Heating & Cooling for Passive Houses and High-Performance Homes

Airtightness and MVHR solve the envelope and the air. The question that follows almost every conversation about a Passive House or high-performance home is simpler: so how do I actually heat and cool it? This guide covers the mindset shift that catches most Australians out, why sizing matters more than brand or system type, and an honest look at split systems, bulkheads, hydronic heating, ducted reverse-cycle and evaporative cooling — including why the last one doesn’t suit an airtight home.

19 min read

1. The biggest mistake Australians make

“My current house has a 16kW ducted system, so the new house needs one too” is one of the most common assumptions in Australian home building — and one of the most expensive. A traditional home leaks air, has comparatively poor insulation, and needs a large system to keep up with that constant heat loss and gain. A high-performance home is a different building entirely: it loses very little heat, gains far less unwanted heat, and requires a surprisingly small system to stay comfortable.

Carrying the old sizing logic across to the new build is the single most common — and most costly — mistake we see. It leads people toward systems many times larger than the home actually needs, with all the downsides that come with oversizing (see section 2).

2. Why system size matters more than system type

A 14kW system isn’t “better” than a 5kW system. It’s only better if the building actually needs 14kW. Every home has a heating and cooling load — the amount of heat that has to be added or removed to keep it comfortable in typical conditions — determined by the envelope’s airtightness and insulation, its glazing, orientation and the local climate. That number, not habit or what the last house had, is what should size the system.

If your high-performance home only needs 2kW and you install a system sized for 14kW, the oversized unit will:

• Short cycle — turning on and off far more often than a correctly sized unit, which is harder on the equipment and less efficient.
• Dehumidify poorly — a system that reaches its target temperature quickly doesn't run long enough to properly remove moisture from the air.
• Run noisier — larger fans and compressors moving more air and refrigerant than the room actually needs.
• Operate less efficiently — most systems are least efficient at partial load, which is exactly where an oversized unit spends most of its time.
• Cost more to buy — a bigger system is a more expensive system, for a smaller job.

3. Split systems: the default that fits

A reverse-cycle split system — one outdoor compressor unit connected to one or more indoor units — is, for most Passive Houses and high-performance homes, the clear starting recommendation. Not because HiPer Haus sells them, and not because they’re the cheapest option in isolation. It’s because the engineering supports it: when heating and cooling loads are as low as a well-built high-performance home’s, a correctly sized split system can comfortably do the job that a much larger, more complex system would otherwise be asked to do.

Many people assume Passive Houses require expensive, specialist heating equipment. In reality, an open-plan high-performance home can often be fully conditioned by a single well-placed, correctly sized indoor unit. Homes with several enclosed rooms, multiple levels, or a layout that limits airflow between spaces may need more than one unit, or a different distribution strategy — which is exactly what a load calculation and a conversation about your specific floor plan will clarify.

This is an educational point rather than a sales pitch: split systems tend to win on the numbers for low-load homes, but bulkheads, ducted systems and hydronic heating all remain valid choices in the right circumstances — covered below.

4. Bulkhead air conditioning explained

A bulkhead air conditioner is often assumed to be a completely different product to a wall-mounted split — it isn’t. It’s essentially the same reverse-cycle technology, just relocated and hidden:

Instead of a visible box on the wall, the indoor unit sits inside a purpose-built ceiling bulkhead and distributes air through short duct runs to discreet linear or ceiling grilles. The trade-off is a cleaner architectural finish in exchange for slightly more install complexity and cost than a standard wall unit, plus the need for an accessible ceiling void. Sizing follows exactly the same logic as any other reverse-cycle system — get the load right first.

5. Hydronic floor heating — and why it can overheat Passive Houses

Hydronic floor heating circulates warm water through pipes embedded in the slab or screed, radiating gentle heat upward. It’s a genuinely comfortable form of heating — but in a high-performance home with good solar access, it has a specific failure mode almost nobody expects: thermal lag.

A concrete floor has a lot of thermal mass. It takes hours to warm up, and hours to cool back down. In a home with generous north-facing glazing, that lag can work directly against you:

The floor was doing its job — responding to a cold morning — but by the time the sun has added its own heat on top, the slab is still releasing warmth it no longer needs to. In a leaky, poorly insulated home this rarely matters, because the building loses heat quickly enough to absorb the mismatch. In an airtight, well-insulated home, that surplus heat has nowhere to go. This doesn’t mean hydronic floor heating “doesn’t work” in a Passive House — it means it needs careful load and thermal-lag modelling against your specific glazing and orientation, not a standard sizing rule. See is underfloor heating worth it in Australia in the FAQs below.

6. Hydronic radiators

Hot water radiators respond far faster than a floor slab, since there’s comparatively little thermal mass to warm up or cool down, which avoids the overheating risk described above. They also allow genuine room-by-room control. The trade-off is cost and complexity relative to a split system: in a home whose heating and cooling load is already small, radiators are usually chosen for a specific reason — a preference for radiant comfort, a heritage or architectural fit, or whole-home distribution without visible indoor units — rather than out of necessity.

7. Ducted reverse-cycle, done right

Ducted reverse-cycle systems can work very well in high-performance homes when correctly designed. The issue is rarely the technology itself — it’s the assumptions behind the design. Many Australian ducted installations are based on standard sizing rules developed for conventional housing. Applying those same assumptions to an airtight, well-insulated home can result in oversized equipment, short cycling and unnecessary energy use — all the problems covered in section 2, at a whole-home scale.

Done properly, a ducted system for a high-performance home starts from an actual heating and cooling load calculation, uses that number to size the unit and design the ductwork, and treats zoning as something to assess rather than assume (see the FAQ on zoning below).

8. Evaporative cooling and airtight homes

Evaporative cooling is common and effective in much of Australia — but it works on a principle that’s fundamentally at odds with an airtight home. An evaporative cooler draws hot outside air across water-saturated pads, cools it through evaporation, and pushes it into the house. For that air to keep flowing and for cooling to continue, it needs somewhere to go — which means windows or doors need to stay open so the air can escape.

An airtight home is built around the opposite idea: a sealed envelope with fresh air delivered and stale air removed through a balanced MVHR system, not through open windows. The two approaches don’t just compete — they actively undermine each other:

Evaporative cooling also requires a large roof penetration for the unit itself — another direct breach of the air barrier that the rest of the building has been carefully designed and detailed to maintain.

9. Comparing your options

There’s no single “best” system — the right choice depends on budget, architecture, climate and personal preference. What follows is a starting point for that conversation, not a ranking.

Frequently asked questions

JH

Written by

Jonathen Hindry

Founder of HiPer Haus. 25+ year plumber turned Certified Passive House Tradesperson — blower door testing, MVHR design and heat pump hot water across Adelaide and South Australia.

About Jonathen & his experience →