That sucks that so much of the season is high humidity, but HPS is not going to help with that either and it actually increases transpiration putting more water in the air.
Yeah it does suck, especially during the fall you can actually notice the decreased quality in coffeeshops, sometimes referred to as 'wet-basement-weed' (sounds better in dutch). You say the HPS is not 'going' to help, yet the reality is it has been successfully doing so for many decades, and again in combination with a tried and proven design in which the HPS+exhaust+space balance is key to creating a workeable environment without having to spend a lot on airco and climate control.
It is high humidity combined with cold that causes rot and mold and some of the abundant airborne spores and plant diseases to thrive. The HPS warmth/heat helps prevent problems that arise from not being able to suck in air that's far less humid than what the plants create by transpiration during lights on. HPS causes more transpiration than LED, of course, the
better temp alone will do that. Although there is a point of no return, generally more transpiration isn't a bad thing unless it becomes too much, on the contrary. Also warm air can retain humid more easily, a good exhaust takes care of that increase. Lowering the exhaust output, as would be necessary to prevent it from getting too cold, is only going to make matters worse.
And of course during lights out the true humidity levels will set in regardless of which light is used.
Indeed, but at least with HPS and the inherit high exhaust output it lowers and warms up significantly during lights on. When replacing that HPS with LED and lowering the exhaust it will remain closer to the 'true humidity level' which is a combination of the environment the plants create AND the air sucked in.
Another reason LED is the best option for a limited space is - it uses vertical space more efficiently. The LEDs can be as close as 6" to the canopy and they have a slim vertical profile. So you could use LED in a shelving situation to really maximize a space.
For example, I stack vegging and cloning boxes. My cloning box uses a Cree XML2 4500K U2 that is running at 300mA <1W and is creating 175 lumens/W. That is about 55% efficient and it is running so cool that it will last indefinitely without lumen depreciation (my lifetime).
So do some large greenhouses here, stacking many layers of clones and tissue culture and using LED, in which case it is specifically meant to replace T5 and T8 and not HPS SON-T. Stacking multiple grows that normally use HPS isn't going to work in our rooms, basements, tents, and attics so I cannot really consider that a valid argument.
The 500W I mentioned is about 4'X5' canopy. It yields 14-16z
That's good enough to match HPS in gpw but again you underestimate the surface area factor. Someone with a 4x5' space here is going to use a 600 watt HPS +10% boast and get 20+ oz instead. Or use a 1000watter with dimmer to save some electricity the first weeks before the canopy fully fills the room and get 30+ oz from it. Saving 100W would effectively mean a pretty huge loss for a typical 'breadgrower' here (based on 6 cycles a year that would be roughly $4500. Not even mentioning the additional investment and climate control costs as I outlined in previous posts. When you actually need to pay bills and put food on the table by growing mj 'efficiency' takes on a more realistic, and practical form.
