November 17, 2005
Re: Residential
Equipment Sizing and Operating Temperatures
Dear Mr. M______,
Thank you for
requesting information concerning some of the generalities associated with
residential equipment sizing and interior operating temperatures.
Specifically, you requested information on the implications of sizing
residential equipment to maintain an interior temperature of 70º at an
outdoor design temperature of 98º. You also requested
information concerning the implications of maintaining an interior
temperature of 70º in general. The following information should help you
identify many of the concerns with non-typical
equipment sizing and/or temperature control.
To provide some
background information, typical design conditions for the Charleston,
South Carolina area include an interior temperature of 75º with a relative
humidity of 50% and an exterior temperature of 92º dry bulb / 77º wet
bulb. These "standard" design conditions have been developed by the
American Society of Heating, Refrigeration and Air-Conditioning Engineers
(ASHRAE) and are incorporated into ACCA’s (Air Conditioning Contractors of
America) Manual J Load Calculation Procedure. The design conditions
reflect average comfort conditions and take into account the limited
moisture removal capabilities of standard residential cooling equipment.
Although it is
certainly conceivable that the summertime exterior temperature in
Charleston can exceed 92º, ASHRAE warns against sizing equipment to meet
the cooling requirements during the most extreme conditions. Rather,
ASHRAE recommends that in addition to temperature control, cooling systems
be sized to provide adequate comfort dehumidification control as well,
particularly in humid climates such as coastal South Carolina. To
accomplish this, they recommend the use of an outdoor design temperature
for which only a small percentage of predicted seasonal temperatures are
expected to exceed. Since standard residential equipment only removes
moisture when the compressor is running (in an
attempt to satisfy the thermostat), this method
provides for improved dehumidification via longer run times and improves
part-load performance.
Assuming an exterior
design temperature of 98º, the resulting equipment size would be
larger and the dehumidification capabilities of a standard system for the
given space would be significantly reduced, particularly during
less-than-peak loads. Loss of humidity control due to "short-cycling" of
the unit could reduce comfort levels and provide conditions suitable for
fungal activity (mold). The system may also experience higher temperature
swings as the larger equipment would quickly cool the space and drop the
temperature to below the thermostat setting before the thermostat sensed
the change. Although the dehumidification capabilities (and temperature
swing) could be improved with the use of variable speed (variable speed
air handler and 2-stage compressor) equipment, the addition of whole house
dehumidification via add-on equipment (such as manufactured by
HealthyAir and ThermaStor) would be recommended
in this scenario. Additionally, although variable speed equipment would be
recommended, air distribution within the structure may be adversely
affected. Although the duct system would be designed for maximum flow at
the peak load, operationally, the system would run at a lower fan speed a
significant portion of the time, possibly affecting the even distribution
of supply air.
Although the loss of
dehumidification capabilities with oversized equipment can be address via
proper equipment selection and add-on equipment, an interior design
temperature of 70º has implications of its own that must be considered.
First, the discharge temperature associated with 70º return air (50º-55º)
may be problematic. In a scenario where humidity is not taken into
account, supply registers and adjacent surfaces (walls, furniture, etc.)
could be quickly cooled to below the dew point temperature of the interior
air and result in condensation or isolated elevated humidity sufficient to
support fungal activity. Even with sufficient interior humidity control,
lesser degrees of isolated condensation and/or high localized humidity
levels might be possible in moisture prone areas or where air flow was
impeded. Assuming interior humidity is addressed, other issues must also
be considered.
Lower interior
temperatures are often associated with crawl space moisture problems. If a
home is maintained at very cool interior temperature, the temperature of
the floor system will often fall to below the dew point of the air in the
crawl space and condensation will occur on the subfloor, joists and
insulation. Similarly, the colder ducts (due to the supply temperature
running through them) will be more likely to experience significant
condensation and result in colder floors adjacent to the ductwork, thereby
increasing the condensation on the floor system. In addition to general
fugal activity, if condensation is allowed to form on the wood structural
components, wood-destroying fungus will become active and damage the
structure. My experience is that naturally ventilated crawl spaces with
interior temperatures maintained at 70º to 72º suffer from significant,
widespread moisture problems. One method that has been used successfully
to address these concerns is to seal and dehumidify the crawl space,
thereby lowering the dew point temperature and limiting the amount of
moisture available to condense on cold surfaces. This can be readily
accomplished with new construction or retrofit to an existing crawl space
as well.
Although to a lesser
degree than in a naturally ventilated crawl space, the potential for
condensation on ductwork and mechanical equipment located in a ventilated
attic is also increased with colder discharge temperatures. Condensation
in wall cavities has also been associated with colder interior
temperatures. Since batt insulation is often installed with the vapor
barrier to the warm-in-winter side, if the back side of the drywall falls
to below the dew point due to cold interior temperatures, condensation or
elevated humidity (and ultimately fungal activity) will occur within the
wall cavity. This situation is exacerbated where supply air blows directly
onto a wall or in overcooled, confined spaces such as a bathroom. Although
various options are available for existing structures, for new
construction, sprayed-in-place closed-cell foam insulation has been
successfully used to provide a satisfactory thermal and moisture barrier
in walls with similar circumstances. Additionally, the same insulation can
be used in the attic to provide a semi-conditioned space, thereby limiting
condensation on cold ductwork and mechanical systems.
Finally, it should be
noted that, in general, buildings are designed with the expectation that
they will be maintained at, and have mechanical
systems designed for, the conditions outlined at
the beginning of this letter. Any deviations, particularly those that have
the potential to promote condensation within the thermal envelope, crawl
space or attic, should be thoroughly evaluated by the design professional.
I hope this general
information has been helpful. Please do not hesitate to contact me if you
have any questions or if I can assist you in any other way.
Sincerely,
Louis Schweers, PE
GLS Engineering, LLC
