Designs That Work

Mixed-Humid 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 case study house was designed with an un-vented crawlspace, so that all of the duct work and mechanical equipment was able to be located inside the conditioned space.

Figure 23: Mechanical Schematic for Mixed-Humid Climate House

Cooling System

The cooling system is designed with a 14 SEER air source heat pump unit (similar to a Carrier Infinity 17 or an American Standard Heritage 16), which is a high efficiency unit. Higher efficiency units are available and will further reduce the energy consumption of the house, however the 14 SEER equipment strikes a good balance between efficiency and cost. Since this is a cooling dominated climate, the efficiency of the cooling system is significant in the overall energy consumption of the house. In addition, proper sizing (right sizing) of equipment through Manual J calculations is done in order to prevent over sizing of equipment. Over sized equipment increases cost and creates other performance concerns (such as reduction in dehumidification through short cycling of the system).

The distribution system is designed to supply air to rooms in the house with the return being through a central return grill. Manual J calculations are done to determine the duct sizing and flow requirements to the various rooms. These flow rates are used in the duct layout strategy. The air handler and all of the duct work is located in the unvented crawlspace.

Heating System

The heating system is an air source heat pump rated at 8.5 HSPF (similar to a Carrier Infinity 17 or an American Standard Heritage 16). The seasonal efficiency of air source heat pumps increases as one moves into warmer climate zones, however, in the Richmond, VA area, the Air Conditioning and Refrigeration Institute (ARI) rating is the efficiency.

Figure 24: HSPF Adjustment Map

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. For the Prototype house, the air handler is located in the crawl space for ease of access with filter changes and maintenance with the duct work running in the unvented crawlspace. The distribution is from ceiling 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 an air-based duct system, there is a central return that is open to the primary living space, with transfer means from bedrooms to the main space. The return path from the bedrooms 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. The flow rates for the Prototype house in the Richmond, VA climate are shown in the duct layout strategy shown in the drawing set.

Figure 25: Over-door transfer grilles
 

Figure 26: Through wall transfer grilles
 

Figure 27: Transfer Grille to Crawlspace

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. This duct draws outdoor air in to the air distribution system and distributes it to the various rooms in the house. The intake duct has a motorized damper controlled by a fan cycler 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" electronically 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 28: Outdoor Air Duct Connected to the Return of the Air Handler

In addition to the central fan integrated ventilation system, 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.

Domestic Hot Water

The Domestic Hot Water system is designed with a Marathon Hot Water Heater. This water heater has an efficiency rating of 0.94 EF. This is the most efficient tank electric hot water heater available, and can only just barely be topped by an electric tankless hot water heater. The Marathon tank is made out of plastic and uses plastic fittings to greatly reduce the thermal transfer from the tank, while preventing difficulties with corrosion. To minimize wasted energy due to hot water left in supply piping, the pipe runs from the tank are kept as short as possible to minimize standby losses in the pipes and length of time for hot water to reach the faucets.

Energy Model Results

The results of the mechanical systems upgrades represented a reduction in energy consumption of 16.0% when compared to the energy consumption of the Building America Benchmark house design.