Was just installed today less than 8 hours ago. The installers said it was all tested and working. I had to take their word for it because since I've been without a furnace for a week I had the old wood stove fired up to keep the house warm. Temp has started dropping and it hadn't kicked in even though it was 5 degrees below the set temp. I went under the house to check. It was plugged in. Pulled off the cover and ..... THE FRIGGIN GAS VALVE IS SHUT OFF INSIDE!

Turn the gas valve on, put the covers on, go reset the thermostat. Still nothing. Go back down. Nope, not on. No burners, nothing. That's when I notice a faint hum from what appears to be a transformer. So I shut the gas back off and unplug the furnace, and then call the company.

Get a answering service. Don't know where the answering service is but when she answered she said "good morning". It's only 10 pm here Tells me they'll have a technician call me. ARRRRGGGGG!.

Sounds like they never tested it before leaving.

Also sounds like they owe you some money back on the install.

ETA:  Check the house's main gas valve.  They may have left that off, too.

Yep, check main gas valve first. If it's a new furnaxe it has a ventor fan that must run, if it;s not then I suspect that the thermostat is not sending a signal back. Make sure it's set on heat and auto, if a programmable one set it to manual override and run up the temp.