oh, right i understand now.

your worried about a mirage ( shimmering air ) caused by rising heat.

if thats a problem you can just run some small ducting outside the house from the vent exit, so the air has chance to cool down outside before it reaches the final exit point.

or if your venting into the atticc just have a nice long vent duct curled up in the corner.

using an open window for vent is fine, but on the scale of 3 1kw lamps you are going to need some kind of custom/improvised ventilation system.

unless you live in alaska or norway, i cant see it even being an issue to be honest....where do you live and what set up have you got?