is there a timer?
Are you 100% sure that thare is not a timer on or near furnace? It would have a small time clock on it, and some brown wires. One of wich would go to a damper in this intake. If not, than you are correct in plugging it off for now, but it should be corrected in spring. And by the way, a gas furnace starts flowing through registers immidietley when the thermostat kicks, but the fire will not start for around 30 to 45 seconds after. Then it will take another 5 minutes at least before heat at registers is at full running temp. Did you turn on the furnace and go check the temp, then go close damper and check again while still running the same cycle? This could explain some of the temp. increase in your test you performed. None the less, if there is no damper, the installers of that one screwed up. If there is a sticker on that furnace, dont call them for repairs. Good luck again...
See my work-Check out my album. "Git 'Er Done":D