It’s no secret that we live in a warm and rapidly warming climate.
And, whilst our aim at Leanhaus is to design and build homes that minimise reliance on additional cooling, the reality is that air conditioning is often necessary as a failsafe. Compared to building walls and roofing that are sufficient enough to withstand the climate extremes, installing air conditioning for the occasional use is far more cost effective and practical. Even if we did build extremely thick, high-performance walls, there would be no additional controls to modify the internal temperature in the event of a heat wave (something that would be very unpleasant).
So, we can safely assume that some air conditioning will be required from time to time, but how should we size and specify the system for a Passive House?
At Leanhaus, we utilise the Passive House system and Passive House Planning Package analysis software, which helps us accurately predict how comfortable and efficient a home will be once built. Our findings with this system suggest that, in a Climate Zone 5 city like Perth, a passively designed Leanhaus will require approximately 20-30W/m2 of cooling load. For a 200sqm house this translates to a 6kW cooling system.
In comparison, a typical 4 x 2 home in Perth will require a 12-15kW ducted reverse cycle system with 4 zones. This translates to a cooling load of around 80W/m2.
The reason for this, is that in a typical home, poor insulation, lots of thermal bridges and AC ducts located in hot roof zones make air conditioning quite inefficient, using around 3kW of power at normal load. On top of this, given the cost of operation, most people wait until they return home at the end of a hot day before turning on the air conditioner. At this point it has to work very hard to bring the interior temperature back into the comfort range in a short period of time. Also, brick homes store heat energy and continue to radiate the day’s heat into the house, long after the outside temperatures have dropped.
In contrast, a Leanhaus home designed with Passive House principles will be well insulated with an airtight envelope, making the home much easier to cool and keep cool. In such a home, a 6kW system rarely needs to operate above 50% load.
The type of air conditioner you choose and where you install it can also impact the efficiency. Generally, we recommend two options, so let’s weigh up the pros and cons of each.
SPLIT SYSTEM AC
The most cost effective and energy efficient option would be to provide two split air conditioning units. One 3kW unit in the main living area and another in a second location such as the bedroom zone or second living zone. The pros and cons of this system include:
Pros
Cost effective
Energy efficient
Easy to install
If one fails, you still have some air conditioning with the second unit
Cons
Head units can look unsightly unless bulkhead type units are used
Only two or three zones are impacted
Floreat Haus stays cool in summer with a single 6kw split system with a bulkhead grille above the kitchen.
A wall mounted AC head unit instead of a grille would have provided a significant cost saving.
DUCTED AIR CONDITIONING
The other option involves a single, smaller ducted air conditioning system that serves multiple rooms. A ducted system physically moves cool air through large ducts around the home. The pros and cons of this include:
Pros
More discrete ceiling vents in lieu of air conditioning head units
Multiple zones can be serviced from one control system
Cons
Costs more to install and operate than two split units
Air ducts are larger in diameter than split system pipe work. This is important to consider early in the design to ensure large enough ceiling and floor voids are built
May prevent the potential for raised or raked ceilings
Silver 300mm AC Air handling ducts use a considerable amount of space and result in dropped ceilings. Note that unlike a typical home, the ducts are installed inside the thermal envelope of the home, not in a hot roof space.
No matter the system you choose, air conditioning should be managed quite differently in a passively designed home than in your standard house. With smaller cooling loads, it can be effective to set the thermostat to your preferred temperature and have the air conditioner running at a very low capacity throughout the day. The highly efficient building envelope will minimise cooling loss and a ducted Heat Recovery Ventilation System will have more time to gradually distribute the coolness from the conditioned to non-conditioned areas. This makes even more sense if you have solar installed to provide free power.
to live comfortably with less impact, it is always important to actively operate your home to minimise heat gain and maximise opportunities for passive cooling:
Install external shading to prevent heat gain through windows.
Open doors and windows to create cross ventilation and flush out heat when outside conditions are suitable.
Utilise ceiling fans to create air movement.
Purchase your own weather station to monitor both the internal and external environment, so you know when the above methods will be suitable, or when you’ll need to turn on the air conditioning. We have been very happy with the Netatmo weather station that provides data logging with remote mobile and web access.
It’s very likely that air conditioning will, at times, still be necessary in a passively designed home. However, they do allow for a significantly smaller system that’s operated less often and at less cost than would be expected in a standard home.