What Aprilaire humidifiers work with horizontal heating systems?

Consumer question:

I just had to return a Model 700 because I have an attic hvac unit that is horizontal. We live in the desert in St. George, UT and have a 1,450 sf house that is 8 years old. What unit would you recommend for our horizontal attic unit?

Aprilaires' response:

Thank you for your email regarding your interest in our Aprilaire humidifiers. The only model that will not work on a horizontal heating system is the model 700. The humidity level is reduced by 50%. We wold recommend the mode 600 for your application.

Thank you for giving us the opportunity to be of service