But I Like It COLD!
The following text is from a letter written to an Air Conditioning Contractor in response to his concerns about oversizing equipment for a customer in order to maintain a low interior temperature, and the implications of the low thermostat setting.
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 Aprilaire 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.