After moving into a new home, I had a licensed HVAC company install a whole house humidifier. A few days later, a guy from the builder's HVAC contractor came out to address an airflow issue. When he saw the humidifier, he told me that because I let another company work on the system, my warranty is now void.
He told me I should have known that because the warranty terms are spelled out in a binder full of paperwork the home builder gave me. After searching through the binder, I finally did find that clause, but I wouldn't call it obvious information. I don't remember signing anything saying I agree to the conditions. The papers were simply handed to me when I bought the house.
To be clear, the manufacturer's warranty still applies. It's only the installer's warranty on their workmanship that's considered void.
Is voiding a warranty like that legal? Would the Magnuson Moss Act protect me? I'm in Idaho, USA.