We have picked our variable speed standard fan especially for its resistance to high heat. It is designed to be able to operate in high-heat temperatures, up to 60C (140F). That is very hot!
The life of the fans is very long: 70000hrs of continuous use: that works out to almost 8 years running non-stop!!
If your outlet temperature is much hotter than this, you might have air-flow impedance in your solar collector making the fan labor, and thus not move the air through quickly enough. Either that, or your solar air heater is Huge, and then you should refer to the FAQ about using multiple fans, inlets and outlets for your giant solar air heater.
The system works best when the fan acts to pull hot air out of the solar collector (through the outlet into the room near floor level). The CFM will always be greater for any fan when it works to pull air out rather than push air in to the solar collector. This is just physics at work.
Many find that in order to get good CFM at their outlet, and they want to blow air into their system at the inlet, they need a really powerful, loud, high CFM fan. However, with that same system, using a less powerful fan pulling the air out of the outlet, you can get better CFM, more air flow.