You use a volt step controller to set the flow in summer. This will automatically speed fans up to your desired input to maintain root temps, it also helps to over size the in/out takes as they will run at lower rpm reducing sound and mechanical tare.
In winter you plug the volt step controller into a thermostatic cut off. The thermo cut off is set to say 68f at root temps, if the temp drops below that in/out take cuts off completely. The heater will kick in to raise the temp to the upper limit of the therm cut off. The time that takes depends on your heater, light heat, humidy unit heat and outside temps. Mine cycles on for around 5 min every 30 min in 0 to -winter temps with oil heater on low setting. At lights off the fans will stay off longer and less heat is also wasted, keeping roots happier. Using a big occolating fan also helps a lot. Point it slightly up and it will then help circulate the heat from the cob/leds from above the units to below. If you have continues airflow with the out take above lighting it will suck that precious winter heat directly out and make no use of it, more so with cob/leds but still so with hps.
You can use hps to heat during lights on with continuous air flow but even in low winter you will still see root temps drop and have to supplement heating unless in a small space or using too many lights than you actually need (or you have a well insulated room). At night you will again need a heater to maintain temps.. and a way to maintain the air for humidity spikes.
I use to think using hps as a source of heating was a bonus but it's not. Leave lighting to efficient lights, leave heating to efficient heaters. And ofc, save/utilize what heat you can to the point it does not effect co2 in take. I know that when it cycles on every 30 min it has no effect in terms of co2, since summer/winter give very similar results.