplied stable temperature control to the rooms.

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