Why does our porta potty smell like ammonia after a hot day?

That ammonia smell is urine breaking down into ammonia gas, which happens faster in heat. The deodorizer in your standard unit works by masking odors and breaking down waste, but hot weather speeds up bacterial action in the holding tank. When the temperature rises, the bacteria consume the deodorizer's active ingredients more quickly, and once they're gone, the urine starts to decompose on its own. By Sunday, three days after the Friday service, the deodorizer is likely spent.

You can ask the rental company to increase the service frequency to twice a week during hot spells. More frequent pumping removes the waste before it breaks down. They might also use a stronger or heat-stable formaldehyde-free deodorizer, but that won't help if the tank isn't pumped often enough. Weekly service is standard, but heat, heavy use, or long weekends often require extra visits.