Air conditioners are sized to keep the house cool, not cool it after the house is too hot. Set the temperature in the morning when it is cooler and let the air conditioner cycle on and off to keep the temperature at the set point. If you open the windows at night after it is cooler outside than inside, don’t forget to close the windows in the morning and re-set the thermostat
Turning the thermostat lower to 15c will not cool the house any faster than setting it at 21c. Using the furnace on heat or air conditioning without a filter or with a dirty filter can allow dirt to pass through the filter and plug the furnace secondary heat exchanger and or the air conditioning coil. This can cause and expensive repair bill.
Spring in Regina is here! Be sure your air conditioner is ready with the following spring start up tips: