A space heater is not radiating directed energy.
An inexpensive diode LASER rated at one (1) Watt of output (optical) power can pop balloons from across the room, and potentially start fires. An eyeball doesn't stand much of a chance against such a device, even at greater distances.
A much higher powered flash pumped LASER can be fabricated for several hundred dollars. A 30 Watt beam is capable of welding, and punching holes in razor blades/hardened steel. If you know what you are doing, you should be able to rig a 30 Watt gas or ruby (yag) LASER in your garage, on a shoestring budget. Such a homemade LASER could indeed be extremely dangerous.
Atmospheric conditions can play a significant role in diffraction but, by its very nature, a LASER beam is capable of maintaining coherency across vast distances. Even if a 10 Watt beam were 90% diffracted by the time it reached its target, you should still have plenty of coherent photons to damage an eyeball.
Your post is off-point. Read my other posts. I wasn't comparing a space heater to a laser in the way you assumed.