September 16, 2020

Author

Suzanne Grant

TYPE

Passive Design

The Lungs of the House

PART 7 - BUILDING A PASSIVE HOUSE

Achieving Passive House standards

Once the house was wrapped, we had achieved a Passive House standard air-tight house with .36 air changes per hour (ACH). From this point we continued building the inside walls and what was one large room started to become a home.

MHRV



The next Passive House element to be installed was the Mechanical Heat Recovery Ventilation (MHRV) system. I call this the lungs of the house because it brings in fresh air into the house, and exhausts the stale out. If a house is tighter than 5 ACH per hour then a mechanical ventilation system must be introduced. Every 'living room' of the home has a supply of fresh air, so for us bedrooms, living, rumpus and office indicated by the blue supply air on the below plan. The bathroom, WC, laundry, ensuite and kitchen have an exhaust outlet which is the red line on the plan.

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Its a fairly simple concept, as the house is air tight you must ventilate fresh air or the house is unhealthy and all sorts of problems arise, such as mould. The amount of fresh air to be supplied to each room is calculated by the volume of the room, so accordingly on our plans you can see that the living and meals area is supplied with 2 ducts of fresh air for example. The dark green lines show the ducts that vent from the outside to the MHRV. Its important to note that these have to be a minimum distance apart from each other, otherwise you would be short circuiting the fresh and exhaust air. This is the same for the living room fresh air and kitchen exhaust.

So as to not jeopardise the airtightness our design kept all the ductwork inside the airtight wrap and used dropped ceilings and bulkheads to hold the ductwork. So the laundry, WC, bathroom and ensuite all have lowered ceiling heights, then above the robes and Rumpus is a bulk head to bring the fresh air ducts to those rooms.

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Fresh and clean

In addition to supplying fresh air, the system as the name suggests utilises the temperature of the air inside the house to heat/cool the fresh air coming into the home. So the basic principle is fresh/supply air comes into the system, it runs through a heat exchanger to adjust the temperature of the fresh air, then goes from the system into the fresh air manifold. Each supply air duct is balanced (this is done when the house is finished) using air flow restrictors at the manifold so that the right volume of air can travel through the ductwork to each room. Then the extract air obviously works in reverse, exhaust coming into the manifold from the house, this goes into the system to utilise the heat and then ducts to outside. In addition to the heat recovery aspect, the MHRV also incorporates two filters, one of the fresh air coming in which removes pollen and dust so it doesn't clog the heat exchanger and the air being supplied to the rooms is fresh and clean. The other filter is on the exhaust so the air being taken from the home is filtered so the heat exchanger is not clogged with dust or debris.

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The MHRV system we went with is a BRINK Renovent Excellent 400 Plus which costs just under $8,000 supplied. It was installed by us and then balanced and commissioned once the house is complete.

Heating and cooling



In terms of other heating and cooling, we are installing a 2kw Mitsubishi Split System which costs a little over $2,000 supplied and installed. Its crazy to think that the size of the split system that is sufficient for our whole house, is what a standard home would need just for one bedroom! We have also installed a solid fuel fire place which costs just over $3,000 for the supply of the fireplace and other materials required to install in.

I remember when Devin was doing the Passive House Tradesmen course, one of my first questions about it was can you have a fire place? Devin was very reluctant to attempt a fireplace on his first Passive House, and initially we didn't include it in our plans. But for me building a traditional house in the hills calls for a fireplace, and so I put my foot down so to speak and it has been one of the most controversial and stressful elements of the house.

The first element to consider is that as the house is air tight so the air supply to the fireplace must be external. That is most coonara style fireplaces use the air from inside the house to feed the fire which is off the table for us. The second element is that the fireplace has to be sealed and airtight to the house envelope. The way Devin approached it was that the fireplace was built on the inside of the air tight wrap and the flue and external air supply were the two points in which it penetrated the wrap and were sealed accordingly.

We decided on the 'Ribe Insert' supplied by Euro Fireplaces

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We initially installed the fireplace temporarily (hence the use of the prop holding the fireplace in the above photo) because you cannot block the supply air vent into the fireplace and Devin was very apprehensive of how much air would leak out of what is essentially a hole in the house. So it was a massive sigh of relief after the first air tightness test was passed and it was able to be installed fully.

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The fireplace works on convection, so there is no fan or electricity to operate the fireplace. Cooler air is drawn into the Skamol hot box through the bottom vent and the warm air comes out the top vent.

When building we had the fireplace on a few times, which the trades were pretty happy about, but since moving in over summer we haven't needed to use it but I'm definitely looking forward to winter nights here and am expecting the extra effort and stress it caused to all be forgotten.

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