What a COP of 1.0 actually means

Coefficient of performance is the least forgiving number in building mechanical engineering, because it cannot be argued with. It is cooling delivered divided by power consumed. Both terms are measurable. There is no modelling assumption to hide behind and no weather normalization to apply.

A water-cooled screw chiller in reasonable condition should return between 5.0 and 6.0. It moves five to six units of heat for every unit of electricity it draws, which is the entire reason the vapour-compression cycle is worth its capital cost. At 1.0, you have spent several hundred thousand dollars on a machine that performs like a baseboard heater running backwards.

We measured 0.88, 1.08, 1.03 and 0.99 across four valid days. Two weekend days were excluded. The spread was 0.2.

That consistency is the important part. A single bad reading is an instrument problem. A bad week is a fouled condenser or a heat wave. Four days inside a 0.2 band is a plant doing exactly what it has been told to do, every day, for a long time.

The arithmetic

Cooling output from a hydronic loop is sensible heat transfer, and it is one line:

Flow is volumetric flow through the evaporator. ΔT is the difference between return and supply chilled water temperature — how much heat the water actually picked up out in the building. And k is a constant that carries the density and specific heat of water.

Divide the result by measured input power and you have COP. That is the whole method.

The unit trap

That constant is where a surprising number of plant assessments quietly go wrong, so it is worth being explicit. k depends entirely on the units your flow meter reports:

• Flow in US gallons per minute: 1 GPM = 0.06309 kg/s, times a specific heat of 4.186 kJ/kg·K, gives k = 0.2641 kW per GPM per °C.
• Flow in litres per minute: 1 L/min = 0.01667 kg/s, giving k = 0.0698 kW per L/min per °C.

The two constants differ by a factor of 3.79 — exactly the number of litres in a US gallon. Pair a GPM reading with the litres-per-minute constant and your calculated cooling output drops to roughly a quarter of reality, and a perfectly healthy plant appears to be running at a COP somewhere around 1.3. Pair a litres-per-minute reading with the GPM constant and a genuinely broken plant looks fine.

Before you believe any COP figure — including one of ours — check which unit the flow meter was reporting and which constant was applied to it. The failure mode is silent and it is directionally plausible in both directions.

At the Calgary plant, an ultrasonic meter clamped to the chilled water supply main logged a steady 350 GPM on weekdays. With the correct GPM constant, and a loop ΔT of roughly 2.4 °C, cooling output lands near 222 kW against a metered input of around 225 kW. That is a COP of 0.99 — and it agrees with the daily energy totals independently. Two methods, same answer, which is the only reason we published the number.

Why the ΔT was 2.4 °C

Here is the diagnosis, and it is not a mechanical fault at all.

A primary chilled water loop is designed to run a ΔT of about 5 to 6 °C. The chiller makes cold water, the building's coils extract heat from air into that water, and the water returns meaningfully warmer. The temperature difference is the work being done.

When ΔT collapses to 2.4 °C, the water is going out to the building and coming back almost as cold as it left. The pumps are working. The chillers are working. Very little heat is changing hands. This is often called low delta-T syndrome, and its causes are almost entirely on the distribution side rather than in the plant:

• Oversupplied flow. Constant-speed pumps at 100% push far more water than the load requires. We observed pump drives installed at this building and not modulating.
• Coil valves not sequencing. Three-way valves or leaking two-way valves let water bypass coils entirely.
• Chilled water supply temperature set too low. Colder water than the coils need means less temperature rise across them.
• Simultaneous heating and cooling upstream. If an air handler is already overcooling supply air, the downstream coils have little left to do.

The last one is worth dwelling on, because at this building it was demonstrably happening. The penthouse air handler was driving 100% outdoor air down to 5 °C supply temperature in August, while floor-level fan coil units reheated it to something occupants could tolerate. The building was paying to make air cold, then paying again to make it warm. That single controls fault suppresses ΔT across the entire chilled water system, and it existed because an outdoor air sensor had been stuck at 24 °C for long enough that no seasonal changeover had triggered.

What we did not recommend

We did not recommend replacing the chillers.

This is worth stating plainly, because a COP of 1.0 reads like a death sentence for a machine, and there is always a vendor willing to agree with that reading. But nothing in the data pointed at the compressors. The evidence pointed at flow, staging, heat rejection and controls — four things you can fix with a service call and a controls engineer.

The sequence we recommended was:

1. Clean evaporator and condenser tubes to restore heat transfer.
2. Inspect the compressor and verify the drives are actually functioning.
3. Tune chiller staging to real load instead of running both machines continuously.
4. Recalibrate the outdoor air sensors so the demand signal means something.
5. Optimize the chilled water setpoint reset and fix coil control.
6. Rebalance the loop.

Only after that would we evaluate permanent flow and power metering, pump variable frequency drives, tower control sequences, and — if and only if the numbers still justify it — replacement machines. The interim target from maintenance and controls work alone was a COP of 4.0 to 5.0.

The reason for that order is not thrift. It is that you cannot assess the payback on a new chiller while the existing one is being fed the wrong flow at the wrong temperature by controls that think it is always 24 °C outside. You would be sizing a replacement against a load profile that is itself an artefact of the fault.

Measure before you specify

The whole campaign that produced these numbers used clamp-on instruments. An ultrasonic flow meter strapped to the outside of the supply main. Current transformers at the motor control centre. Temperature loggers held on with magnets. Nothing was cut, nothing was drilled, the plant never stopped, and no capital was requested to find out that the plant was running at a fifth of its benchmark efficiency.

That asymmetry — days of portable instrumentation against a seven-figure capital decision — is the single best return available in building engineering, and it is routinely skipped.