The design, installation and operation of mechanical systems, such as heating and cooling equipment, ductwork and ventilation can have a huge effect on moisture-control and moisture-related problems in your South Florida home. No single building system acts in isolation, and apparently unrelated building systems can actually impact each other in very significant ways. The size of the A/C may be wrong, the design of the duct work may be inappropriate, or perhaps hidden defects may exist such as leakage or poor insulation.
Oversized Cooling Equipment
Florida building code requires that heating and cooling equipment sizes be based upon a recognized calculation method. Residential cooling systems must be sized based upon a house-specific load calculation. The specific orientation of the house should also be accounted for in this analysis because a change in direction can result in a significantly different design load and equipment size. However, the reality may be far from the truth. A large number of A/C contractors don’t play by the rules. In order to get paid quickly, they by-pass the entire load calculation process since permits and paperwork can increase the sales cycle.
It is not uncommon for many A/C contractors to use the “Rule of Thumb” for sizing methods (such as “1 ton of cooling per 400 ft2 of conditioned floor area”). The method should not be used in place of a building-specific load calculation method. Undersized A/C systems will not cool the house properly and oversizing also has negative consequences. Over-sizing of A/C systems causes increased short-cycling of the cooling equipment. Short-cycling reduces energy efficiency and can decrease the moisture-vapor removal capability of conventional A/C equipment by as much as one-half of the rated latent-heat removal capacity. This has two negative effects. First, indoor relative humidity levels are increased as the A/C system reduces the air temperature but removes moisture to a much lesser extent. Second, due to increased indoor humidity and discomfort, occupants may lower the thermostat set-point temperature to compensate. As a result of cooler indoor temperatures (approximately less than 75º F) and higher indoor RH levels, condensation is more likely to occur on windows and doors, on the inside of wall cavities, on concrete floor surfaces, and on floor sheathing and joists above crawlspaces.
Dehumidification Controlling Indoor Humidity in Damp/Humid Regions
It is not uncommon to find customer in South Florida complaining about mold (driven by condensation) because they set their thermostat at below 70º F degrees. Dehumidification may be necessary in damp/humid regions such as South Florida as a means to control indoor moisture levels. Dehumidifiers systems range in cost, quality and function. A few dehumidification system options are discussed as follows:
• portable dehumidifiers: These are the simplest of supplemental dehumidifiers and generally provide adequate dehumidification for a small volume of air (one room or a small basement). They are relatively inexpensive and can include humidity sensors and controls that regulate their operation. These units may require frequent attention to ensure that they are disposing of condensate properly (unless they are plumbed to a drain).
• stationary dehumidifiers: These systems are available in a range of sizes for small and large jobs. Their designs include stand-alone, single-zone systems, or they may be incorporated into a forced-air duct system. These systems, which also give off waste heat, have the advantage of a plumbed condensate drain that does not require frequent attention.
• dehumidifier ventilators: These systems are also stand-alone systems used to dehumidify air, but they also include capabilities for fresh-air ventilation and air filtering. Their ability to introduce and dehumidify outdoor air with a single unit mitigates the moisture that ventilation air can bring into a house, which is a major concern in hot-humid climates. Furthermore, since dehumidifying ventilators are an independent piece of equipment, they can be used to control indoor humidity in hot-humid climates.
Sealed Ducts Reduce Air Leakage and Moisture
Florida Building codes require that joints in duct systems be made “substantially” airtight by means of tapes, mastics, gaskets, or other approved methods. Unfortunately, again, a large number of A/C contractors in the area, will charge separately for going in your attic to apply mastic in the duct joints. This practice drives the unsuspecting homeowner to question whether or not to pay the price. Homeowners don’t know better and the A/C contractor rather not burn inside of a 100º F attic to mastic the ductwork.
Sealing air leakage from ducts has two important advantages. First, it improves HVAC system energy efficiency and, second, it reduces pressure imbalances that can cause air leakage through the building envelope. This air leakage can transfer a large amount of moisture into building assemblies and cause condensation and related problems. For example, a house with leaky supply ducts in the attic can become depressurized, resulting in warm, damp outdoor air being drawn into the building envelope during the cooling season, a particularly troublesome problem in South Florida. Conversely, building pressurization from leaky return ducts can cause humid indoor air to infiltrate into the thermal envelope where it may condensate on cold surfaces.
Inadequate Air Returns Cause Moisture Problems
Most central return systems do not provide adequate pathways when interior doors are closed because door undercuts do not provide enough flow area for the return air. As a result, some interior spaces become pressurized and others become depressurized. Both of these conditions can drive air leakage and moisture transfer. An ideal return-air system provides unrestricted pathways for return air to travel to return grilles.
• Another important feature of whole-house ventilation systems for hot and humid climates is accounting for the added moisture load introduced into the house. In such areas, the additional latent moisture load should be addressed either through direct dehumidification in the ventilation system, or through the use of supplemental dehumidification.
• Finally, testing and balancing of mechanical ventilation systems is also recommended, especially when contractors are installing whole-house ventilation. Simple devices, such as flow gauges, can help to ensure that systems operate close to their design-flow rates. In many cases, as-built installations achieve only half of the intended air flow, and controls and timers may not be installed correctly. Building diagnostics firms and home energy raters can often provide this type of service.