Why Understanding How Dehumidifiers Work is Vital for NZ Techs …
In the New Zealand restoration and construction industry, we often talk about “drying” as if it’s just about heat. But for a professional water restoration technician or a site manager pushing for a pre-line inspection, the real magic happens through cooling.
It sounds like a contradiction: using cooling coils to create the heat needed to dry out a damp Auckland villa or a rain-soaked slab in Canterbury. However, understanding this cycle is the key to mastering moisture control.
The Science of the “Squeeze”
The primary goal of a refrigerant dehumidifier is to reduce the Relative Humidity (RH) of the air. To do this, we have to physically “squeeze” the water out of the air. In the restoration terms, we are “manipulating the Dew Point.”
1. The Intake: Pulling in the Damp 
The process starts with a high-capacity fan drawing in the moist, ambient air from your project. On an NZ site, this air is often heavy with evaporated moisture from wet timber framing or saturated concrete.
2. The Evaporator: The Cooling Phase
Inside the unit, the air passes over the evaporator coils. These coils are kept at a very low temperature by a refrigerant (like R410A). As the warm, damp air hits these cold fins, it quickly drops below its dew point.
Just like a cold bottle of L&P “sweats” on a humid summer day, the water vapour in the air condenses into liquid droplets. This water is collected and—in professional LGR (Low Grain Refrigerant) units used across NZ—is automatically pumped out through a drainage hose.
3. The Condenser: The Reheating Phase
This is where the “cooling to create heat” happens. After the air has been stripped of its moisture, it is very dry but also very cold. If we exhausted this cold air back into the room, the site temperature would drop, and the evaporation of moisture from building materials (like GIB® or timber) would stall.
To prevent this, the cold, dry air is passed over the condenser coils. These coils contain the heat removed during the cooling phase, plus the heat generated by the compressor’s motor. The air is reheated—typically reaching 10°C to 15°C above the initial room temperature—before being exhausted back onto the site.
Why This Matters for NZ Timelines
By returning air that is both warmer and drier, we create what technicians call “hungry air.”
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Vapour Pressure Differential: This dry, warm air creates a massive pressure difference between the air and the moisture trapped inside 90mm timber studs or concrete. This pressure literally pulls the water out of the material.
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Meeting NZ Standards: This process is what allows a builder to hit the 18% Moisture Content (MC) required for timber framing far faster than natural ventilation ever could. (NZ Building Code Clause E2/AS1 – External Moisture). Refer to full information HERE.
Below is an excerpt from the E2/AS1 document which further explores the management and measurement of construction moisture.
NZ Building Code Clause E2/AS1 – Part 10. Construction Moisture
10.1 Managing construction moisture
- 10.1.1 Moisture in materials
- 10.1.1.1 Moisture contained in the building structure at completion of construction shall not be permitted to damage the building elements.
- 10.1.1.2 Construction moisture includes the moisture contained in:
- a) timber products as a result of a treatment or manufacturing process; and
- b) green timber, and timber or other materials that have been exposed to the weather; and
- c) concrete, mortar or plaster that is not completely cured.
- 10.1.2 Maximum acceptable moisture contents
- 10.1.2.1 The maximum moisture contents shall be:
- a) for timber framing at the time of installing interior linings, the lesser of:
- i) 20% for insulated buildings or 24% for non-insulated buildings, as applicable, or
- ii) as specified in NZS 3602; and
- b) for timber weatherboards and exterior joinery, 20% at the time of painting; and
- c) for reconstituted wood products, 18% at all times; and d) for concrete floors, sufficiently dry to give a relative humidity reading of less than 75% at the time of laying fixed floor coverings.
- a) for timber framing at the time of installing interior linings, the lesser of:
- 10.1.2.1 The maximum moisture contents shall be:
COMMENT: Some manufacturers of timber or other wall or floor components may recommend lower moisture contents for their products. It is advisable to use the manufacturer’s moisture content requirements, if these are lower than those required by this paragraph.
10.2 Measuring the moisture content
- 10.2.1 Timber
Tramex Hammer Pin Probe – 1.5″ - 10.2.1.1 Measurement shall be by the recommended procedure in the Scion (New Zealand Forest Research Institute) publication “Measuring the moisture content of wood” using electrical resistance type moisture meters with insulated probes. Representative samplings of measurements shall be taken:
- a) with meters calibrated to AS/NZS 1080.1 Appendix E; and
- b) by inserting probes to at least 1/3 the depth of timber being measured, at a distance exceeding 200 mm from board ends; and
- c) using correction factors for timber species, temperature, and treatment type (outlined in Scion publication).
- 10.2.1.1 Measurement shall be by the recommended procedure in the Scion (New Zealand Forest Research Institute) publication “Measuring the moisture content of wood” using electrical resistance type moisture meters with insulated probes. Representative samplings of measurements shall be taken:
COMMENT: For convenience of site measurement, readings of moisture content can be compared against a ‘control’ framing sample of known acceptable moisture content. The comparative readings must be taken during the same test period, be of the same framing type, and using the same resistance moisture meter. This method of moisture testing may be appropriate for non-boron treated framing, or processed timber framing.
- 10.2.2 Concrete floors
10.2.2.1 Measurement shall be made in accordance with BRANZ Bulletin 330 “Thin Flooring Materials” using hygrometers calibrated to ASTM E104.
See Cleaning Systems full range of Thermal Imaging Equipment & Moisture Meters (Hygrometers) HERE
Read the BRANZ Bulletin: BU585 Measuring moisture in timber and concrete (2015) – HERE
The Kiwi Climate Factor
While refrigerant dehumidifiers are the “workhorses” of the North Island, they have limits. In the middle of a Central Otago winter, where site temperatures might stay below 10°C, these units can “ice up.” In those freezing conditions, NZ pros often supplement with Desiccant Dehumidifiers, which use a chemical attraction rather than cooling to remove water.
Summary
By using professional dehumidification, you aren’t just moving wet air around; you are physically removing litres of water from the structure. It’s the most reliable way to keep your handover date firm and ensure your build meets the high standards required by NZ building codes.
Further Resource Library articles to read: