DESA recommended annual replacement of the nozzle because they wear out and start producing a spray rather than a fog.

Your eyes burn because the flame is burning too rich or too lean. Always start troubleshooting a fuel delivery problem by setting the pump pressure.

If the pressure is correct (don’t guess), the factory recommended fuel is good, you have checked for a restricted fuel filter, a cracked end cover, air or vac leaks between the pump/nozzle and tank/nozzle then it is time to replace the nozzle with the nozzle that is recommended for your model of heater.

In addition, a fan can cause the same problem if  damaged or mis-matched to your specific model.