4/14/2012 Update. First winter results are being analyzed. Looks good! Students report that the house is comfortable, cozy, and fun to live in even with the interruption of tour groups)!
The last two green building components of TerraHaus I want to add to the blog are the Heat Recovery Ventilator and the Heat Pump.
Our Daikon Heat Pump
The name of the Passive House standard is derived from the emphasis on passive heating. Passive heating means that the space heating requirements are met largely through passive means—the natural sunlight coming through the windows, the heat generated by appliances in the course of routine daily activities, and even the warmth given off by the residents.
The rest of the heat needed to keep the building at 70 degrees is generated by small electric baseboards and by a heat pump. Heat pumps are more common in the southern US than in the north. They have the advantage that they can be used to cool air in the summer and heat it in the winter. They are more efficient than electric heat or combustion heat because they are not creating heat but rather are moving it from one body of air to another.
A heat pump is, in a sense, a type of solar energy because it is the heat from air warmed by the sun that is moved into the home for space heating:
…outside air is heated by the sun (even what we consider cold, winter air contains heat energy given to it by the sun).
…the air is drawn through an evaporator where it warms a refrigerant like freon into a gas even at low temperatures. The refrigerant in a gaseous state is compressible.
…as a compressor reduces the volume of gas, the temperature goes up.
…the gas is transferred to a unit in the house where it condenses and releases its heat.
The heat pump circulation can be reversed to cool the house in the summer.
Given that the heat pump is not creating heat but simply moving the heat energy around, it is very efficient. One kWh of electricity for the fan and compressor results in 2.74 kWh of heat. (As our Sustainable Energy students on campus can explain, COP is the Coefficient of Performance and is calculated as the Btu of output produced divided by the Btus of electricity used. Our COP = 2.74)
In addition to the heat pump, individual bedrooms have small sections of electric baseboard which the occupants can set themselves.
HRV—The Magic Box
Once the heat is captured in the house, from passive means or from the heat pump, we want to hold onto that heat. Air infiltration is one of the major forms of heat loss in most homes, but we benefit from air infiltration too because it ventilates our homes. TerraHaus is so tight that we rely on mechanical ventilation. In order to hold onto the heat though we want to extract the heat and use it to warm the incoming air.
To do this we use heat recovery ventilation (HRV). The stale air passes through an aluminum plenum which absorbs its heat. Air entering from the outside passes through separate passages in the plenum and picks up heat.
Our HRV unit, a Zehnder CA 550, is rated as over 88% efficient meaning that if it is zero degrees outside and 70 degrees inside, the incoming air is warmed to a temperature of 62 degrees before it enters our rooms. No wonder Alan Gibson of GO Logic estimates that 10% of the 90% energy savings of TerraHaus comes from the HRV! (This also explains why he refers to the HRV ventilator as “the magic box.”) The ventilator and flexible tubing ductwork is housed in a space between the ceiling and the bottom of the scissors truss system. The trusses are sealed on the bottom with Zip sheathing. This puts the HRV within the thermal envelope with just two perforations of that envelope for intake and exhaust.
The whole house is ventilated by this system, but the duct work and vents are set up to pull stale exhaust air from bathrooms and the kitchen while supplying fresh air through the bedrooms and living areas. The system is designed to assure that TerraHaus meets the ASHRAE standard of 35% air exchange per hour or 15 cubic feet per minute per occupant. Most homes in Maine, regardless of their age, meet this requirement, but most do so through natural leakage of heated air through the upper portions of our homes and intake of cold outside air from leaks in the lower portions of the building.
Douglas Fox, Director, Center for Sustainability and Global Change, Unity College