Indeed! Properly applied it can shorten the bloom circle or increase your yields. 3w per 2x 2" is enough, so ~20-25w for a 8x 4' area should be sufficient.
Read the first few pages of this thread to get an idea of how it is working and how to apply. In short, far-red goes on 5 mins before lights out and out 5 mins after it switched off.
A well-functioning cycle could look like this:
- week 1+2: 12/12h regime + 10mins far-red end of the day
- week 3-8: 13,5/10,5h regime + 10mins far-red
- week 9-end: back to 12/12h
With a 10 week cycle it would be 840h without far-red and 903h with the light regime above. That's 63h or 7,5% more light and should increase your yield accordingly.
Important!
Too much is not good!!! Too much far-red would trigger SAS, the "Shadow Avoidance Syndrome"! It causes huuuuge stretch, larger bright green leaves, a lot fewer nodes and therefor a significant loss in yield. Really nothing good!
You will also notice slightly more stretch when properly applied but we talk here of only 2-4' depending on strain.
If you want it to be precise, determine how many μMol your far-red diodes produce per second and then determine how many seconds it will take to produce 2000-4000μMol/m². (You can use a QER of 6μMol/J for 730nm)
With 10w/m² it should be between 5 and 10mins!
12 Cree XP-E/730nm or 12 Osram Oslon SSL120/727nm and an APC-25-700 would be ~20-22w. With cheap e3ay diodes I would take 20 or 4 10w/far-red COB's.
10x Cree XP-E/730nm, ..601 binning, 25$US
https://www.ebay.com/itm/10pcs-Cree-XPE-Far-Red-730nm-LED-1-9V-2-4V-350mA-1000mA-20mm-PCB-Board/322722116845?hash=item4b23bcb4ed:g:txsAAOSwKUhZsUG5
10w 730nm-COB(~6,5w @1A), 8,75$US
https://www.ebay.com/itm/10W-High-Power-LED-Light-Lamp-Bead-Infrared-IR-Light-730nm-740nm-Bulb-DIY/222391258876?hash=item33c78d4efc:g:N6sAAOSwcu5UMoCT