High-Efficiency Whole-House Air Filtration System
The United States Environmental Protection Agency (EPA) estimates that indoor air pollution levels can be from two to five times higher than the outdoor air. Along with this, they estimate that some people spend up to 90% of their time indoors. Typically, particles 10 microns in size and larger get caught in the nose and throat, but are expelled by coughing and sneezing. However, many particles, including allergens, can be as small as 0.1 microns. These can become embedded in the lungs and have long lasting negative health effects.
There are many different types of air filters that can clean these smaller particles from the air, and they can be installed in your central forced air heating and cooling (HVAC) system to provide filtration for the entire house. High-efficiency air filters greatly reduce the number of airborne particles when compared to the standard 1-inch thick filter installed on most central HVAC systems. Some high-efficiency models can even filter out tobacco smoke, airborne bacteria and allergens to some extent. Whole house filters are usually placed in the return duct line adjacent to the air handler for easy installation and maintenance. The air handler pulls air through the return ducts, and consequently through the filter. The air-filtration method can vary, and sometimes different methods are combined into a hybrid system.
• Mechanical or surface media filtration is the method by which pleated air filters capture particles. Filtration occurs through a dense matted fiberglass media that captures airborne particles. The fiber material is pleated, allowing considerably more surface area. More surface area improves the filter’s efficiency and useful life. Typically, pleated filters are set in stages (more than one pleated surface) to provide more filtering material. HEPA filters are very efficient pleated versions. They are tested to ensure that they capture at least 99.97 percent of particles that are 0.3 microns in diameter. They are even more efficient at capturing larger particles. Some HEPA filters are combined with activated carbon filters to allow them to absorb pollutants such as cigarette smoke.
• Electronic air filtration makes use of electrostatic precipitation. Electrostatic precipitators charge particles and pull them out of the air stream. It can be a one- or two-stage system. In a one-stage system, a plate or other surface both charges and attracts the particles. In a two-stage system, the particles are charged in stage one as they flow past a set of charged wires or corona fields, and then attracted to an oppositely charged plate or grounded media filter as they flow through in stage two. Many electrostatic precipitators claim to have efficiencies of over 90 percent, with some as high as 98 percent for allergens like pollen.
Many filters combine mechanical and electronic filtration. The flow first passes through a pleated fiber pre-filter to remove some of the larger particles, then through an electronic precipitator, removing smaller particles.
Manufacturers claim varying rates of particle efficiencies for their systems. Particle efficiencies are made based upon ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) testing standards. Ratings are given either in a percentage or a MERV rating (Minimum Efficiency Reporting Value). Percentage ratings must be given for particle size ranges and MERV ratings go from 1 to 16, taking particle size ranges into account. MERV is an industry standard rating, so it can be used to compare air filters made by different companies. Some manufacturers also have their own rating systems. The following table details ratings based on the ASHRAE standards for air filtration.
Maintenance of these filters depends on filter type and particle load, but is typically less frequent than standard 1-inch forced air system filters, which must be changed up to several times a year. Media filters, depending on type, have to be either replaced or cleaned, from once to twice a year. Some HEPA filters require even less frequent replacement (sometimes once every two years). Electronic filters have to be cleaned or wiped down, and the frequency varies by product and manufacturer. One manufacturer recommends cleaning every three to nine months, depending on use. Pre-filters on these devices may have to be changed more frequently than the primary filter needs cleaning.

Installation
Systems should be installed in an easily accessible place between the air handler and the return registers. For electronic filters, close proximity to a power source is recommended. Many installations occur immediately before the return line enters the air handler. Depending on duct cross section geometry, a transition from the duct to the filter and back to the duct may be required in order to maximize the filtering and air flow.
Benefits/Costs
Due to the amount of pollutants and potential allergens in indoor air, filtering the air from a central point can be very beneficial to occupant health. It eliminates the need for multiple portable filtration units throughout the house. Having a central electronic filter will also reduce the power consumed and noise produced from multiple room units. These filters also perform significantly better than typical 1-inch air handler filters, and have fewer maintenance requirements. However, their cost is higher than typical filters and they filter air only when the air handler is on, so individual room filtration control is not possible. Electronic filters are known to create small amounts of ozone, though not in levels typically considered hazardous to human health.
In-Line Fans
In-line fan units can service several bathrooms and provide an energy efficient method of controlled ventilation.
In-line fans are remote fans which can provide ventilation or boost airflow with little detectable noise. They provide a solution to noisy or ineffective bathroom fans, ineffective dryer exhaust, and recirculation range hoods.
Simple single-port or versatile multi-port in-line fans can be used to supply ventilation for most single-family residential applications. A multi-port design allows one fan to provide ventilation for multiple rooms, such as two bathrooms and a laundry. Depending on the configuration, control switches can be manual or automatic.
Because in-line fans are located remotely, there is little noise detectable at intake and exhaust ducts. Rather, the noise occurs outside of conventional living spaces, within attics or other buffered spaces.
Some common uses for in-line fans include:
• General ventilation: An in-line fan can be used in a whole-house, exhaust-only or supply-only ventilation scheme, depending on the climate (exhaust-only is not recommended in hot humid climates, supply-only is not recommended in cold climates). In an exhaust-only system, exhaust ports are placed in bathrooms, laundries, or other areas, and connected together through the in-line fan, typically placed in the attic. Supply ports in a supply-only system must be strategically placed so that occupants are not made uncomfortable from the incoming, unconditioned air (alternately, air can be supplied to the return of a central HVAC system in most climates).
• Bathroom Exhaust: If fan noise is a problem, an in-line fan placed in the attic will operate much more quietly than a typical fan located in a bathroom ceiling. In layouts where bathrooms adjoin or are in close proximity on the same floor, it may be cost effective to use a single in-line fan with ducts connecting to each room. If ductwork is too long for effective moisture or odor exhaust, an in-line fan may augment flow capacity. It should be noted that excessive noise from bathroom fans is often the result of the fan (fans are available with low sound output, or “sone” ratings) or poor installation and duct sealing. Therefore, if an in-line fan is being considered to reduce fan noise, other causes of excessive noise should be investigated before selecting an in-line unit.
• Dryer Vent Boosting: In larger homes or multi-family units, dryers must often be installed far from exterior vent outlets, requiring duct lengths that exceed manufacturer's recommendations, compromising dryer efficiency, and allowing hazardous heat build-up. Some in-line booster fans are specially designed to maintain safe and effective dryer operation.
• Indoor Air Quality, Fresh Air Injection, and Building Pressurization: Buildings which are highly insulated and tightly sealed may require mechanical introduction of fresh air for proper ventilation and pressurization. In-line fans can be used in conjunction with HVAC systems for air supply, air purification, dehumidification, or heat recovery.
• Duct Boosting for Remodeling or Retrofit: When an existing HVAC system is used to heat or cool an addition, new duct runs may stretch too far from the air handler. In-lines fans can increase flow to remote areas and help balance the air supply throughout the system.
• Radon Mitigation: Vent lines placed below slabs for radon mitigation may require an in-line fan to exhaust radon gas.
Note that exhaust-only fans can depressurize a building and draw air into the home. The process, called backdrafting, could lead to dangerous concentrations of lethal exhaust gasses being drawn into the home. Providing an alternative source of make-up air using passive air inlets or balanced exhaust systems can alleviate this threat. Carbon monoxide detectors can serve as an early warning device for backdrafting, and the use of sealed combustion appliances can also eliminate backdrafting concerns.
Installation
Manufacturers usually provide installation instructions to help minimize operational or acoustic problems. Generally, setting up the unit is not too difficult. The fans may be placed almost anywhere. Closets, chases, wall cavities (if wide enough), basements, spaces between floors, and attics are common locations. In-line fans should be located in the duct run near the primary exhaust port. This improves performance and reduces noise. Suspension attachments and other approaches can be used to reduce vibration, structure-borne noise. Externally insulated ductwork can muffle airborne noise and diminish the likelihood of condensation within the duct. Condensation can damage ductwork, fan motor, and indoor air quality. Some fans installed on a horizontal axis require additional fitting with a condensate drain line to remove the water before it can harm the motor.
Ductwork is a key element in proper delivery of the air supply and must be appropriately sized. For multi-port systems, equivalent grille pressures may be difficult to obtain from ducts that are not all the same length. Remote, in-line ventilation systems therefore commonly use flex-duct despite its high airflow resistance because it can be easily routed to intake and exhaust grilles. Backdraft dampers prevent airflow from moving the wrong way in ducts when the fan is not operating, and may be required, depending on the application.
Because of concerns about pressurization and backdrafting described above, it is recommended that overall ventilation needs and requirements be evaluated by a competent HVAC contractor or consultant before installing in-line fans or other mechanical ventilation equipment.
Benefits/Costs
Remote, in-line units are quiet, ventilate well, use little energy, and tend to be maintenance-free if properly installed.

Two Stage and Modulation

Another improvement to gas furnaces has been the addition of staging and modulation. Typical furnaces have operated forever at one capacity, 100% or 0%. A car operated in this manner wouldn't be efficient and difficult to control. We don't operate our cars at full throttle and then no throttle. At least most of us don't. A fireplace can produce variable heat output by simply changing the amount of firewood. We don't put as much wood as possible in a fireplace every time we make a fire. In the days fireplaces were used as a primary source of heat the fireplace would have more wood added in the coldest days and less on milder days. That simple technology of variable capacity didn't exist in gas furnaces until recently. Furnace capacity is selected based on the heat required for the coldest winter days. Anytime the outside temperature is warmer than the coldest winter days the furnace is oversized. Like the fireplace with maximum firewood the standard single stage furnace operates the same way. This method of all or nothing is very crude and primitive to try to maintain comfort or save energy.

Two stage furnaces have become more common for many homeowners today. Two stage heating furnaces provide two capacities for increased comfort, efficiency and control. Typically as an example a 100k btu 92% efficient gas furnace will have a first stage of 60% of the total capacity or 60k btus and full capacity or 100k btus on the second stage. Like having two furnaces in one the two stage furnace runs for longer periods of time with more efficiency and less waste. Since the majority of winter temperatures are significantly less than the coldest winter days, the heat requirement is also significantly less. A single stage furnace of 100k btus and 92% efficiency wastes 8% of it's fuel in the stack or flue gases which is 8,000 btus. Every time the single stage furnace is required for heating 8,000 btus go up the stack. However the same furnace with two stages will operate the majority of time at 60% capacity or 60k btus losing only 4,800 btus or almost 1/2 the amount of the single stage furnace. The two stage furnace will also operate for longer periods of time each cycle maintaining higher operating efficiency than the single stage furnace. The single stage furnace will have more short operating cycles in any 24 hour period resulting in more warm up and shut down cycle losses. During start up of any furnace there is fuel wasted as the energy heats the mass of the heat exchangers to achieve operating efficiency. When the furnace shuts off there is the lost heat that wasn't extracted out of the heat exchanger. In addition there is something occurring that doesn't show up on the gas or electric meter which is the wear and tear caused by excessive starts and stops. Frequent start and stop cycles produce extreme wear and tear on the furnace ignition system, motors and blower from start up electrical surges and thermal expansion in the heat exchanger. The more frequent starts and stops not only waste energy but significantly increase wear and tear. Similar to a car that is driven in city conditions with frequent starts and stops and shorter trips versus one that is used primarily on highway miles for longer durations and a constant speed. All other conditions being equal the car driven at highway conditions is always preferred having the least wear and tear. So what are the advantages and energy efficiency gains of a two stage furnace? More energy saved, increased comfort, longer life span and less maintenance. Comfort is increased significantly by providing longer operating cycles of milder discharge air. These longer more moderate heating cycles allow more close temperature control. An additional increase in efficiency by 6 to 15% can be achieved with less start up and shut down cycle losses and less stack losses during the low fire operation. Actual energy savings depend on the climate and efficiency of the house as well as the correct sizing of the furnace to the actual heating requirements. Longer life span is a benefit as the two stage furnace has fewer cycles, less starting torques for the motors, less firing cycles for the ignition system and fewer expansion and contraction cycles on the heat exchangers. That also translates into less service and maintenance required. Hands down for two stage heating being worth the extra investment. The difference in cost for two stage heating is about $100 and will pay for itself in less than one heating season.

As we highlighted the advantages of a two stage gas furnace there are also new hi tech furnaces with modulating gas valves. Modulation produces infinite stages for capacity creating a furnace that is the equivalent of a throttle on a car. Driving a car similar to the operation of a single stage furnace is the equivalent to flooring the gas pedal every time it's operated. Impossible to control and wastes a lot of energy. A two stage furnace is like having a gas pedal that has off, 1/2 throttle and full throttle. Much better to control, higher energy efficiency and substantially less abuse. A modulating furnace is the same as a throttle on a car, excellent to control, maximum energy efficiency and minimal wear and tear. >From 50% to 100% capacity and anywhere between the modulating gas furnace is the ultimate in heating with a forced air system. Because modulating furnace technology is relatively new, production costs are higher and payback periods are longer than two stage furnaces. At present there are only two manufacturers producing modulating gas furnaces. As more manufacturers produce modulating furnaces the cost will inevitably decline and be more competitive. Modulating gas furnaces are at the cutting edge of new technology and not as cost effective as two stage furnaces. In 2006 and beyond this will change as prices become more competitive. Until then the extra cost of modulation and the smaller benefits derived in comparison to two stage heating do not presently make this feature cost effective. This is not a recommended feature as the payback period is too long and the energy saved is insignificant compared to two stage. There may be a slight improvement in comfort over the two stage operation but this could be considered splitting hairs. A two stage furnace usually provides no more than a one degree deviation in room temperature which is considered near perfect. Could modulation improve this to 1/2 degree and would that be noticeable? Or worth the extra $400? Not recommended at this time by SEER - Solutions for Energy Efficient Results