Evaluating RH: Using Key Factors, Measurements
September 24, 2007
Understanding the relationship between dry bulb temperature, wet bulb temperature, rh, and dew point temperature is essential in all facets of air conditioning. These psychrometric processes play an especially important role in building and materials integrity, occupant health and comfort, and overall IAQ.
The good news, strangely enough, is that poor humidity and temperature levels are likely to cause occupant discomfort. Occupant complaints open a window of opportunity for the HVAC contractor to proactively discover related undesirable psychrometric effects on materials integrity and IAQ, including microbial propagation.
To evaluate rh, wet bulb temperature, and dew point, HVAC technicians traditionally used a sling psychrometer and psychrometric chart. Nowadays they use “humidity” meters that are accurate, more convenient, and usable in confined locations unsuitable for sling psychrometers.
Many states have adopted ANSI/ASHRAE Standards 55-2004 on humidity and 62-2004 on IAQ into their building codes. Since both standards have been newly updated, the following descriptions may help inspectors and contractors update practices to meet new requirements.
RELATIVE COMFORTANSI/ASHRAE standard 55-2004, Thermal Environmental Conditions for Human Occupancy, sets an upper limit to absolute humidity levels (0.012 humidity ratio, or 0.012 x 7,000 = 84 grains moisture/lb dry air, also equivalent to a dew point of 62°F), above which most occupants become uncomfortable.
Since all occupants won’t be satisfied by the same thermal conditions, especially all at the same time, the standard attempts to identify a norm based on a PMV (predictive mean vote) of 80 percent satisfaction.
From that, a PPD (predicted percentage dissatisfied) of 10 percent is calculated for general thermal comfort dissatisfaction and 10 percent PPD from local (“my ankles are cold”) comfort dissatisfaction.
The standard lists six primary factors that affect thermal comfort: metabolic rate, clothing insulation, air temperature, radiant temperature, air speed, humidity.
Understanding the combined affects of these factors can help technicians configure building systems appropriately.
HUMIDITY LEVELSANSI/ASHRAE Standard 62-2001, Ventilation for Acceptable Indoor Air Quality, specifies that “Relative humidity in habitable spaces preferably should be maintained between 30 percent and 60 percent relative humidity to minimize growth of allergenic or pathogenic organisms.”
The updated ANSI/ASHRAE Standard 62.12004, Ventilation for Acceptable Indoor Air Quality, is more specific. Now, rh upper limits are based on peak values. “Occupied space relative humidity shall be designed to be limited to 65 percent or less at either of the two following design conditions:
1. At the peak outdoor dew point design conditions and at the peak indoor design latent load.
2. At the lowest space sensible heat ratio expected to occur and the concurrent (simultaneous) outdoor condition.”
Good HVAC equipment selection practices generally recommend:
• 68° to 70° and 30 percent rh winter design.
• 74° to 76° and 50-60 percent rh summer design at outdoor conditions of 97.5 percent winter and 2.5 percent summer dry bulb. This means that on average, 2.5 percent of the extreme seasonal temperatures will be beyond equipment capacity. The equipment will be effectively undersized during these times.
This is critically important in equipment selection, since only 30 percent of the operating hours of comfort cooling equipment occur within 5 percent of outdoor design dry bulb temperature. Summer latent load control is more difficult to control at part-load conditions, although most commercial equipment is staged or has some form of capacity control.
If comfort-cooling equipment is oversized, moisture-related complaints and problems will increase. Residential heat pumps should be selected according to the cooling requirements, not the heating requirements, especially in geographic areas where dirty socks syndrome is prevalent and air-handling equipment is located in a crawlspace.
FUNGUSWith enough knowledge and measurement, HVAC systems can be set at the appropriate summer and winter psychrometric conditions to discourage fungal growth. Conditions for fungal growth include spores settling on a surface, a microenvironment ensuring oxygen, optimal temperatures, nutrients, and moisture. Four of these conditions are found in nearly every environment. The most controllable variant is moisture.
Relative humidity above 60 percent can support fungal growth on hygroscopic (sorbent) surfaces and hygroscopic surfaces at 80 percent rh are likely to promote fungal growth. Nearly all surfaces are, or can become, sorbent and include painted surfaces, gypsum dry wall, carpets, wall coverings, and masonry products. Even glass with a dirt film and dust on it can support fungal growth.
Masonry products such as brick, cinder block, and concrete are excellent sorbents and can adsorb vast quantities of moisture and become an inviting breeding environment for molds. The vapor pressure within the pores of manufactured masonry can be less than the vapor pressure of the ambient air, which moves moisture from the air into the masonry pores.
As the pores become wetted, capillary action takes over and fills the pores, thus providing an ideal breeding ground for fungal proliferation. This explains why some surfaces above dew point can become wetted.
CONDENSATIONConditions that allow condensation to form on surfaces are more obvious, so action can be taken immediately. When a surface temperature is at or below the dew point temperature, condensation will form. Likely places for this to occur are on basement surfaces, crawlspace surfaces, cold-water pipes, on air-handling equipment and ductwork, and unseen within envelope walls.
Basements typically require supplemental dehumidification equipment, since comfort-cooling equipment can’t control humidity in basements with minimal heat gain. Crawlspaces are particularly difficult and expensive to deal with, but sealing them with vapor barriers up to outside ground level, as well as insulating, and incorporating them into the conditioned space and adding additional means of dehumidification, can control many crawlspace moisture problems, provided standing water or excessive ground moisture is not present (this assumes free crawlspace ventilation air is not required for fossil fuel burning equipment).
Water pipes can be insulated. Air-handling equipment and ductwork must be sealed airtight and insulated with no breaks in the vapor barrier especially when located outside of the conditioned envelope. Ductwork in all walls must be sealed to reduce unseen moisture migration due to air pressure differentials.
In cooling systems, rh in supply ducts can be 95 percent or higher, and evaporators and condensate pans will be wet. So, since moisture control is not feasible, control of airborne spores and food (dust and airborne particles) with good, tight-fitting filtration systems in place is essential to control fungus growth.
If evaporator components are resistant to UV radiation, a UVC “germicidal” light that can see the entire evaporator surface can kill mold and microbes. UVC lights should be selected that do not emit ozone, which is an irritant. Oversized equipment will experience reduced operating times resulting in less condensate production which may actually increase the microbial colonization on the fin surfaces.
TEMP-HUMIDITY METERSFrom dry bulb temperature and rh measurements, temperature-humidity meters can calculate wet bulb temperature and dew point temperature, psychrometric points that are essential for HVAC evaluations and diagnostics.
Wet bulb is very closely related to enthalpy, or the total heat in the air (dry bulb and wet bulb). In a psychrometric chart, the wet bulb lines are nearly parallel to the enthalpy scale values. Return wet bulb temperature is mandatory for accurately charging a cooling system that incorporates a fixed restrictor metering device.
Supply and return wet bulb temperatures across an evaporator can be used with a psychrometric chart or enthalpy table to calculate total cooling capacity, sensible and latent capacity, and S/T ratio.
Total heat may be found by multiplying cfm x 4.5 x enthalpy difference across evaporator (Qt = cfm x 4.5 x Δh).
Sensible vs. latent cooling and S/T ratio can be found by plotting conditions on a psychrometric chart or from a psychrometric calculator.
Dew point is critical in both summer and winter evaluations. Duct surface temperature must be maintained above dew point to prevent condensation whether inside or outside of the conditioned space.
Winter indoor rh must be kept low enough to ensure inside wall and window surface temperatures do not approach dew point. If condensation appears on window or wall surfaces, condensation hidden within envelope walls will be likely.
COMFORT COMPLAINTSWith equipment that does not have capacity control, or is staged, most humidity-related comfort complaints occur at part load conditions when run times based on thermostat dry bulb temperatures are shorter. Less operating time means less moisture removal. Oversized equipment will only exacerbate this as well as increasing occurrences of detrimental coincidental conditions. Changing from a fixed restrictor metering device to a thermal expansion valve will ensure maximum evaporator capacity at part-load conditions and utilize more coil surface for moisture removal.
Most cooling equipment can tolerate reduced air volumes of about 20 percent. If evaporator air volumes are reduced from 400 cfm/ton down to around 325 cfm/ton, the evaporator temperature will fall further below dew point and remove more moisture from the air. This change will also reduce duct surface temperature and register temperature in the direction of dew point temperature and register throw, affecting air patterns in occupied spaces.
A dehumidistat can lower air volumes at increased humidity levels. Another alternative is to use a TimedOnControl device, to provide reduced cfm for the first 510 minutes of cooling demand, and then switch to the design cfm to finish the cooling cycle. A portable dehumidifier can be located in areas of high humidity, such as a basement, reducing humidity, increasing heat gain, and forcing longer cooling cycles. Make sure rooms with intermittent high moisture gain, such as bathrooms, kitchens, and laundry areas are ventilated to the outdoors (not the attic or crawlspace).
DEW POINT COMPLAINTSDucts in unconditioned spaces carrying cool, humid air must be sealed airtight using a National Fire Protection Association-approved duct mastic. Any air leaks in a duct will render the insulation useless at that point and condensation is likely to occur. Duct wrap insulation must not be compressed by hangers. Hangers must be placed underneath duct wrap insulation. Duct wrap insulation barriers must be unbroken and sealed at the seams.
In unconditioned attics, increasing attic temperature may increase heat gain on ceilings below, but will reduce the occurrence of condensation on ducts. Attics in homes of newer construction techniques may result in lower attic temperatures, but this increases the chance of condensation on duct or air handler surfaces. Sealing attic vents and adding humidistat-controlled flood lights to increase attic temperature can compensate for this.
Crawlspaces present unique opportunities. Typical crawlspace vent sizing is inadequate for controlling moisture by ventilation. One hundred percent ground cover vapor barrier up the inside wall to a height equal to the outside ground level, sealing the vents, insulating the perimeter walls, and treating it as a conditioned space is a preferred method of moisture control, often requiring additional supplemental dehumidification.
Air-handling equipment in a crawlspace must have excellent particulate filtration in place with no return side air leaks to reduce microbes and their food sources in the evaporator and supply duct. Humidity levels in basements must be regulated to less than 60 percent rh to discourage microbial growth. Painting the surfaces of hydroscopic masonry (cinder blocks, brick, mortar) will reduce moisture retention, discouraging microbes.
Publication date: 09/24/2007