Tempstar Furnace (3) flashing lead code error? I just installed a tempstar single stage 92% use furnace and the...

I just installed a tempstar single stage 92% use furnace and the thing wont fire up. i can achieve the fan going, but the burners wont fire up. It's giving me a 3 flash LED error...something to do next to the pressure switch.
Were would I receive sandbags?
Answers:    Check your PVC sizes are they sized right per mfg specs.also your discharge pipe should be pitched back toward the oven so condensate can drain out and not lay in the pipe witch would lead to the pressure switch not to make,and the burners not fire.
Why is the floor of my vault other...
That funds there is something wrong beside the way you installed the vent pipes. Check the instructions and engender sure you are within the size limitations. For more info, check out the furnace page at www.hvac-for-beginners.com