More ventilation options now available

22 July 2019 by Jason Quinn

The percentage of New Zealand homes with visible mould is appalling—around half. The NZ Building Code has no requirements for ventilation in residential homes other than windows that are capable of being opened. This is manifestly inadequate*.

Previously, I’ve pointed out that continuous extract ventilation is the cheapest functional ventilation option for New Zealand homes. It’s only the best option if your client can’t afford a mechanical ventilation with heat recovery (MVHR) system—or if their home is too old and leaks too much air for MVHR to be efficient. Continuous extraction is also a reasonable option for rental properties, but if the occupant can disable them, assume they will. It doesn’t matter how good the ventilation system works if it’s turned off.

MVHR is best practice ventilation. It’s superior because it provides filtered fresh air, tempered to prevent cold draughts, to every room of the house. MVHR should be the automatic choice for ventilation in a high-performance home such as a Certified Passive House.

That said, there are finally two continuous extract ventilation options to choose between in New Zealand. Before we get into the pros and cons of those two products, let’s define a couple of terms: ventilation rate and air leakage rate.

First, ventilation rate is the amount of fresh air that enters the building each hour: how many times the air changes per hour (ACH). International evidence indicates we want between 0.2 and 0.3 ACH. This means slightly less than a third of the air in the building will be exchanged every hour. This is sufficient to control the level of moisture and other contaminants in the average home environment.

Second, air leakage rate is how much air leaks into (and out of) the building. This is measured by pressurising the building and measuring how much air leaks out. It’s typically measured at a pressure of 50 Pa, roughly equivalent to a 33 km/hr wind on all the sides of the building (I know, that’s not typically how wind blows, but it serves to create a standard metric). Less than 0.6 ACH at 50 Pa (often written as 0.6 ACHn50) is a certified Passive House level of air leakage. A conventional new build in New Zealand built with typical construction materials and methods would leak about 3 to 6 ACH at 50 Pa. I’ve seen test results ranging from 0.2 to more than 20 ACHn50!

Recently, for a project impacting tens of thousands of existing New Zealand homes, I simulated the energy used for a ventilation rate of 0.23 ACH in three scenarios: MVHR, continuous extract fan and opening windows. I feel obligated to point out again that window-only ventilation is not credible and is a bad idea. It’s only included in the modelling in order to demonstrate this point.

In the graph below, you can see the ventilation heat loss in seven example climate zones** across New Zealand. Ventilation heat loss is the heat loss for a building per square metre of floor inside the building for the whole year—including the air leakage and the fresh air intentionally added. I calculated each scenario twice, first with an air leakage rate of 0.6 ACHn50 (the upper limit of Passive House performance) and second for 3 ACHn50 (the low end of a conventional new house). The candy-striped portion of the bar shows the portion of the ventilation heat losses that comes from air leakage, ie uncontrolled ventilation.

Figure 1: Ventilation Heat Losses annually per square meter of interior floorspace. The bottom candy striped section of the bars is the air leakage portion of the ventilation heat losses.

As the graph demonstrates, the MVHR works really well for a Passive House level of air leakage but when the building is leaking at 3 ACHn50 the energy savings are not that much better than a continuous extract ventilation system. However, that’s only looking at the energy cost, and ignoring the health benefits of filtered fresh air throughout the building and the comfort that comes from an absence of cold draughts. (With MVHR, the incoming air is warmed or cooled as needed on entry, so that it is the same temperature as the air inside the house.)

Remember with continuous extract ventilation the fan is only pushing air out of the house. This means the “fresh” air is being pulled into the house from somewhere through the building’s envelope. For houses built on piles, that “fresh” air will usually be pulled from the crawlspace below the floor. If the house sits on a concrete slab, the “fresh” air is typically pulled from the attic or maybe around the sill plates or the attached garage.

Continuous extract ventilation delivers lower ventilation heat losses than using window-only ventilation. This is because the model assumes the windows are opened sufficiently to control moisture, which can be much more often than typically happens in practice. So when you install continuous extract ventilation at a rate that’s appropriate for the building, you do not use more energy than using window-only ventilation.

Say that you’ve reviewed this graph and decided you’re going to recommend continuous extract ventilation for a refit or renovation. What do you choose? There are now two quite different options I can recommend for continuous extract ventilation in New Zealand homes.

Remember my advice is entirely independent and Sustainable Engineering receives zero benefit from anyone for recommending particular products. I name products I have thoroughly reviewed and am confident perform well, for the benefit of my clients and to improve building standards. I’ll update this article on my website over time as new products arrive, do get in touch if you think I’ve overlooked another option.

OK, to the fans at last! First, the Lunos Silvento: very high performance, quite small and nearly silent (22 to 35 dB(A)). One fan can flow 90 m3/hr [changed from 60 m3/hr on 22 August 2019], which is enough for a home smaller than about 125 m2 [changed from 85 m2]. In a bigger house, use multiple fans.

The Silvento’s power consumption is incredibly low, only about 6.5 watts at the 60 m3/hr flow setting. If you left it on for the entire year (and you should), you’d use less than $16 of electricity to ventilate your house all year.

Figure 2: Silvento extract fan courtesy of The Heating Company NZ. Exhausts 15 to 60 m3/hr and is adjustable in 5 m3/hr steps[override to 90 m3/hr]. Suits 21 to 125 sqm of home at 0.3 ACH.

The Intelli-Flow from Weiss is cheaper to buy (about half the price of the Silvento) but more expensive to run. It will flow enough air at its background ventilation rate of 100 m3/hr to ventilate a 140 m2 home. In a smaller house it would slightly over-ventilate and incur an energy loss, but it’s a reasonable option down to about 80 m2.

The Intelli-flow is very simple and a much easier install (although like the Silvento, it must be installed by a registered electrician). It is noisier but on its lowest setting, still quieter than a modern fridge. It automatically senses high humidity and will ramp up to 650 m3/hr as required, a very high flow rate. The Intelli-Flow uses about $50 per year of electricity on the background setting. This is still cheap when you think about the mould damage that occurs in a typical house without proper ventilation.

With this fan in a bathroom of an even moderately well sealed house, it’s vital to check the level of depressurisation that arises when the fan is operating and that it doesn’t backdraft the fireplace or some other combustion appliance into the building. Need I say, backdrafting must be avoided?

Figure 3: ER08P Intelli-Flow extract fan courtesy of Weiss. Exhausts approximately 100 m3/hr on trickle and 660 m3/hr at high humidity rate. A single fan covers 140 m2 at 0.3 ACH.

Technical notes:

The NZ Building Code G4 references NZS4303-1990, which for residential buildings recommends for bathrooms and toilets a flow rate of 90 m3/hr intermittent or 36 m3/hr continuous extract ventilation. (This is the installed capacity of the equipment and it could be commissioned to run at a lower rate). You can read this yourself as recently MBIE paid to have this standard made available for free.

In well-built homes, any extract fan could, in theory, lower the pressure inside the building enough to cause combustion devices such as wood stoves, gas water heaters and even gas ovens to backdraft. This is potentially dangerous and if you have combustion devices, have a professional HVAC engineer or Air Permeability Testing New Zealand (APTNZ) member check that they are not backdrafting.

The Lunos Silvento unit requires some skill and attention to detail to install. The electrician has to assemble it as he or she goes, as there are many options to adjust. It can be set up to allow background ventilation at an adjustable rate and then ramp-up to full power on activation of a light switch for a set time; this is a useful feature.

Figure 4: Silvento. Here’s what’s in the box for installation in a separate toilet.


Weiss ER08P Intelli-Flow continuously runs at 100 m3/hr background rate and uses a rolling average of relative humidity measurement over 90 minutes. It ramps up to full power when it detects humidity above this rolling average. No other customisation or override is available. It’s plugged into an outlet in the ceiling and it is meant to be left on all the time. At 100 m3/hr in a standard 2.4 m high room, it would cover a 140 sqm home as the only exhaust ventilation at 0.3 ACH. There is no means to adjust this flow. It produces 37dB(A) at the low flow background ventilation to 56 dB(A) on high, which is pretty loud. Remember on high, this flows 650 m3/hr using near 70 Watts (compared to about 20 Watts at the background flow level of 100 m3/hr).

Figure 5: Intelli-flow fan. What’s in the box? Very few parts and a much easier install.

* See pp 12-13 of my book, Passive House for New Zealand: The warm healthy homes we need for an explanation of why relying on occupants opening windows to ventilate is such a terrible idea. If you don’t already have a copy, you can download a digital version of the book at

Get in touch if you’re interested in ordering multiple copies of the printed book to make available to your clients.

** Yes, seven climate zones, not three. See page 12 of Passive House for New Zealand. Passive House designers make use of 18 NZ climate zones.

Comments 1

  1. Post

    Clarification 22 August 2019 from The Heating Company is there is an additional option (not in the manual I read) to run the Silvento up to 90 m3/hr. They provide the Lunos data technical sheet showing this, and the related dip switch settings on the PCB module (you set switches 6&7 to zero). I’ve not tested this but the other data has matched the sheet well and I expect it will perform.

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