Learn About Your System

Refrigerants 
Humidity and construction

Tips, before you call for service
Do you have a humidistat? If you do, it will be located on the wall next to the thermostat. Make sure it is turned all the way to the on or the off position (more on this subject under "Humidistats"). Check your circuit breakers, some homes may have one located outside by the condensing unit (outside portion of your system) as well as in the main panel. Have you waited 10 minutes? Some systems have a built in time delay (anywhere from 1 to 10 minutes). Have you checked your filter?

Humidistats
A humidistat is an energy saving device most useful to people leaving their house unoccupied for periods longer than one week. Humidity must be kept at or below 50% to prevent mold and mildew growth.Without a humidistat when the house is unoccupied the thermostat is usually left set at 80 degrees. The problem associated with this method is when it's 90+ degrees outside the air conditioning system will run until it reaches 80 degrees right? And if its 78 degrees outside the air conditioning system will most likely not run at all. So how do we know that we are not running the A/C more than necessary or not enough? We don't. With a humidistat when the house is unoccupied the thermostat is set to 74 degrees and the humidistat is set to 50%. The air conditioning system will now run until it reaches 50% humidity at which time the humidistat will override the thermostat and shut the system off regardless of the temperature inside the house. Now when it's 90+ degrees outside the air conditioning system may only have to cool the house to 85 degrees to achieve the desired results or if it's only 78 degrees outside it will be able to cool all the way to 74 degrees if necessary. When the house is occupied simply turn the humidistat to the on position and it will no longer have control of the system. Be careful though, if your humidistat was not installed by Houston Air you may have to turn it to the off position as not all companies wire humidistats the same way. If you contact Houston Air we will send you an instruction card suitable for mounting next to your humidistat free of charge no matter where you purchased it.

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Thermostats
Most homes have a simple mechanical mercury bulb type thermostat. They are very reliable and accurate to within 3 degrees. Digital thermostats are now affordable and accurate to within 1 degree. They vary from simple non programmable to fully programmable enabling different temperatures automatically throughout the day. This feature is most helpful if you have a fixed schedule, for example: during the summer if you leave at 8am the temperature would go up to 80 degrees and would return to 76 degrees at 5pm when you return. If your VCR flashes 12:00am you may be the type of person better suited to a non programmable type of thermostat! Also available are large number thermostats for the sight impaired.

Many people wonder whether they should operate their thermostat with the fan switch in the "auto" or "on" position. There is no one best way for every circumstance but there are differences. When the switch is in the "auto" position the fan will shut off when the desired temperature is reached along with the condensing unit (outside unit). Disadvantages: When the air flow stops the thermostat will only sense the air temperature in the area where it is located. So if different areas of the building heat up at a faster rate, the thermostat will not bring the system on until the heat enters the area where it is located. By running it in the on position there is always air flowing past the thermostat from the farthest areas of the building. When the switch is in the "on" position the fan will operate constantly, even after the desired temperature is reached and the condensing unit has shut off. Disadvantages: When the condensing unit stops and the fan continues to run, the evaporator coil (cooling coil) becomes warm and all the water that it was removing from the air when it was cold, now evaporates back into the air causing higher humidity levels. Also the average fan motor will consume 600 watts of electricity, so left running constantly would be equivalent to six 100 watt light bulbs. Houston Air recommends leaving the thermostat set to the "auto" position in most cases.

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Filters
Air filters remove particulates before they soil furnishings, foul critical system components, cause discomfort, or otherwise spoil a comfortable indoor environment. Consumers have a choice of four types or air filters: standard filters, poly filters, media filters, and electronic air cleaners.

Standard filters such as throw-away fiberglass and washable hogshair (the stiff blue material you cut to size) are the least expensive and least efficient of all air filters. Most common sizes are available at hardware stores. Their efficiency is about 3 to 5 percent according to standard industry comparisons. 

Poly filters are more effective than standard filters, but also a little more expensive. They are available from Houston Air and consist of a permanent aluminum frame which holds disposable poly media. Efficiency is about 35 percent. Another advantage over standard filters, they are custom made to the size you need insuring a proper fit. Buying the wrong size filter and cutting it to fit is a recipe for a dirty system, bringing high operating and repair costs.
 
Media filters are the most effective type of disposable filter, and also more expensive.  The media filter traps particles as small as .5 micron including most pollen and plant spores.  Helps maintain cooling equipment efficiency, and can last up to one year before needing replaced.

Electronic air cleaners, the most expensive of all air filters, also are the most efficient. Unlike standard or media air filters that rely on the size of the openings in the material being used to be large enough to let air flow through, but small enough to trap particles, electronic air cleaners attract and retain pollutants like a powerful magnet. When particles reach the electronic air cleaner, the larger particles are trapped by a screen prefilter. Smaller particles pass through to the ionizing section where they receive an electrical charge. These charged particles continue through to the cell to the collecting area where they are attracted to a series of grounded plates. The pollutants are held in this section until washed away during cleaning. Efficiency is about 95 percent, removing particles so small an electron microscope is required to see them.

Summary
Houston Air recommends poly filters for the majority of our customers, for those with allergies, electronic air cleaners. With poly filters costing approximately $1.50 more than standard filters, the only reason standard filters, with their low efficiency and higher long term cost, are still used is due to their wide availability and lack of consumer education on the subject.

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Refrigerants
In 1992, a phase out schedule for refrigerants classified as HCFC’s, which includes Refrigerant 22 (R-22) was agreed upon. This phase out was scheduled for the United States and other technologically advanced countries, such as in Europe and Japan . As mandated by the Montreal Protocol Treaty, the phase out schedule the U.S. must meet includes the following dates.
Refrigerant 22 (Chlorodifluoromethane) contains chlorine atoms, which are ozone depleting and contribute to global warming.
Refrigerant 410A (Difluoromethane/Pentafluroethane) has been developed to serve as a long-term non-ozone-depleting replacement for R-22.
1996    U.S. HCFC baseline cap was established.
2004    HCFC cap is reduced by 35% of baseline.
2010    HCFC cap is reduced by 65% of baseline. In addition Air Conditioners and Heat Pumps that use R-22 will no longer be manufactured or imported into the U.S.
2015    HCFC cap is reduced by 90% of baseline.
2020    HCFC cap reduced by 99.5% of baseline. Manufacturing and importing of R-22 is banned.
2030    Total ban on all HCFC refrigerants.
Keep in mind, the equipment purchased today may encounter costly repairs later on, because of leaks or other repairs that may require the sealed system to be opened and refrigerant to be replaced. Also parts that are related to R-22 may become more costly, since the demand will be lower and not as many are produced. Remember supply and demand always governs the market price.
 

Humidity and Construction

Attic ventilation in humid climates; is it necessary or even beneficial? Experience and past study of structures built near the ocean or intercoastal waterways show that the ventilation of attics can create more problems than benefits. In the past ten years, we have closed off and eliminated attic ventilation from numerous projects without adverse affects to the buildings involved. Several even experienced a drastic improvement in conditions. This year the "1997 ASHRAE Fundamentals Handbook’ chapter 23.10 states "because of the net benefits of attic venting do not always clearly out weight the disadvantages, venting should not be required in a warm, humid climate. Venting should be considered a design option."

The major issue is that the installation of attic vents is required by SBCCI Building code (section 1708.7-1991 Edition.) The purpose of vented openings is a very misunderstood requirement in the code. The primary function of attic ventilation is to control condensation in attics that occurs when moisture migrates from the inside of the structure to the outside in cold (heating) climates. In cold climates, cold attic temperatures can cause this to condense out on metal nail heads and structural plates holding together wood trusses and other construction. This occurs when tile vapor pressure inside the home is higher than that of the outside, which is during the heating season. Moisture migrates from the higher vapor pressure (inside) to the lower vapor pressure (outside). The attic ventilation serves to allow this moisture to diffuse in the drier air before it has the opportunity to condense.

In a humid zone, tile vapor pressure is higher outside than inside backwards to the heating climate. The necessity to control condensation due to cold outside temperatures is invalid in humid climates. A more technical explanation can be found in the ASHRAE (1993 Handbook of Fundamentals) chapter 21 table 1. This table lists recommended practices for what is shown as zones 1,2 & 3. Figure 11 in chapter 21 indicates South Florida is not included in any of these zones, ASHRAE, explains that in the crosshatched area shown by figure 11, condensation is not thought to be a problem, and therefore has not been addressed. It also notes that in coastal climates, attic ventilation becomes less effective. This information translates into the latest information that appears in tile 1997 ASHRAE Fundamentals Handbook, suggesting attic ventilation be a designer’s option.

The chapter also indicates that unconditioned or unventilated spaces should be conditioned or connected by openings to conditioned spaces (not outside or to the attic) to prevent mold growth. In humid zones, homes are either Ventilated or air conditioned most of the year. When cooling, the home will be at a lower vapor pressure than the outside allowing any trapped moisture to equalize to the inside through normal moisture migration as a result of the permeability of construction materials. A vapor barrier should not be located on the inside of an exterior wall or at the ceiling unless specifically designed to vent to the interior from behind the vapor barrier. By excluding the attic ventilation, any normal moisture transfer through materials into the attic space will be handled by moisture migration to the air-conditioned space.

The secondary benefit of soffit vents is heat relief. However, when a load calculation actually accounts for moisture load caused by attic ventilation, it is found that the moisture load from a vented attic can be more than twice the load than that caused by a higher temperature attic.

In numerous homes that have closed up the attic ventilation, rarely do we see over a ten degree increase in light/medium colored roofed attics when these vents are closed. Observation of homes in coastal areas near salt water shows several reasons eliminating attic ventilation is a benefit, these are:
Attic ventilation in humid climates; is it necessary or even beneficial? Experience and past study of structures built near the ocean or intercoastal waterways show that the ventilation of attics can create more problems than benefits. In the past ten years, we have closed off and eliminated attic ventilation from numerous projects without adverse affects to the buildings involved. Several even experienced a drastic improvement in conditions. This year the "1997 ASHRAE Fundamentals Handbook’ chapter 23.10 states "because of the net benefits of attic venting do not always clearly out weight the disadvantages, venting should not be required in a warm, humid climate. Venting should be considered a design option." The major issue is that the installation of attic vents is required by SBCCI Building code (section 1708.7-1991 Edition.) The purpose of vented openings is a very misunderstood requirement in the code. The primary function of attic ventilation is to control condensation in attics that occurs when moisture migrates from the inside of the structure to the outside in cold (heating) climates. In cold climates, cold attic temperatures can cause this to condense out on metal nail heads and structural plates holding together wood trusses and other construction. This occurs when tile vapor pressure inside the home is higher than that of the outside, which is during the heating season. Moisture migrates from the higher vapor pressure (inside) to the lower vapor pressure (outside). The attic ventilation serves to allow this moisture to diffuse in the drier air before it has the opportunity to condense. In a humid zone, tile vapor pressure is higher outside than inside backwards to the heating climate. The necessity to control condensation due to cold outside temperatures is invalid in humid climates. A more technical explanation can be found in the ASHRAE (1993 Handbook of Fundamentals) chapter 21 table 1. This table lists recommended practices for what is shown as zones 1,2 & 3. Figure 11 in chapter 21 indicates South Florida is not included in any of these zones, ASHRAE, explains that in the crosshatched area shown by figure 11, condensation is not thought to be a problem, and therefore has not been addressed. It also notes that in coastal climates, attic ventilation becomes less effective. This information translates into the latest information that appears in tile 1997 ASHRAE Fundamentals Handbook, suggesting attic ventilation be a designer’s option. The chapter also indicates that unconditioned or unventilated spaces should be conditioned or connected by openings to conditioned spaces (not outside or to the attic) to prevent mold growth. In humid zones, homes are either Ventilated or air conditioned most of the year. When cooling, the home will be at a lower vapor pressure than the outside allowing any trapped moisture to equalize to the inside through normal moisture migration as a result of the permeability of construction materials. A vapor barrier should not be located on the inside of an exterior wall or at the ceiling unless specifically designed to vent to the interior from behind the vapor barrier. By excluding the attic ventilation, any normal moisture transfer through materials into the attic space will be handled by moisture migration to the air-conditioned space. The secondary benefit of soffit vents is heat relief. However, when a load calculation actually accounts for moisture load caused by attic ventilation, it is found that the moisture load from a vented attic can be more than twice the load than that caused by a higher temperature attic. In numerous homes that have closed up the attic ventilation, rarely do we see over a ten degree increase in light/medium colored roofed attics when these vents are closed. Observation of homes in coastal areas near salt water shows several reasons eliminating attic ventilation is a benefit, these are:
  1. Corrosion caused by salt deposits left from the flow of salt laden air through the attic is reduced substantially.
  2. There is degradation of the batt insulation’s thermal effectiveness from the salt depositing on the surface of the batt insulation. Observation shows the wetting of the surface of the batt insulation due to the salt deposit, as the deposit acts as a sorbent at high relative humidities.
  3. In vented attics, there is a degradation of air conditioning units and the ductwork vapor barrier. On a recent site investigation, the ductwork materials that had been in an attic for approximately one year were found to have a severely oxidized vapor barrier with extensive pitting. This causes the effective loss of the barrier.
  4. Closing up attic ventilation controls excess infiltration caused by wind impingement on vented soffits, particularly when there is uneven ventilation due to tiered roof levels. Wind effects on structures facing open waterways are three times that of a home in a developed neighborhood.
  5. Closing off the primary source of moisture migration reduces attic moisture load from the inside of the structure over the top plate (ceiling). As a note, the vapor pressure differential outside to the inside conditioned environment is often more than twice the vapor pressure differential inside to outside of a home in a severe heating climate that requires attic ventilation. In South Florida the average dew point temperature from May through September is around 73 degrees F.
  6. Eliminating attic ventilation will reduce the dew point in the attic, reducing the chance of duct sweating, which is prevalent in humid coastal areas where wet bulb temperatures often exceed 80 degrees F. In addition, it raises the attic temperature surrounding the ductwork, keeping the surface ductwork out of the dew point range.
  7. Roofs with light colored tile are often found to be below the outside drybulb temperature and close to the dewpoint of the air. The thermal mass of the roof doesn’t allow the attic to stay warm enough to keep the air conditioning unit and ductwork surfaces above the attics dewpoint, thus sweating occurs.

In conclusion, roof or attic ventilation in residences in coastal regions can create more problems than benefit. This is based on past experience, research by other professionals and references that were used in determining the requirement in the code (ASHRAE, Handbook of Fundamentals). Next month we will address research that indicates that many homes would benefit from raising the attic insulation from the ceiling level to the roof surface and down any exterior walls that form the envelope for tile attic basically turning the attic into a transitional conditioned space.

*Today’s A/C & Refrigeration News- IAQ TECHNICAL REPORT JAN 98

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