I don't remember which spelling I was told, but always use install. I recently discovered the expression a new instalment and my dictionary indicates the spelling installment in the US, which led me to check whether instal also exists. Surprisingly to me install and instal are listed equivalent and without reference to a specific country.
I'm curious about the use of each spelling in British and American English.
By the way, my Firefox spelling dictionary redlines the single l version as I write this question.
Best Answer
Instal is chiefly British variant of install. Merriam Webster defines that.
They both mean same (and are pronounced in the same way)- to make (a machine, a service, etc.) ready to be used in a certain place. Other meanings, uses and examples as mentioned here.