In the Product Data (.pdf) for the thermostat, it states:
VP IAQ comfort systems operate with droopless control. Once the thermostat senses that 1st stage is running at 90% capacity, the thermostat energizes 2nd stage."
As a home owner, this is confusing. My understanding is that each stage has a 'fixed' capacity. What is being referred to (90% of what)?
I'll take a guess (probably wrong). Is it when the home's heat loss is equal to 90% of 1st stage heating capacity? Whatever it is, how does the thermostat sense it?