🕑 Estimated Free Cooling Hours
📏 Heat Exchanger Capacity
Sizing Breakdown
Full Calculation Table
| Component | Basis | Value | Result |
|---|
How to Use the Waterside Economizer Calculator
Choose the Canadian city closest to the project site. Colder inland climates generally provide more free cooling hours than milder coastal climates.
Enter the design chilled water load and the fraction of that load typically present during cold weather hours, since buildings rarely need full design load during winter.
Enter the chilled water supply temperature target and the plate-and-frame heat exchanger's expected approach temperature.
Click Calculate Waterside Economizer to see estimated free cooling hours, required heat exchanger capacity, and the minimum outdoor wet-bulb temperature the economizer needs.
Understanding Waterside Economizer Sizing
A waterside economizer lets a building's cooling tower provide chilled water directly through a plate-and-frame heat exchanger, bypassing the chiller compressor whenever outdoor conditions are cold enough. This differs from an air-side economizer, covered by the economizer control calculator, which brings in outdoor air directly rather than using the condenser water loop.
Why Wet-Bulb Temperature Drives the Calculation
Cooling towers reject heat by evaporating water, so tower performance depends on outdoor wet-bulb temperature rather than dry-bulb temperature, a value the wet-bulb calculator can work out from local dry-bulb and humidity readings if you don't have direct measured data. A colder wet-bulb temperature lets the tower produce colder condenser water, which the heat exchanger then uses to cool the chilled water loop. Every Canadian city has a different distribution of wet-bulb hours through the year, which is why free cooling potential varies substantially by climate zone.
The Plate-and-Frame Heat Exchanger Sets the Approach Limit
The heat exchanger's approach temperature, typically 1 to 2 degrees Celsius for a well-selected unit rated under AHRI Standard 400/401, is the closest the condenser water leaving the exchanger can get to the chilled water supply target. A tighter approach needs more plate surface area and higher upfront cost, but extends the number of hours per year the economizer can fully meet the load without any chiller assistance. The cooling tower calculator covers the tower sizing feeding this heat exchanger.
Partial Load During Cold Weather Changes the Math
Design-day chiller plant load rarely occurs during the coldest weather, since cold outdoor temperatures typically coincide with lower solar and occupancy-driven cooling need in most building types. This calculator applies a load fraction to the design chiller plant load to reflect this, since sizing the economizer heat exchanger against unrealistic full-load winter conditions overstates the required capacity, an oversizing risk ASHRAE Standard 90.1 guards against through its own economizer sizing provisions.
Waterside Economizers Complement, Not Replace, the Chiller
Even in cold Canadian climates, the chiller still handles cooling during warmer months and any cold-weather hours where load exceeds what the heat exchanger can offset alone. The economizer's value comes from eliminating compressor electricity use during a substantial share of annual operating hours, not from replacing the chiller plant, a distinction worth running through the free cooling savings calculator to see the actual energy cost impact rather than only the free cooling hour count. See the chiller plant calculator for full central plant sizing.
Frequently Asked Questions
A waterside economizer uses a plate-and-frame heat exchanger to transfer heat directly from the building's chilled water loop to the cooling tower's condenser water loop, bypassing the chiller's compressor entirely whenever outdoor wet-bulb temperature is cold enough. This lets the cooling tower alone reject building heat during cold weather, cutting chiller electricity use to near zero during those hours.
Free cooling hours vary significantly by city and required chilled water temperature, but many Canadian locations see 1,200 to 3,000 hours per year where wet-bulb temperature is cold enough to fully offset the chiller. Colder inland cities with more winter hours below freezing generally see the highest free cooling potential, while milder coastal cities see fewer hours but often at more moderate temperatures.
Plate-and-frame heat exchangers used for waterside economizing typically achieve an approach temperature of 1 to 2 degrees Celsius between the two water loops, meaning the condenser water leaving the heat exchanger gets within 1 to 2 degrees of the chilled water supply temperature target. A tighter approach requires more heat exchanger surface area and higher upfront cost.
No, a waterside economizer only provides free cooling during cold weather hours when outdoor wet-bulb temperature is low enough to produce condenser water cold enough for the required chilled water supply temperature. The chiller still runs during warmer months and any cold-weather hours where the building load exceeds what the heat exchanger alone can offset. See the chiller plant calculator for full plant sizing.
Related HVAC Tools
Size the tower feeding this heat exchanger
Full central plant sizing
Air-side free cooling alternative
Central air handling context
Freeze protection for exposed loops
Energy cost impact of free cooling