This is for technicians who have the proper tools. Homeowners should not be setting manifold on their furnace, ever. The problem comes when you turn the thermostat up five degrees over the set temperature to get the high fire to come on. Since this is two stage with a W1 and W2 controlled from the thermostat, it means you need to go upstairs, turn the heat down and hope that low fire comes on. Then you have to traipse all the way back upstairs to turn it up again so you can get the heat differential checked properly. Problem is I hate going up and down stairs all the time and getting up can hurt. Here's an easier way to do it.
First, set the high fire in the usual way. To set the low fire, remove the wire from the W2 terminal on the board. There is no way the thermostat can call for second stage and low fire will be the result. Check and set the manifold, which is on the placard inside the furnace. Remember to hook up the wire to the W2 terminal, replace the bottom door, check your amp draws, button up the furnace and check your heat differential and you're done. Maranatha!