I thought these Sunpak units used gas, why do I need a transformer?

The Sunpak series does use gas (natural or propane) as fuel for combustion. Electricity is needed to power the electronic ignition system. The Sunpak units operate on 24 Volts AC power only. A small step down transformer is needed to reduce line voltage ( 120 VAC) down to 24 VAC. The size of the transformer has nothing to do with the radiant output of the heater.