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02 January 2018
How to build a Resilient Home - Mechanical ventilation system with heat recovery (HRV)
A vital element of a Resilient Home is installing mechanical ventilation system with heat or energy recovery (MVHR). In the super-insulated and airtight house, it is critical to provide 100% fresh air all year round for its residents.
In conventional new construction, a ventilation system always comes preinstalled in the form of many small gaps and cracks that drafts and leak air in and out of a house. Some may argue that this is better because it is "free" and no additional mechanical ventilation system is needed.
In reality, it's not free at all. An average house built in the USA right now has around six air changes per hour at 50 Pascals of pressure. And slowly but surely money will leak out of your pockets and disappear into thin air.
It may not seems that way, but contrary to common belief resilient homes are not that much more expensive to build than typical construction. A passive house requires more to be spent on superior materials, high-quality windows, extra insulation, mechanical ventilation, efficient heating, and cooling system, attention to details and high-quality work. But when it comes down to energy systems, additional expense invested in the beginning will pay off.
When building an off-grid house, I think the total price of a correctly done resilient home with all energy systems will not exceed the cost of a conventionally done house of the same size. Energy requirements would be about 70% lower, providing savings on solar panels, batteries, heating, and cooling system.
You just have to decide how you want to spend your money, but it will be spent one way or the other. I don't know about you, but personally, I prefer to invest in a better quality house than into a company that sells solar panels and batteries.
Let me try to explain in simple terms how ventilation should work in an airtight, super-insulated passive house.
With passive houses, builders focus on delivering the maximum airtightness to achieve the desired standard of a maximum 0.6 air changes per hour at 50 Pascals of pressure. That in combination with a high R-rating insulation will significantly cut heat losses in winter and prevent from overheating in summer. Without the appropriate airtightness measures in place, a drafty building will never perform to the optimum level.
One of the most popular misconceptions of a passive house is that its airtightness doesn’t allow the house to breathe. That is correct, partially. The envelope of the house doesn’t allow the house to breathe, but the ventilation system does.
A modern HRV system does much more than just ventilation for a healthy indoor environment. It continually removes stale air out of the kitchen, bathrooms and other rooms with higher air pollution and simultaneously supplies fresh air to living room and bedrooms to substitute the removed air.
Heat from the exhaust air is recovered and applied to the air supply by a heat exchanger. Note that the air flows are not mixed in this process. Best ventilation systems may have a heat recovery rates of 80% to 95%. Basically, you’re recycling the heat in your house without sacrificing your fresh air supply.
The amount of electricity used in this technology to recover the heat is about ten times less than you would use to produce the same amount of heat. This helps dramatically reduce space heating & cooling requirements while providing a high level of comfort.
For many years in conventional construction, the majority of builders used an exhaust-only ventilation. That causes undesirable pressure imbalance inside the home. Air would start finding its way into the house through many cracks and tiny gaps. Drawing air in through the walls brings the risk of moisture condensation inside the walls, especially in cases of big temperature differences between outside and inside air.
It is much better to choose your intake points and add filtration. Excellent indoor air quality is indispensable, and for those who suffer from seasonal allergies, this might be an ideal solution. Especially when it comes in the package with a sizable energy cost savings.