What a constant temperature and humidity specification must state
22 July 2026Selection guide5 min read
A test room that must hold 23 ± 1 °C and 50 ± 5% RH needs four functions in one unit, and a specification that says which band is controlled and how tightly.
Constant temperature and humidity units combine cooling, heating, humidification and dehumidification in one cabinet. They are used where both values matter at once: test rooms, laboratories, material conditioning and metrology spaces.
Most enquiries arrive with a temperature and a humidity figure but no tolerance. The tolerance is what decides the equipment, because it sets the control cycle and the sensor quality.
The four values a supplier needs
A workable specification states the design condition, the tolerance, the load and the air change rate. Without any one of them the selection is provisional.
- Set point and tolerance for both temperature and humidity, for example 23 ± 1 °C and 50 ± 5% RH.
- Sensible and latent loads: equipment inside the room, lighting, people and any process heat.
- Fresh air rate required for occupancy or pressurisation.
- Acceptable recovery time after a door opening or a load change.
Control, not just capacity
Two units of the same capacity behave differently if one uses on/off control and the other modulates. For tight bands the specification should state the control method, the sensor accuracy and where the sensor sits in the room.
It should also state what happens at the edges: whether humidification and dehumidification are interlocked, and how the unit behaves when the room is empty overnight.
Site conditions
Condenser placement, ambient temperature at the outdoor unit and the route of the refrigerant pipe all affect whether the band can be held in summer. They belong in the enquiry, not in the commissioning week.