Tiny Smart Heat Recovery Ventilator Does the Job for Passivhaus Apartments

CC BY 2.0. freeAir unit on display/ Lloyd Alter

This BluMartin freeAir unit has almost no ductwork.

One of the big selling points of Passivhaus design is the air quality. There is almost no air leakage through the walls, so most Passivhaus buildings have a heat recovery ventilator where exhaust air pre-warms or pre-cools incoming air, because in an energy efficient building you don't want people opening the windows when they need fresh air, which is responsible for as much as 50 percent of heat loss.

In apartment buildings, air quality is particularly problematic. Most apartments have ducted bathroom exhausts and actually get the replacement fresh air from under the entry door from the pressurized corridor, along with dust and whatever else people have tracked into the carpet. So a Passivhaus style HRV is a huge plus for comfort, health and air quality.

Minden HRV

Lloyd Alter/CC BY 2.0

But getting that air around a house or an apartment can often mean a mess of ducts, and that can be a particular problem in retrofits, requiring boxing or drop ceilings. Here's an extreme example from a house in Minden, Ontario.

FreeAir units

freeAir units/CC BY 2.0

That's why this FreeAir system from German company BluMartin is so interesting.

section through unit

© BluMartin

It's designed for apartments up to 750 square feet, and has almost no ductwork – just the exhaust from the bathroom, which feeds into the top of the FreeAir unit. It seems straightforward in a studio apartment; a loop is created where bathroom exhaust air is replaced by fresh outside air through the heat exchanger.

FreeAir Plus

freeAir Plus/ Lloyd Alter/CC BY 2.0

Where it gets really interesting is where there are multiple bedrooms. The little FreeAir plus unit gets installed in the wall over the bedroom door. It has built-in temperature, CO2 and humidity sensors; when anything gets above the set threshold, it diverts air from the hall into the bedroom.


© BluMartin

Because the unit is in the actual suite, it can be "demand dependent" and deliver fresh air when actually required, rather than running all the time like most HRVs. In summer evenings it directs air to bypass the heat exchanger.

The decisive difference between the FreeAir and comparable ventilation units lies in the integral sensor control system which measures CO2, humidity and temperature levels at different locations through the living space.
HRV closet

HRV closet/ Lloyd Alter/CC BY 2.0

When I toured The Heights, a Vancouver Passivhaus rental building design by Scott Kennedy, he showed the big HRV units on the top floor that each served five apartments below. That's a lot of ductwork and fire dampers, and a lot of noise in this corridor.

FreeAir pulled out

Guts of freeAir pulled out/ Lloyd Alter/CC BY 2.0

But it has the important feature that building management can service the unit and change the filters without having to enter the residential units; tenants and even owners are notorious for not doing this on schedule. That's a downside with the freeAir, where pulling out the unit to change the filters looks to be a bit of a challenge, but there are compensating benefits.

I talk a lot about Passivhaus being "dumb buildings" but they really are not dumb when it comes to air handling, any more than they are passive. This FreeAir unit is pretty smart and sophisticated, and probably does a better job than a central system that serves a lot of units.

The team in Aveiro

Lloyd Alter/CC BY 2.0

I saw the freeAir unit earlier this year in Munich at the International Passivhaus Conference, but they wouldn't let me take photos; I want to thank their Portuguese team from from NZEBS who were far more generous and friendly.