The furnace installation directions specifies a shutoff valve with a 1/8" NTP plugged hole for test gauge. I bought this valve and am ready to assemble the pipes but I am uncertain about which side of the valve the plugged hole goes on. Is it the supply side or the furnace side? It seems like it should be on the supply side so that the gas pressure at the furnace could be measured with the shutoff valve closed. Someone said that the hole goes between the furnace and the shutoff valve for testing but did not explain how that test would work.
Put the side tap on the furnace side. This will make it much more convenient to connect and remove the measuring gauge because you can shut the valve so that gas won't be pouring out. The reading will be just as useful as having the gauge on the supply side because there will not be any pressure drop across this valve.
If you are worried about air getting into the supply pipe and making a combustible mixture, you could leave the valve open a little so that there is a small flow of gas out the sidetap which will keep air from getting in