Can I use an Ottawa city garden hose to top up my pool or will it affect water balance?

Yes, you can absolutely use your Ottawa city garden hose to top up your pool, and this is the standard method used by virtually every residential pool owner in the city — but the incoming water will affect your water balance, and you need to account for it. Ottawa's municipal tap water has specific chemical characteristics that interact with your pool chemistry, and understanding these interactions prevents surprises after topping up.

Ottawa draws its drinking water from the Ottawa River, treats it at the Britannia and Lemieux Island water purification plants, and distributes it across the city. The treated water that comes out of your garden hose has a pH typically between 7.2 and 7.8, total alkalinity of 20 to 40 ppm, calcium hardness of 30 to 75 ppm, and contains chloramine (combined chlorine) rather than free chlorine as the residual disinfectant. These values fluctuate seasonally — spring runoff tends to lower mineral content while late summer concentrations increase slightly as river levels drop. For pool owners, the most important characteristic is that Ottawa's tap water is relatively soft (low in calcium and minerals) compared to water in many other Ontario cities.

The immediate effect of adding Ottawa tap water to your pool depends on how much you are adding relative to your pool's total volume. A standard residential inground pool holds approximately 55,000 to 75,000 litres. Topping up 2 to 5 centimetres of water to replace evaporation and splash-out adds roughly 500 to 1,500 litres — less than 2 percent of total volume. This small addition has a negligible effect on your water balance and rarely requires any chemical adjustment beyond your normal routine. However, if you are refilling a significant portion of your pool — after a major backwash session, a partial drain for repair work, or a large pool party that splashed out substantial water — the fresh tap water can measurably shift your chemistry.

The pH impact is usually the most noticeable. If your pool is maintained at the ideal 7.2 to 7.4 range and you add a large volume of Ottawa tap water at pH 7.6 to 7.8, your pool's overall pH will creep upward. This matters because pH above 7.6 dramatically reduces chlorine's effectiveness — at pH 7.8, only about 30 percent of your free chlorine is in the active hypochlorous acid form, compared to 66 percent at pH 7.2. After any significant top-up, test your pH within a few hours and add muriatic acid or dry acid (sodium bisulphate) as needed. A litre of muriatic acid costs $8 to $15 at Ottawa pool supply stores and lowers pH in a 60,000-litre pool by approximately 0.3 points.

Total alkalinity is the parameter most affected by Ottawa tap water over the course of a season. Because Ottawa's tap water has relatively low alkalinity (20 to 40 ppm), repeatedly topping up your pool gradually dilutes your total alkalinity downward. Low alkalinity — below 80 ppm — causes pH to become unstable, bouncing up and down in response to small chemical additions or environmental factors like rain, bather load, and CO2 off-gassing. If you notice your pH becoming difficult to control as the summer progresses, test your total alkalinity and add sodium bicarbonate (baking soda) to bring it back into the 80 to 120 ppm range. A 5-kilogram bag of pool-grade sodium bicarbonate costs $10 to $20 and raises alkalinity in a typical Ottawa pool by approximately 15 to 20 ppm.

Calcium hardness tells a more interesting story in Ottawa pools. Fresh tap water arrives with low calcium (30 to 75 ppm), but your pool's calcium level trends upward over the season because evaporation removes pure water while leaving dissolved minerals behind. Every litre that evaporates concentrates the remaining calcium slightly. In a typical Ottawa summer — where a pool can lose 3 to 5 centimetres of water per week to evaporation — calcium hardness can climb from the opening level of 150 ppm to 300 ppm or higher by September if you only top up with small amounts. Paradoxically, adding large volumes of low-calcium tap water actually helps prevent excessive calcium concentration by diluting the pool water. The ideal calcium hardness range is 200 to 400 ppm for plaster pools and 150 to 250 ppm for vinyl liner pools, which is what most Ottawa residential pools have.

One important consideration specific to Ottawa is that the city uses chloramines rather than free chlorine for residual disinfection in the distribution system. Chloramines are chlorine molecules bonded to ammonia, and they are more stable than free chlorine for maintaining disinfectant levels through long distribution pipes. When chloramine-treated tap water enters your pool, it adds to your combined chlorine reading rather than your free chlorine reading. Combined chlorine (chloramines) is a poor sanitizer and causes the "chlorine smell" and eye irritation that people mistakenly attribute to too much chlorine. After a large top-up, running a shock treatment burns off the chloramines and restores your free chlorine level. A bag of calcium hypochlorite shock costs $5 to $15 and treats a standard residential pool.

The practical routine for topping up an Ottawa pool is simple. After adding water, wait 2 to 4 hours for it to circulate and mix fully, then test pH, free chlorine, and total alkalinity. Adjust as needed. If you added more than 5 percent of your pool's volume, also test calcium hardness and cyanuric acid (stabilizer), since both are affected by dilution. Keep a log of how many centimetres you add each week — this data helps you understand your pool's evaporation rate and plan your chemical usage more accurately. Most Ottawa pool owners find they add between 2 and 8 centimetres per week depending on temperature, wind exposure, bather load, and whether they use a solar cover.

Using a hose filter is an optional but worthwhile investment for Ottawa pool owners who top up frequently. A garden hose pre-filter filled with carbon media removes chloramines, sediment, and some dissolved metals from the incoming water before it reaches your pool. These filters cost $25 to $60 and last for approximately 40,000 to 80,000 litres of filtration. While not strictly necessary — Ottawa's tap water is perfectly safe for pool use without pre-filtering — a hose filter reduces the chemical adjustment you need to make after each top-up and can extend the life of your pool's main filter by reducing the load of dissolved contaminants.

Need help dialling in your pool's water chemistry after filling with Ottawa city water? Ottawa Pool Installation connects homeowners with local pool professionals who understand the specific characteristics of Ottawa's municipal water supply and how to balance your pool accordingly.