Chiller selection: efficiency, IPLV and part-load

Choosing a chiller is often treated as a matter of capacity and price, with efficiency checked against a code minimum. That misses most of the story. A chiller sized for the hottest afternoon of the year spends the large majority of its operating hours well below full load, at cooler condenser conditions than design. How efficient it is in those hours determines most of its lifetime energy cost.

This guide explains the efficiency metrics used in chiller ratings and code tables, what they do and do not tell you, and how to compare selections in a way that reflects real operation.

Efficiency metrics

The conversions are simple: COP equals 3.517 divided by kW/ton. A water-cooled centrifugal chiller at 0.55 kW/ton has a COP of about 6.4.

Full-load efficiency

Full-load efficiency is measured at the rated design condition. It matters for peak electrical demand and for sizing electrical service, but it represents only a small fraction of annual operating hours for most comfort cooling plants.

IPLV

IPLV is defined in AHRI Standard 550/590, the North American rating standard for water-chilling packages. It weights efficiency at four load points:

The weightings reflect an aggregate of building types and climates across the United States. They show the important point clearly: most operating hours occur at partial load with relieved condenser conditions. Note that IPLV calculations use efficiency in COP terms for the weighting; consult the standard for exact methods.

IPLV is a useful comparison tool, but it is not a prediction of annual energy for a specific building. A data centre with a flat load profile, a hospital with long operating hours, and a school closed in summer will all differ from the IPLV weighting.

NPLV

When a chiller is selected for conditions other than the AHRI standard, such as a higher chilled water temperature or a different condenser flow, manufacturers report NPLV using the same weightings at those conditions. Code compliance usually allows NPLV to be compared to adjusted minimums.

Code minimums in Canada

NECB 2017 and NECB 2020 set minimum full-load and part-load efficiencies for chillers by type and capacity, and recent editions include two compliance paths: one favouring full-load efficiency and one favouring part-load efficiency, similar to the approach in ASHRAE 90.1. Provinces adopt NECB editions on their own schedules, and some municipalities set stricter requirements, so the applicable edition depends on project location and permit date. See NECB 2020 and HVAC compliance.

Code minimums are a floor. High-performance selections commonly beat them substantially, particularly at part load.

Why part load dominates

Building cooling loads vary with weather, occupancy and internal gains. In Canadian climates, design conditions occur for only a handful of hours a year, and outdoor wet-bulb temperatures are lower than design for most of the cooling season. Lower wet-bulb means colder condenser water from cooling towers, which reduces compressor lift and improves efficiency, especially for variable speed chillers. See part-load performance: why buildings rarely run at design.

Plants with several chillers further change the picture. Individual chillers may run at higher load fractions than the building does, depending on sequencing. The relevant question is not just "how efficient is this chiller at 50 percent?" but "what loads and condenser temperatures will each chiller actually see?"

A practical selection procedure

1. Build a load profile. Use energy modeling or measured data from an existing plant to estimate hours at each load and wet-bulb or ambient temperature.
2. Set the plant configuration. Decide the number and size of chillers, redundancy, and whether chillers are equal or unequal in size.
3. Define design conditions. Chilled water supply and return temperatures, condenser water temperatures and flows, fouling factors, and glycol if any.
4. Request performance data at multiple points. Ask manufacturers for efficiency at a matrix of loads (for example 10 to 100 percent) and condenser temperatures, not just full load and IPLV.
5. Calculate annual energy for each candidate by combining the performance data with the load profile and the planned sequencing.
6. Include auxiliary energy. Pumps and tower fans change with chiller choice, especially when condenser flow or approach changes.
7. Compare life cycle cost. Weigh first cost, energy, maintenance and expected life. See life cycle cost analysis for HVAC options.
8. Check other factors. Refrigerant, sound, footprint, minimum load, turndown, service support and controls integration.

Selection details that affect efficiency

• Chilled water temperature: each degree warmer supply reduces lift and improves efficiency. Supply temperature reset adds savings at part load.
• Condenser water temperature: lower entering condenser water improves efficiency, within the chiller's minimum limits.
• Flow rates and pressure drops: low evaporator and condenser pressure drops save pump energy; higher velocities improve heat transfer. The trade-off should be evaluated.
• Variable speed drives: generally improve part-load efficiency, with the largest benefit where condenser water temperature falls significantly at part load.
• Fouling factors: specifying higher fouling factors than necessary oversizes heat exchangers. Use AHRI standard values unless water quality justifies more.
• Minimum load and turndown: a chiller that cannot turn down far enough will cycle or need hot gas bypass at light load.

Frequently asked questions

Should I choose the chiller with the best IPLV?

IPLV is a good screening tool, but annual energy at your plant's actual load profile and sequencing is the better basis for selection.

Does a variable speed drive always pay back?

Not always. It pays best where the chiller runs many hours at part load with relieved condenser temperatures. Plants with constant high loads or high condenser temperatures see less benefit.

Key takeaways

• COP and kW/ton describe efficiency at one condition; IPLV and NPLV summarize part-load performance.
• Most chiller operating hours are at part load with cooler condenser conditions than design.
• Code minimums under NECB are a floor, not a target.
• Compare chillers using annual energy calculated from a real load profile and sequencing plan.
• Plant-level choices on temperatures, flows and drives matter as much as the chiller model, and energy modeling is the best way to test them.