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laurenhvacPE

SAT reset vs humidity: lessons from Charlotte

I keep seeing supply air temp reset sequences that look great in energy models but fall apart in our humidity. We push SAT up to 62-65°F to cut reheat, then latent removal tanks and zones drift to 60%+ RH. Operators crank minimum OA or force colder SAT, and we end up worse than before.
What’s worked better for us: tie SAT reset to outdoor dewpoint and return RH. If OA dewpoint is above ~55°F, cap SAT at 55°F and let reheat do its job. Only allow SAT to float up when OA dewpoint drops and return RH stays below 50-55% for a while. Use enthalpy or dewpoint for economizer changeover, not dry-bulb. Keep VAV minimums realistic so you don’t starve zones during dehumidification. Also, block night purge on sticky nights; use low-OA mechanical dehumidification instead.
Curious how others are writing these guardrails into sequences and getting them implemented. Are you using dewpoint locks, zone-level dehumid calls, or something else? Any trends with sensor placement or BAS defaults that have bitten you?

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dee
Jun 22 at 4:00 PM
We stopped chasing SAT and just control to discharge-air dewpoint: if OA dewpoint >55 F, hold leaving-coil dewpoint at 52 - 54 F, lock out SAT reset, and only release it after return RH averages <52% for 30 minutes. High RH override (>60%) clamps OA to design minimum, enforces realistic VAV minimums (not the 30% default), slows the fan slightly to help latent, and enables reheat - just interlock it. Biggest bite has been sensors: put OA humidity in an aspirated, shaded intake, add a leaving-coil dewpoint sensor (not just return RH), and alarm if economizer runs on dry-bulb or return RH drifts >5% from a handheld… well, there’s your problem.

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