Designs That Work
Hot-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 and directly vented 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 - duct leakage values of 20% of system flow are common.
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 enclosure or
extending the thermal enclosure 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 conditioned and semi-conditioned
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 unvented conditioned
attic, so that all of the duct work and mechanical equipment was able to
be located inside the conditioned space of the attic.
Figure 22: Mechanical Schematic for Hot Humid 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, and any
upgrades to the system provide good payback terms. 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 lack of
proper dehumidification through short cycling of the system).
Heating System
The heating system is an air source heat pump rated at 8.5 HSPF (again
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, since the outside temperature is higher for a larger
portion of the year, and rarely drops to freezing.
Figure 23: HSPF Adjustment Map
While the standard ARI rated efficiency is 8.5 HSPF, the Air
Conditioning and Refrigeration Institute has a climate zone map that shows
adjusted efficiencies for different areas of the country. Hot humid
climates are in either Zone 1 or Zone 2, which increases the HSPF rating
by 0.8 to 1.3, meaning that the actual seasonal efficiency will be between
9.3 to 9.8 HSPF when the unit is used in climates such as New Orleans.
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 living space for ease of access with filter changes and
maintenance with the duct work running in the unvented attic. The
distribution is from ceiling registers in each of the rooms.
Figure 24: Air Handler Schematic
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 New Orleans, LA climate are shown in the duct
layout strategy shown in the drawing set.
Figure 25: Overdoor transfer grilles
Figure 26: Through wall transfer grilles
Figure 27: “Jump Duct” over interior partition wall
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 when the air handling unit is running. 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.
Dehumidification
In more energy efficient building enclosures the air conditioning loads
are reduced, especially the “sensible” (non-humidity) heat gains. Since
there is less of a sensible air conditioning load, there also tends to be
a reduction in the amount of dehumidification from regular air
conditioning operation. Therefore, for hot-humid climates, it is
recommended to provide supplemental dehumidification to avoid times of
uncomfortably high indoor humidity.
The house is designed with a stand alone dehumidifier located in the
base of a mechanical closet behind a louvered door and the air handler
located directly above the dehumidifier. This configuration places the
dehumidifier directly in the return air path so that the air will be drawn
past the dehumidifier during the fan cycling periods. This system has been
shown to provide reliable dehumidification while still maintaining
affordable installation costs. With this system it is important that the
house have proper mixing and redistribution of interior air, therefore
central fan cycling is required for distribution of the dehumidified air.
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 18.1% when compared to the energy consumption of
the Building America Benchmark house design. |
