Thought I'd share my water cooled COB build here. I've built 8 light bars with cheapo chinese COBs. The idea being to initially build them cheaply and then swap chips to something more efficient after a couple of grows. As this is a bit of an experiment I didn't want to spend thousands and thousands on expensive COBs from the outset (I knew there was a chance this would fail miserably as nobody else has jumped on water cooling yet). Anyway I made a steel frame from 4cm flat steel sections. The frame is 100cm long and 25cm wide. I epoxied the water blocks onto a steel frame and then made "clamps" out of flat steel sections and 4mm bolts (epoxy alone was slightly dubious). The water blocks aren't going anywhere when clamped! I then used thermal glue to mount the COBs onto the water blocks. The glue can be separated with a razor blade easily which would make upgrading straightforward. I've used Meanwell HLG-600H-36A drivers because I have no interest in dimming the lights. They're just over 96% efficient at full power. Each COB is running at 0.835A (16.7A total for 20 COBs). I've used 8mm ID (14mm OD) hose on the 10mm barbs with 12mm hose clips. Here's the hoses mounted and wiring done: I've alternated cheap warm and cool COBs. If I were doing this again I'd pick one and stick to it because the power draw of different chips is unpredictable when running off a HLG-600H in parallel. Certainly when I upgrade the COBs later this year I'll be going for 1 uniform type. Anyway here's the money shot of the lights in action (excuse the lonely auto): I made an earlier version of these with voltage controlled drivers and current limiting resistors. That's the 8 COB bar you can see at the top. I learnt a lot from building those which I used to make better lights with version 2 (these 20 COB lights). The water gets pumped around the system with an Eheim 1250 water pump. All the water flows through the water blocks and then into a car radiator mounted on a timber frame where it's cooled: I have a 16 inch 1800cfm car radiator fan running off a 12V PSU. The main advantage of this setup is temps inside the tent are dramatically lower than when running air cooled lights as I'm not relying on the air in the tent to remove the heat any more. When running an 8x4 tent at full blast at 2100W (2x600W and 2x450W lights) the tent stays at 25C under lights and 20C in the shade (room where the tents are located is at 19C). If I run only the 2x600W lights so 1200W total, the temp stays at 22C in the tent under lights. COB surface temp is around 45-55C (measured with FLIR camera). I could make some improvements but this will do for now Promised myself there'd be no more tinkering with lights for 2 grows and have put my tools away! Edit: I should probably mention cost. £167 for the Meanwell HLG-600H, £60 for water blocks, £10 for hoses, £5 for hose clips, £20 for wiring and terminal blocks, £44 for COBs, £10 for steel, sections, £10 for bolts (inc eye bolts for hanging). £326 approx for each light bar. £15 for car radiator, £15 for radiator fan, £12 for PSU, £65 for water pump. £107 total for radiator and pump.