Cool. I actually like that furnace.
The time delay relays are actually called sequencers in the HVAC biz. They start the elements in a sequence which varies in time so they all don't start at once and overload the circuits or dim lights. Has a bi-metal warp switch at the bottom pushing on a rod on contacts.
You probably want some magnetic contactors rated for 20 amps and I am not sure about how they rate them for duty cycle etc and full load. There are differences in quality so I would not go cheap. Each element is 5000w/240 volts approx 20 amp draw.
If it is very humid in there I would be concerned about corrosion of the motor etc as it may rust to pieces quick. I worked in greenhouses and we had huge 275,000 BTU furnaces running w/o chimneys to provide CO2 and heat for the plants. Not sure what you need if your are growing "medicinal" marywanna or "tomatoes".:biggrin2:
Once you alter the factory wiring any UL or CSA rating goes away. If it starts on fire you may not have insurance or could be liable if it damages anybody or anything. It may be possible to get a special electrical permit? Would need a licensed electrician to take it out and get it inspected?
There are other issues depending on when you plan to use it. You cannot run a AC unless it is 50F or 10C and if it does not have enough load in the other times it can freezeup and damage the compressor. Not necessarily as simple as it sounds.
I don't think I need anything, I'm not really changing the way the furnace works aside from disabling a few heating elements since heat really won't be needed except to dehumidify and maybe for night time temperature control.
I'm not actually growing anything, I'm just a guy with too much time and skill on my hands. I saw some similar units and thought to myself "self, I'm sure I could build one of those for much cheaper and probably better"
I wasn't really going to bother with it until I saw this electric furnace come up for sale nearby like 2 days later, so I took it as a sign that I should probably give'er a whirl.
In this furnace those really are just time delay relays, it even says so in the manual, there is no bimetallic disc in those or anything like a sequencer. It appears the time delay isn't adjustable by any means though.
The unit I saw was basically just a furnace without heating elements, and with a quick disconnect style of AC unit controlled by thermostats. there might be some custom programming involved, but the basic unit sold for like $5000, then you could buy additional components like filters sterilizers, dampers all for an additional cost.