That is a nice size for an aviary. Do you know if the shed is insulated?
This is important for both summer and winter in a temperate climate. Tropical parrots do not tolerate extreme temperature shifts and will do best in a climate-controlled environment. You mentioned a heat fan. Is that a dish heater like this or something similar?
You will definitely want to provide heat at night and during fall/winter/spring. Just be sure to confirm that the heating unit is bird safe and not in a position that would allow the birds to get burned or electrocuted.
In the summer, you will need to make sure the shed has adequate ventilation and cooling. Insulation can help here too. A macaw should be fine with temperatures between 20 to 25 Celsius, but if you are looking at temperatures over 30 inside the shed, that is not okay. The bird will likely show signs of over-heating at those temperatures.
Ideally, you would want to be able to provide consistent comfortable temperatures, day and night, through out the year. This could be accomplished by fully insulating the interior and installing a central heating and cooling unit, similar to what you would use in a human dwelling. Or by setting up a smaller air conditioner and heater in a location that is protected from the birds, for example in the safety porch.
A safety porch is a small entry space into an aviary or bird room which has two doors - one leading into the aviary and one to the outside. It acts like an "airlock" to prevent birds from flying out through an open door. You will want to build a safety porch either inside or directly outside of any doors that lead into the shed. Since this area will not be normally accessible to the birds, it is an ideal location for heaters, fans, and other potentially hazardous electronics or other equipment.