I use tube heaters with a thermostat for small grows like this in the winter. Especially at lights off. I always tell people a decent fan controller is essential for any size grow when controlling temps, you can literally dial it in.It's spring I n the UK and in any well insulated house temps only drop to 18 Without central heating .my LED raises the temp by about 5 c so no need for a heating most of the year.Temps are 21 low and 29 high. In a closet or small space air flow is important to control high temps. But essential in any situation to provide fresh air and co2. I have a passive vent installed in the door and 4" fan and filter extracting. Laters