Finally got the heat under control in my gear alcove — what worked for me

I went through the same head scratching with the cupboard under my stairs where my Sunsynk and a rack of cells live. Mine was worse in summer becuase the cupboard door was pretty much airtight and the whole thing acted like an oven. I fitted a pair of 120mm fans side by side on a 40 degree thermostat switch, one blowing in through a vent near the floor and one pulling out near the top, and it dropped the internal reading from about 38 down to the low 30s on hot days. Not as dramatic as some setups but good enough for my cells.

One thing I learned the hard way is to put a dust filter on the intake side. Mine runs through a garage with a concrete floor and after a few months there was a surprising amount of fine grit building up on the fan blades and over the inverter heatsink. A simple washable filter mesh on the intake vent sorted it and it only costs a couple of dollars. Also keep an eye on how the fan behaves at night if your area gets cool. Mine used to pull cold damp air through and I noticed some condensation on the battery terminals during a humid spell, so I raised the thermostat cut in point to stop it cycling when the outside air was cooler than the cupboard.

I tried the fan and vent route first and it half worked for me, mainly because of where the intake air was coming from. My gear lives on the garage wall and when I cut the low vent I realised the air feeding it was coming straight off the driveway, which hits mid 30s by lunch in summer. So the fan was just cycling hot air around and the batteries never really dropped. The exhaust side was doing its job, the intake side was the weak link.

What actualy fixed it for me was less clever than any of this. I pulled the batteries off the enclosed shelf and put them on an open rack against the north facing wall of the garage, shaded by the fence line, and left the inverter where it was with its own fan setup. Batteries seem far happier sitting in open air than sealed in any box, no matter how well vented. The rack was maybe 80 bucks of shelving from the hardware store and it dropped the cell temps more than a month of fiddling with fans ever did. Might not suit everyone if you have kids or pets around, so mounting them up high on the rack solves that. But if your intake air is drawing from a hot spot like a driveway or a roof cavity, no amount of extraction will fix it. Worth tracing where the air is actually coming from before buying anything.

Done something similar with the shed wall where my off-grid gear lives, except I skipped the thermostat in the end. First summer I had it cutting in and out and the space kept slowly climbing between cut outs because the fan never ran long enough to clear the hot pocket at the top. Wired it to run whenever the system is awake and the temps have been rock steady since. Draws so little I honestly can't measure a difference on the daily numbers, and it means one less thing to fail. If your fan is quiet enough you might find you don't need the thermostat either.

Good result. One thing that bit me with a high side vent like yours was rain driving in during a storm, even with the fan set back a bit. I stuck a simple louvre cowl over the outside of mine and angled it down, problem gone. Also worth checking where your fan actually dumps the air, mine was blowing straight into a covered corner at first so the hot air just curled back round and got pulled in again.

Reading this as someone who's only just joined and still working out my own setup, mine's a bit backwards from yours. The cabinet with my charge controller sits against an inside wall in the hallway, so if I vent it the way you did I'd just be dumping the warm air into the house, which the wife will absolutely not tolerate in summer. Wondering if anyone has tried venting into the roof space instead, or whether that just creates its own problems. My plan so far is to cut through into the garage behind the wall, but I'm nervous about drilling until I know what's in there.