Heat pump dryers and laundry ventilation in NSW homes

A heat pump dryer is not a vented dryer. It does not push moist air out through a duct to the outside. Instead, it condenses moisture from the clothes and releases that moisture — as water or as vapour — into the laundry. In a new home, that changes the moisture load in a room that is often small, closed off, and lined with materials that do not like being wet.

This article is about laundry moisture source control and dryer integration. It is not about hot water heat pumps. The two appliances share a name but sit in different parts of the build, and the ventilation strategy for a laundry is its own problem.

If you are a licensed builder or site supervisor in NSW, you already know that moisture is the slowest and most expensive defect to fix. It does not show up at practical completion. It shows up months later as swollen timber, bubbling paint, musty smells, or a call from a homeowner who has found mould behind the washing machine. By then, the frame is closed, the tiles are laid, and the cost lands on you.

Why a heat pump dryer changes the moisture balance in a laundry

A conventional vented dryer takes air from the room, heats it, tumbles the clothes, and exhausts the moist air outside through a duct. The laundry itself stays relatively dry because the moisture leaves the building. A heat pump dryer works differently. It uses a refrigeration cycle to cool the air, condense the moisture, and then reheat the air to dry the clothes. Most of the water ends up in a condensate tank or drain, but a significant amount of moisture remains in the air inside the machine and is released into the laundry when the door is opened or through the appliance’s own casing.

The result is that the laundry becomes the moisture source. In a well-sealed new home, that moisture has nowhere to go. It raises the relative humidity of the room. When the room cools — overnight, or in winter — that moisture condenses on the coldest surfaces: window glass, external wall corners, tiled floors, and any exposed metal. Over time, repeated wetting and drying cycles affect the building fabric.

WoodSolutions materials on timber and moisture explain that timber expands when it absorbs moisture and shrinks as it dries. That movement is normal in service, but repeated cycles from a persistently damp laundry can cause swelling, cupping, and joint separation in timber flooring, architraves, and even the framing if the moisture migrates into the wall cavity. The damage is not sudden; it accumulates.

What the NCC and BASIX require for laundry ventilation

The National Construction Code (NCC) sets out performance requirements for moisture management in buildings. It does not prescribe a single ventilation rate for every laundry, because the requirement depends on the room volume, the moisture load, and the climate zone. BASIX, the NSW sustainability index, adds an energy and water target that can influence the choice of exhaust fans and the overall ventilation strategy.

In practice, you need to demonstrate that the laundry can remove the moisture generated by the appliances and activities in that room. That means mechanical exhaust ventilation, not just an openable window. A window is useful for purge ventilation on a fine day, but it is not a controlled solution. Homeowners close windows when it is raining, when it is too hot, or when they are not home. The moisture load continues regardless.

The NCC also requires that moisture from wet areas does not cause harm to the building fabric or the health of occupants. That is a performance outcome, not a prescriptive fix. Your job is to select and install a system that meets it. If you are unsure how to interpret the current provisions, check the NCC and the BASIX requirements directly, or speak to your certifier. Do not rely on a rule of thumb that was common before heat pump dryers became popular.

Sizing the exhaust for the appliance and the room

An exhaust fan must be sized for the moisture load, not just the room volume. A small laundry with a heat pump dryer can have a higher moisture load than a larger laundry with a vented dryer, because the moisture is not being ducted away at the source. The fan needs to change the air often enough to keep relative humidity below the level where condensation forms on surfaces.

Manufacturers of heat pump dryers publish moisture release rates in their technical data. Use that figure to estimate the peak moisture load. Then select an exhaust fan that can remove that moisture under the expected conditions. The fan duty is affected by duct length, bends, and the type of termination. A fan rated at a certain flow rate in free air will deliver much less through a long, convoluted duct. Keep the duct run as short and straight as possible, and terminate it outside the building envelope, not into the roof space or subfloor.

Make-up air is just as important. If the laundry is sealed tight and the exhaust fan runs, the fan will struggle to move air and may draw air backwards through other flues or gaps. Provide a dedicated make-up air path, such as a louvre or a transfer grille from an adjacent space, sized to match the exhaust. The make-up air should be filtered if it comes from outside in a dusty or polluted area.

Containing moisture within the laundry

The laundry should be treated as a wet area and separated from the rest of the house as much as practical. A self-closing door helps contain moisture when the dryer is running. If the laundry opens directly into a living area or hallway, consider a door with a seal or a vestibule. The goal is to prevent moist air from migrating into bedrooms or living rooms, where it can condense on cooler surfaces and create a broader problem.

Seal penetrations around ducts, pipes, and cables. Gaps in the building envelope allow moist air to enter wall cavities and roof spaces, where it can condense on timber and metal. That is the same principle that drives condensation risk in roof spaces, which is covered in our article on roof ventilation and condensation risk in NSW. The laundry is a concentrated moisture source, so the detailing needs to be tighter, not looser.

If the laundry is on a suspended timber floor, moisture can also affect the subfloor. Our article on subfloor ventilation for suspended timber floors explains how ground moisture and poor airflow under the floor can compound the problem. A heat pump dryer adds to the moisture load, so the subfloor ventilation needs to be adequate.

Drainage and condensate management

Most heat pump dryers can be set up to drain condensate directly, rather than collecting it in a tank. If a drain is available, use it. A tank that fills and overflows, or that the homeowner forgets to empty, will spill water onto the floor. Even a small spill repeated over time can damage flooring and skirting.

Where a drain is used, ensure the connection is correct and the hose is not kinked. The drain should be trapped if it connects to the sewer, and it should be accessible for maintenance. If the dryer is installed in a cupboard or a tight space, leave enough room for the hose to run without being pinched.

If the dryer must use a condensate tank, make sure the homeowner knows to empty it. That sounds obvious, but in a new home, the homeowner may not read the manual. Include it in your handover.

Interaction with other ventilation systems

Many new homes have a mechanical ventilation system, such as a heat recovery ventilator or an exhaust-only system. If the laundry exhaust interacts with that system, you need to avoid short-circuiting or depressurising the house. A laundry exhaust fan that runs continuously can unbalance a balanced system. In that case, the laundry may need its own make-up air or a interlocked control that coordinates with the central system.

If the laundry has a split system air conditioner, as covered in our article on split system air conditioner installation in NSW, the air conditioner can help dry the air, but it is not a substitute for exhaust ventilation. Air conditioners cool and dehumidify, but they do not remove moisture from the room; they condensate it and drain it, which is helpful, but the moisture load from a dryer can overwhelm a small unit. Use the air conditioner as a supplementary measure, not the primary control.

Also consider the interaction with cooking and bathroom exhaust. If the laundry exhaust shares a duct or a termination with other exhausts, the pressure balance can change. Keep systems separate where possible, or have a mechanical engineer confirm the design.

Commissioning and handover

Commissioning is where you prove the ventilation works. Run the dryer on a normal cycle with the exhaust fan on and the laundry door closed. Measure the relative humidity in the room at the start and after the cycle. If the humidity stays high or condensation appears on surfaces, the ventilation is inadequate. Adjust the fan speed, reduce duct resistance, or add make-up air.

Check the airflow at the exhaust grille with a anemometer or a flow hood if you have one. Compare it to the design flow. If it is significantly lower, find out why. Common causes include a blocked or crushed duct, a closed damper, or a fan that is not rated for the actual static pressure.

At handover, explain to the homeowner how the system works. Tell them to run the exhaust fan whenever the dryer is in use and for a period afterwards. Show them how to clean the lint filter and check the condenser. Give them a written note in the handover pack. If they understand why it matters, they are more likely to do it.

Worked example: a new home in western Sydney

A builder in western Sydney is completing a new four-bedroom home. The laundry is a small room off the hallway, with a heat pump dryer installed under a bench. The room has an openable window and a standard ceiling exhaust fan that was installed for the bathroom next door, but it is not ducted to the outside; it exhausts into the roof space. The builder runs the dryer for a test and notices condensation on the window and the tiled floor.

The builder checks the NCC and BASIX requirements and realises the exhaust fan is not adequate for the moisture load. The fan is also too small and the duct is not connected to an external termination. The builder replaces the fan with a higher-duty unit, runs a short duct to an external wall, and installs a transfer grille in the laundry door to provide make-up air. The builder also seals gaps around the dryer hose and the door frame.

After the changes, the builder runs the dryer again. The relative humidity in the laundry stays below the level where condensation forms, and the window remains clear. The builder records the commissioning results and includes them in the handover pack, along with instructions for the homeowner.

If the builder had not caught this, the homeowner would have noticed mould on the ceiling within the first winter. Fixing it would have involved removing part of the ceiling, treating the mould, and repainting — a cost that would have come out of the builder’s margin. Worse, the moisture could have affected the timber framing, leading to a more extensive repair.

Regulatory context and your CPD

In NSW, licensed builders must keep their knowledge current. The NCC is updated regularly, and BASIX targets change. If you are unsure how the current requirements apply to a project, check the source documents or ask your certifier. Do not rely on what you did five years ago. Heat pump dryers are becoming more common, and the ventilation requirements are not always well understood.

For more on how NSW builder CPD works, see our guide at NSW builder CPD. The PointCPD course catalogue includes short courses on moisture management and ventilation that count towards your hours. If you are new to the platform, start at the PointCPD home page.

Working in a hot laundry during commissioning can also be a work health and safety issue. Safe Work Australia’s guidance on working in heat applies to any indoor environment where temperature and humidity combine to create a risk. If you are running a dryer for testing in summer, take breaks and stay hydrated. The hot work permit template is not directly applicable to laundry ventilation, but it is a reminder that heat and moisture are hazards that need to be managed on site.

Water efficiency is another consideration. The WELS scheme rates appliances for water use, but heat pump dryers do not use water in the drying process, so WELS is not directly relevant. However, if the laundry includes a washing machine, check its WELS rating as part of the overall water efficiency of the home.

Design details that make a difference

Small design decisions have a big impact on moisture control. Position the exhaust grille high on the wall or ceiling, away from the dryer, so it captures moist air as it rises. Avoid installing the exhaust directly above the dryer, where it can short-circuit and pull air from the make-up path without sweeping the room. If the laundry has a door, undercut it or install a transfer grille to allow air to move from the rest of the house into the laundry when the exhaust runs.

Use moisture-resistant materials in the laundry. Plasterboard should be moisture-resistant, and any timber should be treated or protected. Seal the floor-to-wall junction if the floor is tiled, and use a waterproofing system that extends up the wall behind the washing machine and dryer. These measures do not replace ventilation, but they buy time if there is a temporary moisture event.

Consider the condensate drain. If the dryer drains into a standpipe, make sure the standpipe is properly trapped and vented. If it drains into a laundry tub, make sure the tub cannot overflow. A blocked drain can cause water damage that looks like a ventilation failure.

Finally, think about maintenance access. The exhaust fan, duct, and dryer all need to be accessible for cleaning and repair. If you bury the duct behind a tiled wall, you are creating a future problem. Leave access panels where needed.

Common mistakes and how to avoid them

The most common mistake is treating the laundry like a dry room. It is not. It is a wet area, and the heat pump dryer adds to the moisture load. Another mistake is installing an exhaust fan that is too small or ducted incorrectly. A fan that exhausts into the roof space is worse than no fan at all, because it moves moisture into a cold, unventilated cavity where it will condense on timber and metal.

Another mistake is forgetting make-up air. A fan cannot move air out of a sealed box. If the laundry door is closed and there is no make-up air path, the fan will stall or draw air from undesirable places. Always provide a dedicated make-up air path.

Finally, do not assume the homeowner will run the fan. Some people will, but many will not. Make the system as automatic as possible. A fan interlocked with the dryer, or a humidity sensor that runs the fan when needed, reduces the reliance on occupant behaviour. If you cannot automate it, make the controls obvious and explain them at handover.

By treating the laundry as a moisture source and designing the ventilation accordingly, you protect the building fabric, the homeowner’s health, and your own reputation. The cost of getting it right at construction is small compared to the cost of fixing it later.

Frequently asked questions

Can I just use the existing bathroom exhaust fan for the laundry?

Not unless it is sized for the laundry moisture load and ducted correctly. A bathroom fan is typically sized for a smaller room and may not have the duty to handle a heat pump dryer. It also may not have a make-up air path. Use a dedicated fan or upgrade the existing one.

Does a heat pump dryer need a vent to outside?

No, it does not need a duct for the dryer itself. However, the laundry still needs mechanical exhaust ventilation to remove the moisture that the dryer releases into the room. That exhaust must terminate outside the building envelope.

What if the laundry has no external wall?

You can still duct the exhaust through the ceiling or floor to an external termination. Keep the duct run as short and straight as possible. If the laundry is internal, make-up air can come from an adjacent hallway or living area via a transfer grille.

How do I know if the ventilation is adequate?

Commission the system by running the dryer and measuring relative humidity. If condensation forms on surfaces or the humidity stays high, the ventilation is not adequate. Adjust the fan, duct, or make-up air until the humidity is controlled.

Can I rely on the homeowner to open a window?

No. Windows are not a controlled ventilation solution. Homeowners may not be home, or the weather may be unsuitable. Mechanical exhaust with make-up air is the reliable approach.

What about mould? Is it a health risk?

Mould can cause respiratory problems and allergic reactions. Moisture control is the primary prevention. If mould appears, it indicates that moisture is not being removed effectively. Investigate the ventilation and fix the source.

For more detailed guidance on moisture management and ventilation, see our related articles on roof ventilation and condensation risk and subfloor ventilation for suspended timber floors. These cover other parts of the building where moisture can cause problems, and they complement the laundry strategy described here.

Questions builders ask

• Heat pumps
• Moisture
• Ventilation

Published 26 July 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.