Thermostat for a Whole-House Dehumidifier in a Humid Climate: ecobee vs. T10 Pro vs. Standalone Humidistat

Your AC in Florida runs for 8 minutes, hits the temperature setpoint, shuts off, and leaves the house at 68% RH. You want it at 55%. Running the AC harder will not fix this. You need a different piece of equipment, and a thermostat that knows how to control it.

This article covers the one decision most people miss: whether a thermostat can send a signal to a standalone whole-house dehumidifier based on a humidity setpoint you control, versus the common workaround of “overcool to dehumidify,” which chills the space below comfort to force more latent removal. In IECC climate zone 2/3, those two strategies are not interchangeable.

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Why AC alone struggles in zone 2 and zone 3

An air conditioner removes moisture as a side effect of removing heat. The evaporator coil gets cold, humid air passes over it, water condenses out, and the drain pan carries it away. The problem is that latent removal only happens while the compressor is running.

In zone 2 and zone 3 (Florida, Gulf Coast, much of the Southeast), two things work against you.

First, a correctly sized AC for sensible load runs short cycling on mild days. When outdoor temps are 68-72°F and sensible load is low, the AC may satisfy the temperature setpoint in 6-10 minutes. A typical residential system needs 15-20 minutes of runtime to pull meaningful moisture from the air. Short-cycling means the compressor shuts off before the latent job is done.

Second, oversized AC is endemic in the South. An oversized unit cools even faster, shortens runtime further, and can leave homes at 65-70% RH on overcast days even while holding 76°F.

A standalone whole-house dehumidifier (Aprilaire, Santa Fe, Honeywell TrueDRY) bypasses this entirely. It runs a refrigeration cycle dedicated to moisture removal, recirculates dry air without conditioning the temperature, and can operate at any outdoor temperature including a mild spring day when the AC never turns on. It runs until the RH setpoint is satisfied, then stops, regardless of what the thermostat is asking the AC to do.

The question is whether your thermostat is the one setting that RH target, or whether the dehumidifier is running on its own.

The metric everyone checks vs. the engineering blind spot

Every modern smart thermostat advertises “humidity monitoring.” That is the onboard sensor showing you a percentage. It tells you the room is at 64% RH and does nothing about it.

The blind spot is whether the thermostat has a dedicated dehumidifier output, a numeric RH setpoint you set, and the logic to fire the relay when humidity exceeds that setpoint regardless of whether the AC is also running. Those three things together are what controls a whole-house dehumidifier in a zone 2/3 mild-day scenario. Any one of them missing and you are back to overcool.

How the three approaches compare

The blind-spot delta: mild days are where this breaks

The core problem in zone 2/3: at 70°F outdoor temperature, the AC’s sensible load may be close to zero. The compressor does not run. But outdoor humidity is 80% and indoor RH is climbing toward 65%. You need dehumidification with no cooling call. An overcool strategy cannot help here because overcool requires the AC to be running to begin with.

ecobee Smart Thermostat Premium with a dehumidifier on ACC+/ACC-: When RH exceeds your setpoint, ecobee fires the ACC relay and signals the dehumidifier. HVAC School and installer reports confirm the relay fires on humidity demand regardless of whether AC is also calling. The critical configuration is setting “Dehumidify with Fan” to OFF; if left ON, the air handler blower can run continuously whenever there is a humidity demand and no cooling call, which is common on zone 2/3 mild days and wastes energy. The AC Overcool Max feature (up to 5°F past setpoint) is a separate, parallel setting for when the AC is already running.

Honeywell Home T10 Pro with ISU 9000 “Whole House Dehumidifier”: This is the cleaner setup for zone 2/3. ISU 9000 exposes a distinct “Whole House Dehumidifier” equipment type, separate from the AC-based modes, and the relay fires across all system modes including Heat and Off. ISU 9130 lets you choose whether the thermostat also runs the air handler fan (“Tstat Controls Fan”) or lets the dehumidifier handle its own fan (“Equip Controls Fan”). For a zone 2/3 install with a ducted whole-house unit, “Equip Controls Fan” is typically the right call. Over-cooling on the T10 Pro is limited to 0-3°F (vs. ecobee’s 5°F ceiling) and applies only in AC mode. The explicit whole-house dehumidifier mode with mode-agnostic firing is a hardware and firmware distinction worth noting.

Aprilaire Model 76 standalone humidistat: This is a wall-mount digital control that wires directly to the dehumidifier with two wires and needs 24VAC from the equipment. The thermostat is completely out of the picture. The 76 senses RH at the wall, compares it to your setpoint, and energizes the dehumidifier. It runs in any season, any mode, any outdoor temperature, with no thermostat integration required. What it cannot do: smart scheduling, away setback, or smartphone access. If your dehumidifier is in a utility room or crawl space and you want to set a target and forget it, this is the lowest-complexity path.

Installation and wiring traps

Power isolation on the ACC or U relay. A whole-house dehumidifier typically has its own 24VAC transformer. Wiring a self-powered unit incorrectly to ACC+ and ACC- can back-feed two transformers against each other and pop the system fuse. On ecobee, configure the accessory as “Single Transformer” only if the dehumidifier and air handler share one transformer. If the dehumidifier has its own transformer, the ACC relay provides an isolated dry contact. Check the dehumidifier’s installation manual before landing wires. The same isolation rule applies to the T10 Pro U relay: ISU 9050 sets whether the contacts are normally open or normally closed; match that to the dehumidifier’s input requirement.

Don’t run the fan on humidity-only calls in zone 2/3. On ecobee, the “Dehumidify with Fan” setting controls whether the air handler blower runs during dehumidification calls. Set it to OFF for a standalone dehumidifier. The dehumidifier recirculates air through its own coil; running the main air handler simultaneously does not help and runs up your electric bill on mild days when there’s no thermal reason for the blower.

The Aprilaire 76 needs 24VAC at the wall. The model 76 is not battery-powered. It pulls 24VAC from the dehumidifier or HVAC system. If you’re mounting it in a finished room away from the unit, run low-voltage control wire back to the dehumidifier. The unit ships with wiring instructions for both local and remote installation.

Zone 2/3 sizing note. Whole-house dehumidifiers are rated in pints per day at AHAM conditions (80°F, 60% RH). In a zone 2 climate, size for the latent load at partial sensible load conditions, not peak cooling day. An undersized unit will run continuously on humid spring days and never satisfy the setpoint.

The verdict

Best control for zone 2/3: Honeywell Home T10 Pro if you want thermostat-integrated control of a whole-house dehumidifier. ISU 9000’s explicit “Whole House Dehumidifier” mode fires the relay in all system modes including Off and Heat, which is exactly the right behavior for a climate where humidity can be high on cool days. The numeric setpoint and the ISU 9130 fan control give a field-adjustable setup that works without a cooling call.

ecobee Smart Thermostat Premium is a solid alternative. The ACC relay fires a standalone dehumidifier on humidity demand independently of the AC when configured correctly (“Dehumidify with Fan” OFF). The 5°F overcool ceiling gives more range than the T10 Pro’s 3°F when AC does run. The setup is slightly less explicit than the T10 Pro’s distinct whole-house mode, but it works.

Skip thermostat-integrated control entirely if: you already have a whole-house dehumidifier with its own humidistat, your thermostat replacement is not otherwise warranted, or you don’t need smart scheduling. An Aprilaire 76 wired directly to the dehumidifier costs around $40, sets an RH target, and runs the unit any time RH exceeds it. No new thermostat, no accessory terminal configuration, no risk of fan interaction.

When it flips: overcool-to-dehumidify (using AC to remove moisture by running past setpoint) is a reasonable approach in zone 4/5, where sensible load is high enough that the AC runs long cycles anyway. In zone 2/3, it fails on the mild days that are the most common source of humidity complaints. That is when a dedicated dehumidifier relay on its own setpoint earns its place.

DIY line: Wiring a standalone dehumidifier to a low-voltage thermostat relay (ACC or U terminal) is within reach for a homeowner comfortable with 24V AC control wiring. Photograph existing wiring, kill the breaker, confirm power isolation before landing wires. Sizing and duct integration for the dehumidifier itself, or any work involving the dehumidifier’s 120/240V power feed, is a licensed HVAC tech job.

Frequently asked questions

Can the thermostat control my whole-house dehumidifier on a cool day when the AC isn’t running? Yes, if configured correctly. The Honeywell T10 Pro in ISU 9000 “Whole House Dehumidifier” mode fires the relay in all system modes including Off. The ecobee fires the ACC relay on humidity demand when “Dehumidify with Fan” is set to OFF. Both work on a 70°F day with no cooling call, which is exactly the scenario where zone 2/3 homes struggle.

What’s wrong with just using “overcool to dehumidify”? Overcool requires the AC to be running. If outdoor temps are mild (65-72°F) and sensible load is low, the AC may not run at all, or only for short cycles. Overcool cannot fire on a non-existent cooling call. On those days in zone 2/3, indoor humidity climbs even though the house is at temperature setpoint. A standalone dehumidifier wired to a thermostat relay or a direct humidistat runs whenever RH is high, regardless of outdoor temp.

Does the ecobee run the air handler fan when the dehumidifier is running? Only if you configure it that way. The “Dehumidify with Fan” setting in ecobee installation settings controls this. Set it to OFF for a standalone whole-house dehumidifier. A standalone unit recirculates air through its own refrigerant coil and does not need the main air handler blower. Leaving this set to ON in zone 2/3 means the blower runs on mild days whenever RH is elevated, which is most of the time.

Is an Aprilaire 76 humidistat a real alternative to thermostat control? Yes, for a straightforward install. The Aprilaire 76 wires directly to the dehumidifier, senses RH at the wall, and controls the unit with no thermostat involvement. You lose smart scheduling and app control, but you gain simplicity and a setup that works with any thermostat brand. It’s the right call if your thermostat does not have a dehumidifier relay or if you just want to add dehumidification without changing the thermostat.