It's almost assured that you don't have a dual stage thermo. I'm not an
HVAC guy, not even close... but if your thermostat is 6 years old, and
they didn't replace it or rewire it - I'd bet your system was not
It seems ridicuosly common for new install of dual stage systems to not
be put in right. I've no idea why this is. But in order for you to
truly get what you paid for, and the benefits of dual stage heating -
you need a dual stage stat.
I'd call the company up, ask why they didn't install a 2 stage stat,
and get them back out to finish the job properly. I'm sure an HVAC tech
will jump in here soon with other advice. Bubba, you out here?