Exhaust Air Heat Pumps for Hot Water
What You Need to Know
A heat pump water heater pulls warmth from the air and uses it to heat water. Most units draw outdoor air. An exhaust air heat pump draws warm, humid air from bathrooms, kitchens, and laundries instead. That warmer source air raises the COP (coefficient of performance), especially in winter.
AS/NZS 3500.4 (Plumbing and Drainage, Part 4: Heated Water Services) governs installation. NCC 2025 Section J gives them credit in energy compliance modelling because they use 60–80% less energy than electric resistance heaters.
The Rules
- Store hot water at 60°C minimum. Deliver from the heater at 60°C minimum (AS/NZS 3500.4)
- Keep circulation return lines at 55°C or above (AS/NZS 3500.4)
- Cap delivery to personal hygiene outlets at 50°C. For healthcare, aged care, early childhood, and schools, cap it at 45°C (NCC Volume Three B2D5)
- Insulate all heated water pipes to the R-values in NCC 2025 Table J6D9a. Insulate at least the first 500 mm of inlet and outlet pipes (AS/NZS 3500.4 Cl 8.2.1)
- Install a TPR (temperature/pressure relief) valve drain with no taps or restrictions. The drain must fall continuously to its discharge point (AS/NZS 3500.4)
- Heated water supply systems must comply with NCC Volume Three, Part B2 (NCC 2025 J8D2)
- Heat pump water heaters must be registered under GEMS (Greenhouse and Energy Minimum Standards) before supply in Australia
What This Means in Practice
How exhaust air sourcing works. A standard heat pump water heater has an evaporator coil, a compressor, and a condenser wrapped around or inside the storage tank. The evaporator draws air across it, extracts heat, and discharges cooler air. Most units have 200 mm duct connections on the air inlet and outlet. By connecting the inlet to a bathroom or laundry exhaust duct, the heat pump draws warm indoor air (typically 22–25°C) instead of outdoor air. In winter, when outdoor air drops to 5–10°C, this warmer source air keeps the COP higher.
Expect a COP of 2.5–5.0. COP depends on the source air temperature and the target water temperature. CO2 refrigerant systems (like Reclaim Energy) achieve an average COP of 5.0. R290 and R134a systems typically run at COP 2.5–3.5. With exhaust air at 22°C rather than outdoor air at 8°C, COP improves by roughly 10–20% in cooler months. Below about 5°C, most units switch to a backup electric element, but exhaust air sourcing avoids that trigger in most Australian climates.
Airflow matters. A heat pump water heater’s evaporator fan moves roughly 200–250 L/s of air. A single bathroom exhaust fan moves about 25 L/s. You cannot just tap the heat pump into one bathroom fan duct. You need to collect exhaust from multiple wet areas into a common duct sized for the heat pump’s airflow, or provide a louvred makeup air path so the heat pump can draw a mix of exhaust air and ambient air. Size the ductwork for the heat pump’s rated airflow, not for the bathroom exhaust rate.
The system doubles as exhaust ventilation. When the heat pump runs, it pulls air out of the wet areas, which helps meet AS 1668.2 exhaust requirements for bathrooms and laundries. But the heat pump runs only when heating water, not continuously. A separate exhaust fan is still needed for code-compliant continuous or intermittent ventilation. Do not rely on the heat pump as the sole exhaust system.
Key Design Decisions
Dedicated Exhaust Air vs Ambient Air Source
For buildings with large volumes of warm exhaust air (apartments, hotels, swimming pool halls), ducting the heat pump to exhaust air boosts winter COP by 10–20%. For small commercial buildings with limited exhaust, ambient air is simpler and still efficient.
Refrigerant Selection: CO2 (R744) vs R290 vs R134a
CO2 systems deliver the highest COP (average 5.0) and heat water to 65–90°C without backup elements. R290 (propane) is a low-GWP natural refrigerant with good efficiency. R134a works but has a GWP of 1,430, and Victoria’s VEU scheme requires GWP below 700. Pick CO2 or R290 for new installations.
Centralised vs Individual Units
A central heat pump with a large storage tank (315–400 L) serves multiple fixtures from one location. Individual units (one per apartment or per floor) cut pipe runs, reduce heat loss, and lower Legionella risk. For buildings over 4–5 storeys, individual units often win on energy and maintenance.
Integration with Mechanical Ventilation Design
Coordinate the exhaust air heat pump with the building’s mechanical ventilation system early. The heat pump’s air intake must not starve other exhaust fans or create negative pressure in occupied spaces. Size the common exhaust duct to handle the heat pump’s airflow plus any simultaneous exhaust fan operation.
Who Needs to Know What
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References
- AS/NZS 3500.4:2021, Plumbing and drainage — Part 4: Heated water services
- National Construction Code 2022, Volume One, Part J8 — Heated water supply and swimming pool and spa pool plant
- National Construction Code 2022, Volume Three, Part B2 — Heated water services
- NCC 2025, Volume One, J6D9 — Pipe insulation
- AS 1668.2-2012, The use of ventilation and airconditioning in buildings — Ventilation design for indoor air contaminant control
- ASHRAE Handbook - HVAC Applications, Chapter 51: Service Water Heating (2019)
- CIBSE Guide G: Public Health and Plumbing Engineering (2014)