Designs That Work

Cold Climate

The Basic House - Mechanical Systems

As with the building enclosure design, working towards energy efficient mechanical systems is also very important in reducing the overall building energy consumption. Creating efficient mechanical systems is not just a matter of using high efficiency units; the overall system strategy, the location of the equipment and ducts, and the design of the distribution systems all impact the efficiency of the design. This section examines the impacts of efficient mechanical systems through examining the design of the cooling, heating, ventilation, dehumidification, and domestic hot water systems.

Prior to deciding on the specific system design for a house, a calculation should be made as to the maximum heat loss and heat gain of the house to determine how much energy the mechanical system needs to transfer to provide indoor comfort. The Air Conditioning Contractors of America has developed a methodology titled Manual J, which calculates the heating and cooling loads by taking into account the characteristics of the building enclosure. With this information, the system type and size can be determined depending on other constraints.

There are numerous methods for creating and distributing heating and cooling energy within homes, each with their own set of benefits and compromises. The primary decisions about mechanical systems tend to be controlled by available fuels, and by programmatic considerations. In general, there are two types of distribution systems - air based systems and water based systems. While heating can be accomplished with either system, cooling has thus far primarily been provided by air based systems due to the considerations with humidity.

With a tight building enclosure, mechanical ventilation and pollutant source control is also required to ensure that there is reasonable indoor air quality inside the house. A further consideration with the space conditioning system is how it might inter-relate with the mechanical ventilation system. Ventilation air flows are relatively small, and could be accomplished with smaller ducting, but there are certain advantages to coupling the space conditioning and ventilation systems. Exhaust fans located at potential pollutant sources can minimize the need for ventilation, but make-up air must also be considered for the air exhaust fans remove from the house.

In order to ensure good indoor air quality, all combustion appliances are recommended to be sealed combustion to the outdoors. These systems are completely decoupled from the interior environment through the use of dedicated outdoor air intake and exhaust ducts connected directly to the unit. Not only are the combustion products decoupled from the interior environment and concerns of back-drafting of the unit removed, but the usual make up air ducts soft connected to an area near the combustion appliance are eliminated. These make up air ducts (required for naturally aspirated units) are a source of uncontrolled air leakage through the building enclosure, and therefore increase utility use. Finally, the sealed combustion appliances tend to be more efficient than the naturally aspirated units.

Forced air systems can integrate the heating and cooling requirements as well as the ventilation requirements into one system, and therefore are often more cost effective than other specialized heating systems. Intermittent central-fan-integrated supply, designed to ASHRAE 62.2 ventilation requirements, with fan cycling control set to operate the central air handler is recommended to provide ventilation air, distribution, and whole-house averaging of air quality and comfort conditions.

Also, an integrated space conditioning and ventilation system is more likely to be serviced, and provides whole house mixing of indoor air. However, if a cooling system is not being installed, then a water based distribution system can be used instead, with smaller ventilation system ducting, and potentially a Heat Recovery Ventilator (HRV) to economize on heat used for ventilation air.

Typically, cooling requires a ducted air conditioning system, and the use of electricity. Depending on the climate, it may also make sense to use electricity and the ducted system to provide heating, in the form of an air source heat pump (ASHP), or ground source heat pump (GSHP). Where there is significant heating required, and natural gas is readily available, the performance of an ASHP or cost of a GSHP may prove to have a higher life-cycle cost than a condensing furnace. In the case where a cooling system is not desired, the duct system can either be downsized, or deleted and a hot water or radiant system can be used instead.

The location of the duct system can have a significant impact on the overall performance of the system, both the utility use and the ability to provide comfort. The energy loss from the ducts for forced air heating and cooling systems can be significant depending on the location of the ducts, and how well the ducts are sealed against air leakage. Though it is conceptually easy to imagine sealed duct systems, it is uncommon to find tight duct systems, and more common for duct leakage values of 20% of system flow. In many houses, the distribution duct work is located either in a vented crawl space or in a vented attic - effectively outdoors. With the ducts located exterior of the thermal envelope of the home, any leakage and conductive losses from the duct work is lost directly to the outside.

Moving the duct work and air handlers inside the thermal envelope or extending the thermal envelope to include areas such as crawl spaces and attic as part of the conditioned space of the house can be used to help prevent this energy loss to the exterior.

In general, the placement of the mechanical equipment will depend on the design of the house. For houses with conditioned crawlspaces and basements, it is often logical to place the air handler or furnace in those locations. For slab on grade designs or elevated floors, space can become a concern, in which case unvented attics provide for a convenient location for the mechanical equipment and ducts. Otherwise, placement of the equipment and / or ducts in a dropped ceiling or in closets is sometimes necessary. Consideration for space requirements for the mechanical equipment should be made early in the design. The following case study house was designed with a basement, so that the duct work and mechanical equipment was able to be located inside the conditioned space.

Figure 22: Mechanical Schematic for Cold Climate House

Cooling System

Part of the America Benchmark Protocol requires the inclusion of a central cooling system on both the Benchmark and Prototype designs. To this end, the energy simulation calculations reflect the use of a central cooling system. Looking at the loads however, the cooling load is only 6% of the total yearly heating and cooling loads for the house located in Pontiac, MI, with the heating makes up the remaining 94%. Since the cooling is such a small portion of the load, no cooling system was actually included in this design.

Heating System

The heating system chosen is a 92% AFUE sealed combustion furnace. These high efficiency condensing furnaces (similar to a Lennox G51 gas furnace) are readily available on the market. The selected unit should be a sealed combustion unit with the dedicated intake and exhaust ducts connected to the outside.

Duct Distribution System

A ductwork distribution system is designed to supply air to rooms in the house with the return being through a central return grill. The Manual J calculations typically yield the duct sizing and flow requirements to the various rooms to satisfy the loads therein. These flow volumes are used in the duct layout strategy. The furnace is located in the basement with the duct work running in the floor joist of the first floor and up through mechanical chases to the second floor. The distribution is from floor registers in each of the rooms.

As with any distribution system, there must be a return path for the energy distributing fluid. In the case of a air-based duct system, the return path needs to be able to allow sufficient return flow to prevent room pressurization and allow supply flow. While door undercuts can account for some of the return air path, wall transfer grilles or jump ducts should be installed to provide acceptable means for return air.

Figure 23: Over-door transfer grilles

Figure 24: Through wall transfer grilles

Ventilation

The ventilation system for this house is designed as a central fan integrated system, which is made up of a 6 inch outdoor air intake duct connected to the return side of the air handler. The air handler draws outdoor air in to the return side of the air distribution system, as well as return air from the rooms and distributes the mixed air to the various rooms in the house. The outdoor air intake duct has a motorized damper controlled by a fan cycling controller to close the damper to prevent over ventilation of the house during times of significant space conditioning demands. Below is schematic example of the central fan ventilation system with 6" electrically operated damper.

Filtration

It is generally considered good practice to provide for some filtration of the distributed air in the house. It is common to place a filter on the return side of the air handler flow. Standard furnace filters will provide some amount of air cleaning; however in some instances it may be warranted to install a high efficiency 3 to 5 inch filter instead. Even if the high efficiency filter is not added originally, leaving enough room at the return side of the air handler (approximately 12 inches) would allow for the filters to be added to the design at a later date.

Figure 25: Outdoor Air Duct Connected to the Return of the Air Handler

Provision is also made for point source pollutant control. Exhaust fans located in the bathrooms and kitchen are used to remove the localized odors and higher humidity levels created in these areas.

In addition to indoor ventilation and point source exhaust, indoor air quality can be impacted by issues with soil gas diffusion into the living space. In order to avoid potential soil gas introduction into the living space, a sub-slab to roof vent is installed to allow a path of least resistance from below slab to roof without passing through the living space. The preferred vent system consists of a perforated PVC pipe installed in the gravel bed below the slab that is connected to a PVC vent stack that runs from under the slab all the way through the roof. The whole assembly must be sealed to prevent soil gasses that are venting up the stack from leaking into the living space. In addition, any penetrations through the slab and roof assembly must be sealed to prevent air leakage as well. The sub-slab to roof vent system handles conditions that are difficult, if not impossible, to assess prior to completion of the structure - resultant confined concentrations of air-borne radon, soil treatments (termiticides, pesticides) methane, etc. The cost of this "ounce" of prevention is well balanced against the cost of the "pound" of cure.

Domestic Hot Water

Traditional Domestic Hot Water systems use gas or electric heating to heat up water that is stored in a central water tank. Many gas heated tank systems have an efficiency factor around 0.54 EF, or roughly 50% efficient. There have been modest increases in the efficiency of standard tank hot water systems have improved over the years; however, due to the intermittent use of the water, and design of the tank's heat exchanger, much of the energy is lost to standby losses. Using better insulated tanks can reduce the energy transfer somewhat, thereby reducing the effects of standby losses; however, they cannot be completely eliminated. With more efficient tanks, the rating can increase to between 0.62 EF and 0.65 EF.

Instead of a standard tank water heater, this house was designed with a tankless gas domestic hot water heater (similar to Takagi Flash T-KD20). Sealed combustion tankless gas hot water systems can have efficiency factors in the 0.84 EF range due to more efficient heat exchangers and the elimination of standby losses from the system, with some new premium condensing systems that are 95% efficient becoming available on the market.

A well designed hot water distribution system minimizes the length of pipe runs to the various faucets, to provide shorter wait times for hot water, and less wasted heating of water that will cool in the pipework. Potentially, two smaller instantaneous units could be used to service different areas of the house, if long runs from a single unit are encountered.

Energy Model Results

The results of the mechanical systems upgrades represented a reduction in energy consumption of 14.9% when compared to the energy consumption of the Building America Benchmark house design.