Struggling to keep temps up! The

Hi! Close all the passive intakes of your box and use an inline heater (ex: Winflex NK***). Regular heaters don't work well in a negative pressure chamber, because the air you try to heat move out too quickly. Heating incoming air inside a restricted space is more efficient.

Can you turn your light intensity up? The driver and LEDs will get warmer might raise temps up a bit. Raise the light up if needed.

A dehumidifier inside the tent will lower RH and raise Temps. Most people have to take the dehumidifier outside of the tent due to high temperatures. In your situation it would help by raising the Temps. You could also buy a small electric heater to put in the tent as well.

Space heater. You could burn a propane burner down there. That will give you heat, and co2 as a added bonus. The downside is humidity increase.

Is this an auto or photoperiod ? Can you change it to lights out in the daytime instead of night to keep temperature up. Most plants are okay so long as it doesn't drop below 18c

I had a small similar situation, my vpd wasnt staying within range during lights out so I added a heater and brought the lights out temp closer to my lights on temp and that has fixed that issue for me. So I guess adding another heater? How big is your dehumidifier, maybe put that inside the tent if you can and use that warm air to help raise the temps