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How to Use the Condenser Sizing Calculator
Pick air-cooled, water-cooled or evaporative, then the refrigerant. The refrigerant sets the saturation pressure fit used to report condensing pressure, and the condenser type sets the default approach and the wording of the temperature inputs.
Enter the cooling load in BTU/hr and the compressor COP, which is cooling capacity divided by compressor input power. The condenser must reject the load plus the compressor heat, so a COP of 3 means 133% of the cooling load and a COP of 2.5 means 140%.
Enter the design outdoor dry-bulb for air-cooled, entering water for water-cooled, or design wet-bulb for evaporative. Keep the approach at the default or enter your own, and keep subcooling at or below the approach.
Select a condenser with at least the heat of rejection plus the 10% margin at your design TD or approach. For water-cooled mode, enter the water flow to see the temperature rise and leaving water, and compare the condensing pressure with the equipment limits.
Condenser sizing for Canadian HVAC systems
The condenser has to reject everything the refrigeration circuit picks up: the heat absorbed at the evaporator plus the heat the compressor adds. That total is the heat of rejection, and it is what a condenser is selected against. An undersized condenser pushes condensing temperature up, which raises compressor power, trims capacity and can trip the high-pressure cutout on a hot afternoon.
Start from the cooling load and COP
Heat of rejection equals the cooling load times one plus the reciprocal of the compressor COP. At a COP of 3 that is 133% of the load, and at 2.5 it is 140%. The example, a 36,000 BTU/hr system at COP 3, rejects 48,000 BTU/hr (4.0 tons of rejection). If you are still working out the load itself, the cooling load calculator produces the number to enter here.
The compressor COP means cooling divided by compressor input power. A hermetic or semi-hermetic compressor puts its motor heat into the refrigerant, so it belongs in the heat of rejection. An open-drive compressor does not, so its COP should be based on shaft power. Fan and pump power are not included either way.
Air-cooled condensers, approach and TD
For an air-cooled coil the approach is the TD, the condensing temperature minus the entering air temperature. Manufacturers publish capacity per degree of TD, so dividing the required heat of rejection by the design TD gives a number you can read straight off a selection chart, as the ACHR News guide to choosing an air-cooled condenser shows. The example at 95°F air and a 25°F TD condenses at 120°F and needs 1,920 BTU/hr per °F of TD.
AHRI 460 rates remote air-cooled condensers at 95°F entering air and 125°F condensing, a 30°F TD. Scaling the example linearly to that basis gives 57,600 BTU/hr, which is useful when a catalogue lists only rated capacity. The calculator adds a 10% margin on top, and the scaling is an approximation, so confirm against the manufacturer's TD correction. Use the design temperature for your own city, since peaks vary a great deal between regions.
Condensing temperature, pressure and efficiency
Condensing temperature is entering temperature plus approach, and the calculator converts it to a saturated pressure with a fit calibrated against real-fluid data. R-410A at the example's 120°F condenses at about 420 psig. Higher condensing temperature costs efficiency: across R-410A, R-454B, R-22, R-404A and R-134a, raising it 15°F cuts ideal cycle COP by about 21-25%, roughly 1.5% per °F.
That is why a bigger coil often pays for itself in a long cooling season, and the annual energy cost calculator turns a COP change into dollars. The pressure fits cover 60-150°F condensing, and the calculator warns outside that range.
Water-cooled condensers
Water-cooled condensers reach lower condensing temperatures than air-cooled ones. The AHRI 450 standard rates them at 85°F entering water, 95°F leaving water and 105°F condensing, a 20°F approach to entering water and 10°F to leaving water. The calculator therefore defaults to a 20°F approach in water-cooled mode and also reports the approach to leaving water.
Water flow follows heat = 500 x GPM x temperature rise, where 500 is 8.33 lb/gal x 60 min/hr x 1 BTU/lb-°F. For the example, 48,000 BTU/hr at a 10°F rise needs 9.6 GPM, or 3.2 GPM per ton of cooling. The common 3 GPM per ton rule corresponds to a COP near 4. If your entered flow pushes leaving water up to the condensing temperature, the calculator stops with a message.
Evaporative condensers
An evaporative condenser approaches the design wet-bulb rather than the dry-bulb, and it can never condense below wet-bulb. A common ammonia base rating is 96.3°F condensing at 78°F wet-bulb, an 18.3°F spread, so the calculator defaults to 18°F. A smaller approach to wet-bulb means a larger condenser.
Coil fouling and selection margin
A coil that starts the season clean can drift several degrees of TD higher as dirt, cottonwood fluff or crop dust collects. The 10% margin shown in the results gives some room for that and for load uncertainty, but it is a judgement call, not a code requirement. Cleaning the coil is cheaper than a larger condenser. Pair this page with the heat of rejection calculator and the compressor sizing calculator to keep the whole circuit consistent.
Frequently Asked Questions
The condenser must reject the heat of rejection, which is the cooling load plus the compressor heat input. At a compressor COP of 3 that is 133% of the cooling load, so a 36,000 BTU/hr system needs 48,000 BTU/hr. Next choose the design condition for your own location, add a TD of 15-30°F, and compare against the manufacturer's capacity per °F of TD. At 95°F outdoor air and a 25°F TD, condensing is 120°F, and the same system needs about 1,920 BTU/hr per °F of TD, or about 57,600 BTU/hr when converted to the 30°F TD basis that AHRI 460 uses for ratings. Add a margin of about 10% for coil fouling and load uncertainty.
Approach is the condensing temperature minus the entering air, water or wet-bulb temperature. For an air-cooled coil it is also called TD. A smaller approach needs a larger condenser but lowers condensing temperature. Real-fluid property data put the cost of higher condensing at roughly 1.5% of ideal cycle COP per °F, so 15°F of extra approach costs about 21-25% for R-410A, R-454B, R-22, R-404A and R-134a. Higher condensing temperature also raises compressor power and discharge pressure, which is why the calculator reports the saturated condensing pressure.
Water-cooled condensers run at lower condensing temperatures. AHRI 450 rates them at 85°F entering water, 95°F leaving water and 105°F condensing, against 125°F condensing for an air-cooled coil rated at 95°F air under AHRI 460. That 20°F gap is worth roughly 30% in ideal cycle COP on the 1.5%-per-°F rule. The saving is largest on big systems with long running hours, which can justify the capital cost, water treatment and tower maintenance. Small residential and light commercial systems almost always stay air-cooled, because the tower's added complexity does not pay back at that scale.
Fouling raises the approach needed to reject the same heat. If a clean coil runs at a 20°F TD and dirt pushes that to 30°F, condensing temperature rises 10°F, which costs about 15% of cycle COP on the 1.5%-per-°F rule and raises discharge pressure toward the high-pressure cutout on the hottest afternoons. Cottonwood fluff, crop dust and industrial fallout all speed this up. Inspect the coil at least once each cooling season and more often in dirty locations, and select with the 10% margin the calculator shows so the unit still meets the load when the coil is partly fouled.