Well it depends on the temp of the air that you venting out side and if its to hot or to cold for around that time that its getting vented out side it can become a big heat target if cops thermal test it.
If your only concern is the cops hitting you with the infared gun, please stop worrying. The police don't randomly test houses for heat sigs. If they did, every house with a dryer duct would be getting raided. The only time the police use thermal detection is if they have a reason to suspect that you are growing and need probable cause for a search warrant. If it's gone that far (the cops suspecting you), you're already fucked. The best security is, and always has been, nobody knowing that you're growing. According to our attorney (who only represents collective marijuana grow ops and dispensarys), 95% of all grow op busts are the result of someone who knows of the grow telling the police. The other 5% are the police just getting lucky (pulling over a guy with 20lbs of pot in car, grow equipment, and a $2,000 electric bill with a warehouse address...that is a true story!).
Here is California many of the large scale grow ops are ratted out by former employees of the grow. We really can't fire our workers in this biz. With your size grow, you have very little to worry about. Be careful, by all means. But, try not to get worked up about things that arten't realistic. Legalflying is totally correct. You need to get the hot air out of the house one way or another. Get creative and have some fun solving this problem. Good luck!
https://www.rollitup.org/indoor-growing/407048-20-000-watt-medical-grow.html