Designs That Work
Very 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. In this case, there is
essentially no cooling required, so a radiant heating system was chosen.
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 radiant heating system and small
ventilation ducting, so that the duct work and mechanical equipment was
able to be located inside the conditioned space.
Figure 22: Mechanical Schematic for Very 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 much
less than 1% of the total yearly heating and cooling loads for the house
located in Juneau, AK, with the heating makes up the remaining over 99%.
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 an 85% AFUE sealed combustion oil fired
hot water heater, both for the availability of oil for heating, and the
small size of the components of the system. The high efficiency oil boiler
(in this case a Toyotomi Oil Miser OM-180) is somewhat of a specialty
item, but is a good option for the cost and sealed combustion. The
selected unit should be a sealed combustion unit with the dedicated intake
and exhaust ducts connected to the outside to avoid any potential for
back-drafting combustion products into the house.
The choice of a heat distribution system in the case of this prototype
isn’t impacted by a need for cooling, and space is at somewhat of a
premium, so baseboard finned tube radiators are being used for heating.
Heat will be distributed around the house using baseboard finned tube
radiators, which has been sized for a lower water temperature to allow
integration with the hot water system, and higher efficiency. Standard
baseboard radiators similar to Slant Fin BaseLine 2000 could be used with
length shown on the drawings in the Appendices.
Duct Distribution System
With no need for cooling duct flows, the duct system can be
significantly downsized to meet only the modest ventilation needs of the
house. Small ducts are run from the outdoor air intake and exhaust hoods
to the HRV, with supply air to the bedrooms of the house, and exhaust air
from the common space. With the small flows expected from the HRV, the
undercut on doors can easily handle the return air flow, avoiding the need
for any further means of return.
Ventilation
The heart of the ventilation duct system is an HRV with flow ratings in
the ~40-50 CFM range. Using the duct system described above, the objective
is to turn over air throughout the house by locating the supply and
returns on opposite sides of the house. The HRV fan is a particularly
efficient means for providing the small ventilation air flows, with the
added benefit of gaining heat recovery in the process
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.
Filtration
It is generally considered good practice to provide for some filtration
of the distributed air in the house. In the case of a house with a Heat
Recovery Ventilator, a small filter could be installed in the system for
the inlet air. Some HRV’s are designed to re-circ and filter house air,
though their power use tends to be higher than a simple ‘once-through’
model. Higher levels of filtration generally require larger fan sizes than
are found in HRV’s.
Domestic Hot Water
The base system for domestic hot water would be direct heating of the
domestic water using the oil water heater. In this way, the firing rate of
the appliance leads to the higher efficiency for hot water. However, some
building codes don’t allow using potable water in the house heating
system, in which case an indirect tank water heater similar to Amtrol
Boiler Mate or Heat Transfer Products SuperStor tank could be added in a
parallel zoned system through the boiler. While there is some loss of
efficiency on the hot water side of things, since the boiler is within the
conditioned space, and the need for heating is an overwhelmingly large
part of the year, most ‘stand-by’ losses directly offset heating needs,
and are not actually losses.
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.
Energy Model Results
The results of the mechanical systems upgrades represented a reduction
in energy consumption of 6.3% when compared to the energy consumption of
the Building America Benchmark house design. |
