Are these gas fired radiant heaters or water tube . If so at what temperature.
Some gas radiant systems have no flue (a bit dodgy-applicable to the UK ) but if you ventilate the space to about 2AC (Don't quote me on that exact figure), then no flue is required.
I generally use flues and in addition, If I need fresh air, then this is tempered before supply to the space.
The heat output from a radiant panel is mostly radiant (depending on the surface temperature) Typically 90% radiant and 10% convective for the higher temperature models. If a 'draft' is incurred, then a greater output on the convective component will be realised...i.e. the heat output could go up slightly.
You should supply tempered air to offset the potential effect of cold drafts.
Friar Tuck of Sherwood