Pool Cover Evaporation and Heat Loss Savings

On most outdoor pools it is the largest item in the heating bill. Every pound of water that leaves the surface takes its latent heat of vaporisation with it -- about 1,046 BTU -- so a quarter inch a day off a modest pool is over a million BTU a day, before any consideration of the air temperature. EVAPORATION DOMINATES, AND THE INTUITION DOES NOT. Radiation, convection and conduction to the ground are all real and all smaller, and an owner reasoning about pool heat loss reasons about how cold the air is. That is why the most effective single measure is a cover and the second is a windbreak: wind drives evaporation hard, so a sheltered pool and an exposed one at the same air temperature lose quite different amounts. The evaporation rate is entered rather than derived precisely because it depends on wind, humidity and the water-to-air temperature difference together, and a single figure for a site is worth more than a formula applied to the wrong conditions. A COVER ONLY WORKS WHILE IT IS ON, which is the arithmetic owners skip. Effectiveness and hours multiply: a 90 percent cover on for two thirds of the day saves 60 percent of the evaporative loss, not 90. The hours that matter most are overnight, when the air is coldest and driest and evaporation is fastest, so a cover pulled on at dusk and off in the morning captures more than its hours alone suggest. THE FUEL SAVING IS LARGER THAN THE HEAT SAVING, by the heater's efficiency. Heat not lost is fuel not bought, and dividing by the efficiency is the step that turns a thermal figure into a bill -- an 82 percent heater means every MMBTU of pool heat saved is 1.22 MMBTU of fuel. And the water is a second saving the heat figure hides: the gallons evaporated are treated, balanced water, replaced with cold make-up that then has to be heated and re-balanced. This computes from an entered evaporation rate. It does not predict evaporation from weather conditions (which needs wind speed, humidity, and water and air temperatures, and is the largest uncertainty here), model the other loss paths or the pool's total heat load, size a heater, account for splash-out, backwash or leaks -- which a make-up water figure cannot distinguish from evaporation -- evaluate a cover's insulating value when the pool is warmer than the air, or address the safety requirements a pool cover carries. ASHRAE Applications for pool evaporation, the cover manufacturer's data, and the pool professional govern.

evaporative loss = surface area x evaporation depth x 7.481 gal/ft^3 x 8.34 lb/gal x 1,046 BTU/lb of latent heat; a cover's saving is that loss times its effectiveness times the fraction of the day it is on, and the FUEL saving divides by the heater efficiency.

Pool evaporative heat loss on the latent heat of vaporisation. The evaporation RATE is ENTERED because it depends on wind speed, humidity and the water-to-air temperature difference together, and is by far the largest uncertainty in the result.

One latent heat constant and two unit conversions.

Estimate. AHJ and licensed professional govern.

Field names used by the API: surface_area_ft2, evaporation_in_day, cover_effectiveness_pct, cover_hours_per_day, heater_efficiency_pct, fuel_cost_per_mmbtu, season_days, gallons_per_day, pounds_per_day, mmbtu_per_day, cover_saving_mmbtu_day, season_saving_mmbtu, fuel_mmbtu, season_saving_cost

• Evaporation rate is entered it depends on wind, humidity and the temperature differencea site measurement or ASHRAE Applications
• Cover effectiveness times hours a cover off the pool saves nothingthe cover manufacturer's data
• Make-up water is not all evaporation splash-out, backwash and leaks read the samethe pool professional