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. |
