Mechanical system is nearly installed
Posted from: MD, USA
After the building envelope, the key element of any Passive House is its ventilation system. Because the envelope is so tight, fresh air must be brought in mechanically during the heating and cooling months when the house is closed up. We do this with an energy recovery ventilator (ERV)—a simple fan system that continually brings in fresh air at very low velocity and exhausts stale air, exchanging energy and humidity between the two air streams in the process. This is integral to every Passive House.
The energy recovery ventilator is hooked up to a duct system that carries the fresh air to all the living spaces in the house and pulls stale air from all the bathrooms, the kitchen and the laundry. This creates basically a one-way trip through the house for the air, which is the reason why Passive Houses have proven to be so healthy—they don’t simply churn the air around and around as do standard forced-air heating and cooling systems.
The Zehnder Comfo-Air 350 ventilator we are using is 86% efficient in exchanging heat from outgoing and incoming air, and around 50% efficient in exchanging humidity. Its core is basically a matrix of straws that run two air streams past each other, as shown in the picture below of a small section of a core:
Its efficiency is further improved by the pretempering ground loop, described in an earlier post. To make up for the small amount of heat that is lost through the envelope and the exhaust air, we are running a water line from a heat exchange coil in the domestic hot water tank to another heat exchange coil in the fresh air supply line downstream of the ERV. This will transfer the small heat we need from the heated water into the air supply.
duct heat exchanger, above
water tank heat exchanger, below
In summertime, we must cool and dehumidify the air coming in through the ERV. Most projects in the past have used a very small hi-efficiency Mitsubishi heat pump (sometimes called a mini-split) to do this. They generally hang on a wall as shown in the picture below.
The difficulty with such a system in a house with five bedrooms is getting the cooled air to all of the spaces—after all, air doesn’t move through closed doors very well. What makes our project unique among Passive House projects we know is that we are linking the heat pump to the ERV duct system so that we can distribute that cooled air to every living space. This took a lot of careful planning on the part of Dan Foley, our mechanical contractor, and Michael Lebeau, who led the mechanical component when I took the Passive House training.
Because this is a large house on four levels, we have broken up the mechanical system into parallel systems. One handles the basement and first floor spaces; the other handles the attic and second floor spaces. This allows us much shorter duct runs and provides the ability to zone the house into two parts.
The final piece of the system is the hot water heater. Remember, it is giving us not just our hot water, but our winter heat as well. We are using a Viessmmann Vito-Cell hot water storage tank linked to two heating sources: a 96% efficiency Lochinvar gas fired burner and a solar hot water heat exchange coil.
The water tank, with water boiler on the wall to the left. Lines not yet run:
And finally a couple of progress pictures.
Keith installing the stairway:
The siding going up: