Should I use a pool cover between swims to keep heat in during Ottawa's cool nights?

Yes, using a solar cover (also called a solar blanket) between swims is one of the most effective and cost-efficient ways to retain pool heat during Ottawa's cool nights, and it can reduce heat loss by 50 to 70 percent overnight while also cutting water evaporation by up to 95 percent. A solar cover for a standard residential pool costs $75 to $250 depending on size and thickness, and it typically pays for itself within a single season through reduced heating costs and lower water and chemical consumption.

Ottawa's climate makes overnight heat retention a genuine challenge for pool owners. Even during July and August — the peak swimming months — overnight low temperatures in Ottawa regularly drop to 14°C to 18°C, and it is not unusual for nighttime lows to dip to 10°C to 12°C during cooler stretches. A pool at a comfortable swimming temperature of 26°C to 28°C can lose 2°C to 5°C overnight through a combination of evaporative cooling, radiative heat loss to the clear sky, and convective cooling from wind across the water surface. Without a cover, your heater or heat pump must work every morning to recover this lost heat, consuming energy and money that a simple cover would have preserved.

Evaporative cooling is the dominant mechanism of heat loss from an uncovered pool. When water evaporates from the pool surface, it carries heat energy with it — approximately 2,260 kilojoules per litre of water evaporated. On a typical Ottawa summer night with moderate humidity (50 to 70 percent) and light wind, an uncovered residential pool loses roughly 25 to 50 litres of water to evaporation, carrying away a substantial amount of thermal energy. A solar cover sitting on the water surface creates a physical barrier that virtually eliminates evaporation, and because evaporation accounts for roughly 70 percent of total heat loss, stopping it has a dramatic effect on overnight temperature retention.

The type of solar cover matters for Ottawa's specific conditions. Solar covers come in several thicknesses, typically measured in mil (thousandths of an inch) or millimetres. A 12-mil cover is the budget option at $75 to $125 and provides basic heat retention and evaporation prevention but degrades faster under UV exposure and Ottawa's freeze-thaw temperature swings at the beginning and end of the season. A 16-mil cover costs $125 to $200 and offers better insulation and durability, lasting 3 to 5 seasons with proper care. The premium option is a liquid solar cover — a non-toxic alcohol-based product that forms an invisible monomolecular layer on the water surface, costing about $30 to $50 per season — but testing shows it reduces heat loss by only 15 to 25 percent, far less effective than a physical cover.

For Ottawa pool owners who heat their pools, the energy savings from nightly covering are substantial. A natural gas pool heater running to recover 3°C of overnight heat loss in a 60,000-litre pool consumes approximately 3 to 5 cubic metres of natural gas per recovery cycle, costing roughly $1.50 to $3.00 per morning at Enbridge's residential rate. Over a 120-day season, that is $180 to $360 just in heat recovery costs. A solar cover reducing overnight heat loss by 60 percent brings the daily recovery cost down to $0.60 to $1.20, saving $110 to $215 per season. An electric heat pump is more efficient than gas but still benefits enormously — the reduced recovery demand extends the heat pump's lifespan by reducing compressor cycling and wear. Even unheated pools benefit from covering because the solar cover retains daytime solar gains, keeping the water 2°C to 4°C warmer than it would be without a cover.

The practical challenge of using a solar cover daily is handling it. Removing and replacing a large solar blanket manually is a two-person, 5 to 10 minute job that quickly becomes tedious. This is the primary reason many Ottawa pool owners buy a cover and then stop using it by mid-July. The solution is a solar cover reel — a wheeled aluminum or stainless steel roller that sits at one end of the pool and allows one person to roll the cover on or off in under 2 minutes. A reel costs $150 to $500 depending on pool width and build quality, and it transforms the daily cover routine from a chore into a quick habit. Consider the reel a mandatory companion purchase with your solar cover — without it, the cover will likely end up folded in the corner of your pool deck by August.

Using the cover during the day as well as at night provides additional benefits, though with some trade-offs. A clear or blue solar cover allows 70 to 85 percent of solar radiation to pass through to the water, actively heating the pool during sunny days by trapping solar energy — this is the "solar" in solar cover. Leaving the cover on all day except during swimming sessions maximizes heat gain and retention. However, if your pool receives full sun and the cover stays on during a stretch of hot weather with daytime highs above 32°C, the water temperature can climb above 32°C itself, which becomes uncomfortably warm for swimming and creates conditions that accelerate algae growth and chlorine consumption. In those conditions, remove the cover during the hottest afternoon hours to allow some heat dissipation.

Ottawa's shoulder seasons — late May to mid-June and September — are when a solar cover makes the biggest difference to your swimming season length. Many Ottawa pool owners close their pools in mid-September because the water gets too cold to enjoy, but a solar cover combined with a modest heater can extend comfortable swimming through the entire month of September and sometimes into early October. Daytime highs of 18°C to 22°C are common in Ottawa's September, and a covered, heated pool retains enough warmth for pleasant evening swims that an uncovered pool simply cannot match. The cover effectively adds 2 to 4 weeks to each end of your swimming season, which over the years represents dozens of additional swim days that justify the modest investment many times over.

Chemical savings from using a solar cover are the hidden bonus that most Ottawa pool owners underestimate. Chlorine degrades rapidly under UV light — an uncovered pool can lose 3 to 5 ppm of free chlorine per day to photodecomposition, even with stabilizer (cyanuric acid) in the water. A solar cover blocks UV from reaching the water, reducing chlorine consumption by roughly 30 to 50 percent. Over a full Ottawa season, this translates to $50 to $150 in chlorine savings. Reduced evaporation also means less frequent top-ups with fresh water, which means less dilution of your existing chemical balance and fewer adjustments — saving both time and money on balancing chemicals.

Looking for the right solar cover and reel setup for your Ottawa pool? Ottawa Pool Installation connects homeowners with local pool suppliers who carry covers sized for every pool configuration and can recommend the best option for your heating system and usage pattern.