Why have energy recovery ventilation in a positive house?
How to recover heat from inside your positive house?
May you don’t know us yet, let’s have a quick introduction: we are ERA architects.
Actually, I’m Esther Rovira, architect and passive house designer in Barcelona, Catalonia and Andorra for either passive houses, efficient houses and positive houses.
In this post ‘Why have energy recovery ventilation in a positive house’?, You will find our professional recommendation.
I hope it helps you!
Let’s go!
Positive houses and passive houses use energy recovery ventilation systems for two main reasons:
REASON 1) To renovate the air
First, in order to secure the air renovation inside.
Which means that the air inside has CO2 levels lower than 1.000 ppm (parts of carbon dioxide in one million parts of air).
Fun Fact: To give you an idea, the CO2 particles in a forest are lower than 300ppm during the day.
In a school classroom of 30m2 with no ventilation and 25 teenagers inside (with their active hormones); will be easily above 1.500ppm within the first 10 minutes of class.
REASON 2) To recover heat
Second, to recover the heat inside during winter with a double vent tube system.
On the side, there is a set of ventilation tubes taking out the foul air inside.
These tubes take away the air from the kitchen and bathrooms since it’s where the air is getting wet and smelly.
So this unhealthy, used air also happens to be at the inside temperature (20º-25ºC). Then, during winter, this temperature is recovered to heat the fresh air that comes from outside without mixing them to each other.
So, on the other side, before pushing fresh air into the rest of the rooms, the outside air goes into a heat exchanger box. This box puts the different air flows into small tunnels in a specific order.
That order forces the different airs to be neighbors on the other side of the tunnel.
Then the fresh air from outside (at 10º) is the neighbor of the air at 20-25ºC.
Since the only thing dividing these neighbors is a thin aluminum wall, and aluminum has a very high thermal conductivity value. Then the airs exchange their temperatures.
This causes the fresh air to get almost the inside temperature even before it’s been pushed into the living room and bedrooms.
The magic of this system is that it uses the inertia of the heat inside to create more heat. Resulting to be a very efficient system running at a very low energy during winter.
You may ask yourself:
What about when it’s -15º outside?
You’re right!, then you may preheat the air before it’s being pushed into the bedrooms.
However these systems are like an electrical toaster and consume lots of energy.
So, we usually use other systems that are more efficient to heat the air inside and give that difference of temperature we need.
Usually an efficient heat pump for the cooler days.
How about summer?
This system has a bypass during summer not to pre-heat the air.
Meaning it takes a detour to go around the box exchanger, so the air comes at an outside temperature directly.
What if the outside temperature is 35ºC?
Then, there are other systems to pre-cool the air before it’s pushed into the rooms.
However it forces you to insulate the fresh ventilation tubes to avoid water condensation.
And if you have 35ºC outside, the passive simulator will force you to have a cooling system apart from the ventilation system.
So, considering there are more efficient systems to cool the air, why spend money and energy insulating the pushing in ventilation tubes?
Our professional recommendation:
Energy recovery ventilation system work best if you have an airtight house.
Airtightness is better achieved in CLT wood houses.
You may also be interested in:
Our other posts:
– Airtighness in a passive house
– CLT wood house price
Hope this post ‘Why have energy recovery ventilation in a positive house?’ is helpful!
Best of luck on this adventure.
See you on the next post,
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