Was the big vent put in the living room wall, sited wrongly?

We had cavity wall insulation put in last June. We have a back boiler, & so had to have this vent put 20 inches up from floor for it to burn properly. All the summer it was fine because we didn't have any gas on. In September we switched on & got headaches. Called the gas fitter out & he checked it thoroughly--- said nothing was wrong. must have been - because head aches have gone. We find the room cold & extremely draughty especially when wind is in wrong direction. I sit with a blanket on me & turn the heat up. Kind of defeats the object doesn't it! Has anyone had this same problem? Can I do anything about it?