Passive house utilizes integrated HVAC, solar system

Passive house utilizes integrated HVAC, solar system

MILLCREEK TOWNSHIP, UTAH — The Breezeway House, a passive 2,800-sq.ft. home located here, near Salt Lake City, was completed last December, taking approximately eight months to build. Since energy efficiency is at the heart of passive houses, many energy-efficient technologies and systems were used, including an efficient HVAC system with an energy recovery ventilator and a photovoltaic system.

HVAC systems can be minimized in passive houses because these houses focus on conservation first, employing the strategy of minimizing losses and maximizing gains through super insulation, air tightness, high performance doors and windows, and ventilation with highly-efficient heat recovery, according Katrin Klingenberg, executive director of the Passive House Institute of the U.S.

“The remaining peak loads are around 1-1.5 Watts/sq.ft.,” explained Klingenberg. (1W equals 3.412 Btuh). “A very tiny HVAC system can take care of that. Most of the year the building is fine ‘passively,’ not needing any conditioning.

“Ideally, all space conditioning would be transported through a ventilation system that is needed for indoor air quality anyway,” added Klingenberg. “Passive home balanced ventilation systems provide usually continuous air supply at very low-flow rates, between 50-80 cfm. That requirement limits the heating and cooling that can be transported through the system significantly. The envelope has to be designed to reduce the losses to a point that the peak heating load is very small and doesn't exceed 1 Watt per square foot. For cooling that number is even smaller, around about 0.8 Watt/sq.ft.”

According to Mark Fisher of Fisher Custom Building, based in Salt Lake City, the project’s general contractor, the Breezeway House’s mechanical system was the biggest challenge.

“Our heating load was only 12,000 Btuh, less than a tenth of a code built house,” said Fisher. “A passive house is very specific on the amount of energy the house uses per square meter. You can meet these criteria by adjusting the amount of insulation type, of glass type and of the heating system.”

“Air-tight construction is a crucial component of a passive house system,” said Dave Brach, owner of Brach Design Architecture, based in Salt Lake City, the project’s architecture firm. “The standard is less than 0.6 air changes per hour at 50 pascals of pressure. This is very difficult to achieve, but Fisher Custom Building was instrumental in their dedication to realizing this goal and was able to ultimately get the house down to 0.46 air changes per hour.”

HVAC, solar system

Heliocentric, an energy and environmental engineering firm based in Salt Lake City, was a key member of the project team during the construction documents phase. The firm designed and installed the HVAC system. Installing the system took approximately three months.

“They [Heliocentric] were the only mechanical contractor in the whole state of Utah that was able to cost effectively design and install the integrated HVAC and renewable energy system — a system that utilizes the mechanical ventilation system to efficiently deliver the small amount of space heat required by a passive house,” said Brach.

When designing the HVAC system, a thorough energy analysis was done by running a 3-D model of the house in a simulated climate for the Salt Lake City area. Heliocentric used EnergyPlus to look at all of the dynamic aspects of the building, including mountain shading, seasonal tree shading, mass thermal storage, natural ventilation and detailed thermal loss analysis, according to Troy Harvey, principal engineer, Heliocentric.

The HVAC system consists of a 2 kW photovoltaic system; solar thermal panels and a 120-gal. solar tank; a water-to-air heat exchanger; an energy recovery ventilation system; radiant floor tubing in the basement and entry of the home to provide a heat boost in these areas if required; motorized dampers; a CO2 sensor and a digital controller, and an indirect-direct evaporative cooler. The solar system should provide approximately 75% of the house’s energy needs per year.

“The solar thermal panels provide heat for domestic hot water and for space heat by heating the liquid in the storage tank,” explained Brach. “The remaining space heat demand is provided by an electric resistance element in the storage tank (and only indirectly by the PV panels). The excess energy created by the PV panels flows back into the grid, causing the meter to run backward and giving the owners a credit on their electric bill.”

Water from the solar tank is circulated via the water-to-air heat exchanger inline with the RecoupAerator 200DX, an energy recovery ventilation system. Only 210 cfm of air is used to heat the house. The ventilation system exchanges the stale air with clean fresh air about once every couple hours. The ventilator captures temperature and moisture from the outgoing air and transfers it to the incoming air stream, and is programmed to regulate heat exchange automatically to prevent frost buildup.

The RecoupAerator has two fans. One fan draws in fresh air while the other pushes the stale air out. The two air streams cross and pass through the patented heat exchanger that transfers both heat and moisture from one air stream to another, so the heat from the air being exhausted is transferred to the air going into the building or home. Also, when outside air enters the RecoupAerator, it is also forced through a MERV 12 filter, capturing incoming contaminants.

To seal off outside air, the motorized dampers close off the home’s ductwork to the outside when the heat recovery ventilation is not operating. To monitor occupant level in the home, CO2 sensors are utilized, which also modulate the heat recovery ventilation rate. If heating demand is greater than ventilation needs, the heat recovery ventilation speeds up to meet that demand. A digital controller manages the HVAC and solar, so occupants in the home can change set points and monitor the system via Web based controls.

For the cooling system, the Oasys, an indirect-direct evaporative cooler was installed. When the cooler meets its set point, a damper opens to the house and two windows upstairs open, so the air has an exit path. The OASys produces up to 3.5-tons of cooling while using less than 600 Watts. The cooler consists of an Indirect Cooling Module, which cools incoming fresh air without adding moisture, and a Direct Cooling Module, which cleanses the air and optimizes humidity. The water used in this process is renewed by a self-purging reservoir, so the waste water from this process can be used to water landscaping.

PHPP

Brach Design Architecture oversaw the Passive House Planning Package (PHPP), the main design tool used for the Breezeway House. PHPP, developed by German physicist Wolfgang Feist who built the first passive house in Germany in 1990, is used by architects and designers to evaluate design solutions immediately without the need to wait for dynamic simulations to run.

According to Brach, with a passive house, energy modeling is done at the same time as the architectural design in a fully integrated process, often being the most crucial distinction of passive design.

Brach was trained by Klingenberg in 2008 and became the first fully certified passive house consultant in the country with the completion of this project.
PHPP energy modeling is climate-specific and allows the architect to make critical decisions during schematic design, regarding the form of a house; the size, location and type of windows; and the amount of insulation to put in the floors, walls and roof. It also allowed the architect to assess different shading strategies, which resulted in the use of custom-designed wood shade devices over all the south windows and the shade structure for the outdoor dining area.

By using PHPP, all of the formal components of the house were optimized to achieve passive house levels of energy use and to heat and cool the house using primarily passive strategies.

“It [PHPP] gives the designer the ability to manage the complexity, to optimize each part of the building envelope and mechanical system to each other in regards to energy consumption and with that also in regards to the cost effectiveness of a certain measure,” said Klingenberg.

The house recently received certification from the Passive House Institute of the U.S. (PHIUS), and it is the first building in the Western U.S. to be a certified passive house and one of the first 10 nationwide.

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