A greenhouse without a controller is a building with opinions about killing your plants: it freezes them on clear nights and cooks them by 10 a.m. A thermostat or climate controller is the brain that turns sensor readings into action — switching heaters, exhaust fans, vent motors, humidifiers and dehumidifiers so you don't have to sprint outside at every weather change. The market now runs from ten-dollar mechanical dials to app-driven multi-channel systems computing vapor-pressure deficit. Here's the landscape, and which brain your greenhouse actually needs. (For the sensors-only side — thermometers and hygrometers — see our earlier monitoring roundup; this guide is about devices that switch equipment.)
Controller Types in Plain English
A plug-and-play thermostat outlet is a box between the wall and your equipment: plug a heater into the heating socket, a fan into the cooling socket, set a target, and a relay clicks them on and off. Dead simple, dead reliable. A combined temperature-humidity controller adds humidity sockets for a humidifier or dehumidifier. A multi-channel smart controller runs two to four devices independently on schedules, timers, cycles and climate triggers — including VPD, the temperature-humidity combination metric serious growers target — with app monitoring and phone alerts. The right buy is the simplest device that covers your actual equipment list.
The Picks
Inkbird ITC-308 / ITC-308 WiFi
The ITC-308 is the most recommended piece of climate gear in home growing for a reason: a pre-wired dual-stage outlet (one heating socket, one cooling socket) rated 110V/10A/1100W, with a probe on a lead, simple setpoint-plus-differential programming, and years of field reliability behind it. The WiFi version adds app monitoring and the frost alert that saves seedlings. Its limits are honest ones: 1100W caps the heater size, and it knows nothing about humidity. For a small greenhouse running one heater and one fan, nothing else is worth the extra money.
Inkbird ITC-608T
The ITC-608T folds temperature and humidity into one pre-wired box — dual probes, dual-stage logic, and a 120VAC/15A/1800W rating that comfortably runs a proper greenhouse heater plus an exhaust fan or dehumidifier. ETL listing and the same set-and-forget Inkbird programming apply. This is the sweet spot for growers whose problems are “frost at night, humidity all winter” and who don't want an app in the loop. Pair it with a circulation fan from our fan roundup and most hobby houses are fully automated.
AC Infinity Controller 69 PRO (+ UIS ecosystem)
The 69 PRO is the brain of AC Infinity's UIS ecosystem: four channels driving the brand's inline fans, clip fans, lights and outlet adapters with ten-step variable speed (not just on/off), programmed by timers, cycles, schedules and climate triggers including computed VPD, with app graphs and push alerts that catch an overnight freeze before it happens. The catch is ecosystem gravity — it speaks UIS ports natively, so third-party gear needs the brand's outlet adapters. For a greenhouse already running UIS ventilation, it's the obvious brain; the companion Outlet AI / Controller AI+ class brings the same intelligence to standard plugs at up to 1800W.
VIVOSUN GrowHub E42A+
VIVOSUN's GrowHub E42A+ answers the 69 PRO at a friendlier price: four-channel control with temperature, humidity and VPD logic, timer/cycle/schedule programming, and WiFi app oversight, aimed at the same fan-light-heater-humidifier stack. It's the value play for growers who want climate-triggered multi-device automation with graphs and alerts but aren't invested in a specific hardware ecosystem. As with all app controllers, check that your equipment's wattage and plug types match the controller's outputs before buying.
High-load & specialty controllers
Two niches the mainstream boxes miss. First, high-amperage duty: 15A/1800W-class controllers (DEWENWILS and similar heavy-duty outlet thermostats, or greenhouse-specific units like the BioGreen TER2 class) for larger heaters that would trip a 10A relay — always match controller rating to heater draw with headroom. Second, single-purpose reliability: heating-only thermostats (BN-LINK class) for heat mats and the humidity-only Inkbird IHC-200 for propagation and mushroom houses. Simple relays doing one job are the most failure-proof automation there is.
Matching Controller to Greenhouse
| Your setup | Buy this class | Why |
|---|---|---|
| Small house, one heater, one fan | Inkbird ITC-308 (WiFi if you want alerts) | Dual-stage covers both jobs; nothing to configure |
| Frost + winter humidity problems | Inkbird ITC-608T | Temp and humidity in one 1800W box |
| Multiple fans/lights, VPD targets, remote alerts | AC Infinity 69 PRO or VIVOSUN E42A+ | Four channels, climate triggers, app oversight |
| Big heater (over ~1100W) | 15A/1800W heavy-duty class | Relay headroom; undersized relays fail closed or welded |
| Heat mats / propagation bench | Heating-only thermostat + IHC-200 for humidity | Single-job relays are the most reliable automation |
Setup Rules That Save Plants
- Probe placement decides everything. Mount sensors at canopy height, shaded from direct sun, away from the heater's plume — a probe in sunlight reads fantasy numbers.
- Set a real differential. A degree or two of hysteresis stops relays from machine-gunning on the threshold, which is what wears relays out.
- Respect wattage. Heater nameplate draw must sit comfortably under the controller's rating; sustained max-load operation is how budget relays die.
- Alerts are the killer feature. A WiFi controller that texts you at 2 a.m. when the heater fails pays for itself the first time. Pair with the heaters in our heater roundup and the automation logic in our automation guide.
Power and Safety in a Damp Building
A greenhouse is a wet room that happens to contain electricity, and controllers sit at the middle of that marriage. Feed everything from a GFCI-protected circuit — either a GFCI outlet at the house end or a GFCI-equipped outdoor extension lead rated for the total load. Use one appropriately gauged outdoor cord rather than a daisy chain, hang connections off the ground where irrigation can't find them, and mount the controller body itself out of drip lines and spray paths (probes are built for the humidity; the relay boxes prefer not to be watered). Check nameplate amperage arithmetic once, when you set the system up: heater plus fan through one controller must sit under the controller's rating and the circuit's. None of this is exotic — it's the same discipline as outdoor holiday lighting — but it's the difference between automation and an incident report.
Alerts, Logging, and the Case for WiFi
The honest argument for spending up on a WiFi controller isn't remote control — you'll rarely change a setpoint from the couch — it's alerting and evidence. A push notification when the greenhouse crosses a low-temperature threshold catches the failed heater, the tripped breaker, or the door left open at 2 a.m., while there's still time to act. And the temperature/humidity graphs answer questions memory can't: how cold did it actually get last night, how fast does the house heat after sunrise, is the humidifier keeping up at 4 a.m.? Growers who run the data for one season size their heaters better, vent earlier, and stop guessing. If your greenhouse contains anything you'd be upset to lose — a citrus collection, a bench of grafted tomatoes, February's entire transplant crop — the WiFi tier is cheap insurance with a graphing habit.
Our Verdict
Most greenhouses need less brain than the marketing suggests: the Inkbird ITC-308 remains the default answer for one-heater-one-fan houses, and the ITC-608T is the upgrade when humidity joins the fight. Go AC Infinity 69 PRO if you're building on UIS hardware or want VPD-driven multi-channel control, VIVOSUN E42A+ for the same idea at value pricing, and a 15A heavy-duty unit whenever the heater is big. Whatever you buy, put the probe in the right place — it's the cheapest upgrade in this entire guide.
Frequently Asked Questions
What is the best thermostat for a small greenhouse?
The Inkbird ITC-308 class is the default: a pre-wired dual-stage outlet with heating and cooling sockets, 1100W capacity, and a probe on a lead. The WiFi version adds app monitoring and frost alerts. It covers a one-heater, one-fan greenhouse with nothing to configure.
What does a dual-stage thermostat mean?
It controls both heating and cooling from one setpoint: equipment in the heating socket switches on below target, and equipment in the cooling socket (typically an exhaust fan) switches on above it, with an adjustable differential between them.
What is VPD and do I need a VPD controller?
Vapor pressure deficit combines temperature and humidity into one number describing how hard air pulls moisture from leaves. Multi-channel smart controllers like the AC Infinity 69 PRO and VIVOSUN GrowHub can trigger equipment on VPD targets — valuable for high-value crops and propagation, unnecessary for basic frost protection.
Can a thermostat controller run any size heater?
No — match ratings. Common 10A/1100W controllers suit small heaters only; larger greenhouse heaters need 15A/1800W-class controllers with headroom above the heater's nameplate draw. Undersized relays are a common failure and fire risk.
Where should I put the temperature probe in a greenhouse?
At plant canopy height, shaded from direct sunlight, and out of the heater's direct airflow. A probe in sun or in the heat plume reads incorrectly and drives the whole system to wrong decisions.