Spent the weekend testing temps after my neighbour put their heat pump right opposite my AC

Summer cooling ussually isn't the problem with side-yard setups becasue that hot air dissipates pretty fast if you get any kind of natural draft. I had an almost identical panic when the folks next door dropped a 3-ton Daikin Fit about nine feet across from my Trane XR14. Like you, I went out with temp clamps during a 95-degree heatwave and barely saw a 1.5-degree rise on my intake.

Where it acutally caught me off guard was mid-January. In heating mode, thier heat pump blew freezing cold exhaust right across that narrow passage, and the defrost cycles kept dumping damp vapor that drifted over and froze on my flagstone walkway. If you do build that cedar screen, just make sure to leave wide spacing between the slats—aim for at least 40 to 50 percent open free area. If you box it in too tight, you run the risk of bouncing their discharge air right back into their coil, wich can trigger nuisance defrost cycles all winter long.

Be careful with that cedar screen idea. I ran into a mess trying to do the exact same thing between my Carrier unit and my neighbor's Mitsubishi side-discharge unit. We had about a seven-foot gap between the foundations, and thermally everything tested fine at first thanks to the natural wind tunnel between the houses. The second I put up a six-foot horizontal slat partition to cut down on the inverter whine, it completely killed that cross draft. Without the steady wind sweeping down the fence line, the exhaust air started bouncing off the wood and pooling right in the setback. Come late July, my unit's head pressure jumped and it struggled on 95-degree afternoons because the air between our walls couldn't escape. On top of that, the solid wood acted like an acoustic mirror, reflecting the low-frequency vibration straight into my home office window instead of letting it dissipate.

If the noise does end up bugging you this winter, look into getting an aftermarket compressor sound blanket for their unit (offer to pay the $70 for it) before putting up physical barriers in that narrow corridor. Restricting that airflow channel usually causes more problems than it solves.

What kind of thermocouple probe did you use for that? I just bought my first place with central air and I've been dying to test outdoor temps like that around our yard!

Also totally off topic, but do your lights flicker at all when both units happen to kick on at the exact same second? We only have a 100-amp main panel and I really want to switch our old furnace to a heat pump next year, but I'm super curious if neighbours sharing close lines ever see big voltage drops when both compressors spike at startup!

If you want to kill that direct airflow path without building a partition, look into directional wind deflectors on their chassis. If it's a side-discharge unit like a Bosch IDS 2.0 or a Gree Flex, you can usally snap on an OEM deflector hood—like the Daikin KPW kits or standard 45-degree louvers—and throw that discharge straight up into the open air instead of horizontal across your yard. I went that route when a neighbor's 3-ton Fujitsu was pointed at my fence line. Diverted the airstream up at an angle, eliminated cross-draft turbulence, and saved having to cram an ugly barrier into an 8-foot breezeway.

Totally agree that winter is where this actualy bites you, but watch out for the water runoff too. Everyone focuses on the blowing air, but during heavy defrost cycles a heat pump can easily dump several gallons of melted frost off the coils every single day.

If their pad is pitched even slightly toward your property line, that water pools right in that narrow shaded gap and turns into a sheet of black ice. If you do put that cedar screen up, leave at least two or three inches of clearance underneath it so you don't accidentally create an ice dam right against your own AC pad.

Quick update on this — definitely glad I read the warning about the cedar screen messing with the draft. Instead of building a partition, I chatted with my neighbour and we grabbed an OEM angled wind deflector for thier fan exhaust. It directs everything 45 degrees up and away from my coils. No trapped air, noise is dampened, and the whole issue is put to bed. Thanks for the tips!