Night Purge Cooling Savings Calculator
See how quickly cool evening air can reset your indoor temperature, how many kilowatt-hours that translates to, and the dollar value of a nightly window-flushing routine.
See how quickly cool evening air can reset your indoor temperature, how many kilowatt-hours that translates to, and the dollar value of a nightly window-flushing routine.
When a heat wave lingers for days, conventional wisdom suggests closing every window, sealing curtains, and relying on mechanical cooling. Yet in many climates, the outdoor temperature still dips well below indoor comfort levels after sunset. A night purge—sometimes called night flushing—capitalizes on that gap by drawing cool outdoor air through the home before the next day begins. Doing so removes sensible heat stored in the air, furnishings, and lightweight interior surfaces. The Night Purge Cooling Savings Calculator quantifies exactly how much heat leaves the building, the resulting indoor temperature by sunrise, and the kilowatt-hours of compressor runtime that the purge can offset. Instead of guessing whether it is worth cracking windows or running a whole-house fan overnight, you receive a data-backed answer expressed in temperature, energy, and dollars.
The tool asks for details you already know or can estimate: floor area, ceiling height, and a simple thermal mass factor that represents how much furniture, drywall, and masonry store heat per degree of temperature change. Combine those with the ventilation rate you can achieve (whether by a dedicated fan, stack effect, or strategic window placement), the difference between indoor and outdoor temperatures, and the duration of the purge, and the calculator simulates how fast the interior approaches the outdoor night air. We also incorporate your cooling system’s efficiency and utility rate so the results connect directly to your electricity bill. For households managing peak pricing programs or renewable energy enthusiasts trying to minimize air-conditioner use, this quick analysis transforms night breezes into actionable savings.
The math behind night purging follows the same exponential decay that governs any first-order thermal system. As fresh air enters and mixes with indoor air, it transports heat away at a rate proportional to the temperature difference. We treat the home as a single thermal node with two sources of heat capacity: the air itself and the furnishings, walls, and flooring represented by your thermal mass input. Let denote total heat capacity in BTU per degree Fahrenheit and the volumetric airflow rate in cubic feet per minute. The governing differential equation is
because every cubic foot per minute of airflow carries roughly 1.08 BTU of sensible heat per degree of temperature difference. Solving that equation yields an exponential approach to the outdoor night temperature. After a purge lasting hours, the indoor temperature becomes . Multiplying the resulting temperature drop by the total heat capacity reveals how many BTU of heat left the home. Divide by your air conditioner’s efficiency (expressed in BTU per watt-hour) and we obtain kilowatt-hours avoided. It’s a straightforward energy balance presented in plain English so you can make decisions quickly.
Picture a 1,800-square-foot bungalow with 8.5-foot ceilings, a modest mix of drywall and furnishings that equate to about 35 BTU of thermal mass per square foot, and an indoor temperature drifting up to 80 °F by evening. The outside air cools to 62 °F overnight, and the owners can run a whole-house fan that averages 900 CFM for six hours. Plugging those numbers into the calculator shows the interior dropping to roughly 66 °F by sunrise, removing about 120,000 BTU from the building. With an 11 EER air conditioner, that savings equals around 10.9 kWh, or $1.74 at a $0.16 per kWh tariff. The bungalow starts the next day already cool, delaying the time when the compressor must kick on. If the heat wave lasts a week, the household pockets more than $12 and roughly 76 kWh of avoided consumption, all by opening windows and running an efficient fan before bedtime.
| Airflow (CFM) | Final indoor temp (°F) | Heat removed (kBTU) | Cooling avoided (kWh) | Estimated savings ($) |
|---|---|---|---|---|
| 600 | 69.9 | 92.1 | 8.4 | 1.34 |
| 900 | 66.2 | 120.3 | 10.9 | 1.74 |
| 1,200 | 64.0 | 136.7 | 12.4 | 1.99 |
The table illustrates a key insight: once airflow becomes high enough to achieve several air changes overnight, each incremental CFM produces diminishing returns. You can use the calculator to test whether investing in a higher-capacity fan or adding additional window openings will pay back through reduced energy use. It also helps renters understand how much difference crack-opening a bedroom window plus a box fan can make compared to keeping everything sealed.
Night purging works best when paired with daytime shading, controlled internal gains, and intelligent thermostat settings. If you also pre-cool during the late evening using mechanical cooling, compare strategies with the building-pre-cooling-energy-savings-calculator.html. Once the sun rises, ceiling fans and smart thermostats help maintain comfort with less compressor time; the smart-thermostat-vs-manual-savings-calculator.html highlights how automation can lock in those gains. If you are planning a broader retrofit, pair this analysis with the net-zero-home-retrofit-roadmap-calculator.html to see how envelope upgrades and electrification amplify the benefits of night flushing.
Implementing a night purge safely requires attention to filters, insect screens, and security. Whole-house fans should be equipped with backdraft dampers to prevent heat gain during the day. If you use window fans, secure them against tipping and ensure only screened windows are opened. In wildfire-prone regions, avoid purging when outdoor smoke levels are high or install MERV-rated filters designed for high airflow. Monitor humidity as well; in muggy climates the added moisture may counteract sensible heat gains, making the practice less effective. The calculator does not account for latent loads, so use it as a first-pass screening tool and adjust for humidity if you live in a coastal or tropical environment.
The model treats your home as a single thermal zone with uniform mixing, an approximation that breaks down in large or multi-story layouts. Real homes have rooms that cool faster than others depending on exposure and ventilation pathways. The airflow input assumes a steady average during the purge; if wind gusts fluctuate dramatically, actual results will vary. We also ignore internal heat gains from appliances, people, and electronics during the purge window. If you run a clothes dryer or leave lots of equipment on overnight, the energy removal calculated here will overestimate reality. Latent heat from humidity removal is omitted, so in humid climates the savings may be lower because the air conditioner must still wring moisture out the next day. Finally, the EER input assumes constant equipment efficiency. Variable-speed heat pumps can perform better at part load than their nominal rating, while older units may degrade in efficiency during long cycles. Use the tool as a planning baseline, then refine with measured data from smart thermostats or whole-house energy monitors.
Despite those simplifications, the Night Purge Cooling Savings Calculator provides a transparent, physics-based snapshot of how much value lies in evening breezes. Whether you are trying to ride out a grid alert without overloading the system or simply want to sleep cooler without cranking the AC, quantify the gains here, experiment with different airflow strategies, and revisit the model whenever your home layout, fan capacity, or energy prices change.