Every heated greenhouse has a control problem before it has a heating problem. The heat is being supplied to a long thin space with cold surfaces at both ends, and the temperature at the appliance bears very little relation to the temperature at the far bench.
A built-in thermostat is inside the appliance, which is the warmest point in the house whenever the heater is running. It therefore satisfies early.
The comparison
| Measure | Built-in thermostat | External controller |
|---|---|---|
| Probe location | Inside the appliance | Wherever you place it |
| Measures the crop temperature | No | Yes, if sited at plant height |
| Setpoint precision | A dial, often unnumbered | Digital, to a degree |
| Differential control | Fixed and usually wide | Adjustable |
| Runs a fan as well | No | Dual-output models do |
| Extra cost | None | Modest |
| Works with any heater | Not applicable | Yes, within the relay rating |
Where to put the probe
At plant height, in the body of the house, out of direct sun and away from the heater outlet. A probe in the sun reads the sun rather than the air; a probe next to the heater reads the heater. A probe hanging above the crop in the middle of the house reads roughly what the crop experiences, which is the point.
An Inkbird ITC-308 Digital Temperature Controller ($36.00) has two relay outlets, one for heating and one for cooling, so a single box can run the heater on the low setpoint and the extract fan on the high one. That means the heater is not fighting the fan, which is a genuinely common and expensive mistake when both have their own separate thermostats.
Rating, and the mistake worth avoiding
Controllers are rated for a maximum load, and a 1500 W heater draws about 12.5 amps. A bayite BTC211 Temperature Controller, 1650 W ($39.99) rated 1650 W has almost no headroom above that, which is fine for exactly one 1500 W heater and nothing else on the same outlet. Check the rating against the connected load rather than assuming, and never run two heaters through one controller outlet.
If you would rather not assemble a controller and a heater separately, a bundled unit such as a Greenhouse Heater with Digital Thermostat Control Kit ($89.99) at least cycles on a setpoint rather than running flat out, which is better than an uncontrolled heater even if the probe placement is still not ideal.
Humidity deserves its own controller
Temperature is only half the winter problem. Running an extract fan on a humidistat such as an Inkbird IHC-200 Humidity Controller ($42.99) vents the house when moisture rises rather than on a timer, which is the difference between controlling botrytis and merely losing heat on a schedule. That is covered in humidity and disease control.
The decision, summarised
- Any heated greenhouse: an external controller, probe at plant height.
- A heater and a fan both running: a dual-output controller so they share one setpoint scheme.
- One 1500 W heater and nothing else: a single-output controller is sufficient, check the rating.
- Winter disease problems rather than cold: add a humidistat on the extract fan.
- Before buying either: confirm what the house actually needs to hold, because the setpoint costs more than the hardware.
