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What are heat recover ventilators?(HRV'S)


Heat recovery ventilators, energy recovery ventilators (ERVS), heat exchangers or air exchangers as they are sometimes called, improve the energy efficiency in a balanced ventilation system. Inlet and exhaust airflows are controlled by fans so that house pressures are balanced. Heat is recovered from the exhaust air by transferring it to the cool inlet air. Energy recovery ventilators transfer house moisture as well as heat from the exhaust air into the fresh air coming into the home. For simplicity, we'll call them all heat recovery ventilators or HRVs.

The components of the heat recovery ventilator include

  1. A cabinet
  2. A heat exchanger
  3. Inlet and exhaust fans
  4. A duct system
  5. Flow measuring stations
  6. Controls
  7. Air filters
  8. A condensate system
  9. A defrost system

The heat recovery ventilator is typically a sheet metal cabinet in a basement or closet area.

Heat recovery ventilators are typically the size of small furnaces. They are often hung from ceilings and are usually longer than they are tall. They might be 30 inches long, 18 inches tall and 18 inches wide, for example.

Heat recovery ventilators are usually located in conditioned spaces, often in basements. Putting them in garages or attics is considered poor practice as this reduces the energy efficiency of the systems.

Types of Heat Exchanger There are six types of heat exchangers used in heat recovery ventilators:

  1. Flat plate
  2. Rotary wheel
  3. Concentric tube
  4. Heat pipe
  5. Capillary blower
  6. Heat pump

Flat-plate heat exchangers are made of metal, aluminum or plastics such as polypropylene. Fresh air and exhaust air usually flow in opposite directions. Exhaust and supply air are kept separate. There is no moisture transfer in most units. Some have paper cores with limited moisture transfer. Rotary wheel. This type has a slowly rotating wheel. Fresh air moves in one direction; exhaust air moves in the other. A stationary baffle separates the warm side from the cold side. Heat collects on the warm side and as the core rotates to the cold side, heat is released to the cold air. The wheel turns at about 30 rpm. There is some cross leakage between exhaust and indoor air. Some moisture is transferred. Concentric tube-type ventilator. This is a pipe within a pipe. The fresh air flows in opposite direction to exhaust air. Fresh air may flow through the core and second ring; exhaust air may flow through the first and third rings, for example. Heat is transferred from the exhaust air to the fresh air. Heat pipe ventilator. This unit has a core with aluminum or copper fins on tubes filled with a refrigerant such as Freon. A baffle separates the warm side and the cold side. The cold side is slightly higher than the warm side of each tube. Warm air flowing over the outside of the tubes boils the refrigerant from a liquid to a gas. The gas migrates to the high side. Cool air passing over the high side takes the heat from the refrigerant. The refrigerant condenses and flows back to the low side. Capillary blower-type. This system has a foam ring that rotates at high speed. It's like a doughnut with a stationary baffle across the middle of the hole. Cold air comes in to the center of the doughnut on one side of the baffle and exhaust air comes into the hole on the other side. The rotating ring picks up heat and moisture from the warm exhaust air and transfers it to the cold air exhaust outlet. Heat is transferred to the cool air before it is discharged through the indoor supply duct. The ring is permeable and some moisture transfers from the exhaust air to the intake air. Heat pump-type. The heat pump system has a compressor and an evaporator and condenser coil. Heat from the warm exhaust boils a refrigerant in the evaporator, giving off its heat to the refrigerant. The refrigerant goes to the compressor where it is heated before being released into the condenser. The cool inlet air passing over the warm condenser picks up heat. The refrigerant in the condenser cools and condenses to a liquid from a gas. The liquid refrigerant moves to the evaporator and the cycle repeats. The illustration shows how these systems work.

The Goal of Heat Recovery Ventilators The different types of heat exchangers are trying to accomplish the same thing. As the warm air is exhausted from the house, cold air is brought in. The warm air is moved through one side of the heat exchanger and the cool, outdoor air is moved through the other side. Heat moves from the exhaust air into the intake air. This cools the exhaust air before it is dumped outside, and warms the fresh intake air before it is distributed through the house.

Fans are used to move the exhaust air through the heat recovery ventilator, out of the house, and to move the fresh air from outside through the HRV and into the home. There may be two fans and two motors, or two fans driven by a single motor, in a typical heat recovery ventilator. Fans are typically 120-volt, but may be 240 volts if an electric preheater is used for the fresh air.

Efficiency HRVs are typically 55 to 85 percent efficient, with an average of about 70 percent. Efficiency varies with outdoor temperature and the design and performance of the unit.

The amount of ventilation There are a number of ways to look at how much ventilation a house should have. Common recommendations include

  • One-third air change per hour (0.3 to 0.35 ACH). Note: Levels should be higher if people smoke or if there is a hot tub or swimming pool in the home. Or,
  • There should be 15 cfm (cubic feet per minute) per person. This is adequate for a person at rest to avoid excess carbon dioxide levels. Or,
  • There should be 65 cfm total for the kitchen, dining room, living room, one bathroom and a master bedroom. Add 10 cfm for each additional room, and 20 cfm for an unfinished basement. We need 65 cfm for the base group of rooms, and 10 cfm each for the second bathroom and three other bedrooms, and 20 cfm for the basement. This totals (65+10+30+20) 125 cfm.
  • There is a second set of criteria to calculate. The total number should include 50 cfm for each bathroom and 100 cfm for the kitchen. For example, a house with four bedrooms, a family room, two bathrooms and an unfinished basement will need 100 cfm for the kitchen and (50x2) 100 cfm for the bathrooms. This totals 200 cfm. The first criteria called for 125 cfm. We use the larger, so we require 200 cfm.
This total ventilation is typically achieved with exhaust fans and an HRV. The HRV typically would handle about 50 percent of the total demand. An average design for a house might by 200 cfm. The HRV might operate in the 70 to 100 cfm range on normal speed, and have a maximum capacity of 200 cfm on high speed. High speed numbers in the area of 200 cfm may only be required if the system is also used as a kitchen or bathroom exhaust fan.

Over-Ventilating and Under-Ventilating. We want to provide enough ventilation, but not too much. If we don't provide enough, moisture levels in the house will rise and indoor air quality will suffer, as the levels of other pollutants rise. Over-ventilating reduces moisture levels in the house to an uncomfortable level and may use too much energy. Exhausting air does cost some energy, even if the HRV is operating at eighty percent efficiency.

It's important the HRV be balanced so the house will not be pressurized or depressurized. Pressurizing the house drives moisture into the structure. Depressurizing the house may cause combustion appliances to backdraft.

In some jurisdictions, houses with mechanical ventilation systems require carbon monoxide sensors in rooms with wood stoves or fireplaces. The risk of depressurizing the house and causing backdraft of these appliances warrants the installation of these sensors. Obviously, there's considerable indoor air pollution and a risk to health if wood-burning appliances backdraft.

Ducts running through unconditioned spaces to or from heat recovery ventilators should be insulated to prevent heat loss and condensation. The ducts on the cold side of the HRV (the exhaust duct running from the HRV out through the wall and the fresh air intake running from the outdoors to the HRV) should be insulated. Flex duct with one inch of fiberglass insulation (R-value of 3.0) is usually considered adequate. Where ducts are longer than ten feet, some recommend additional insulation.

The ducts on the cold side of the HRV should be kept as short and straight as possible. Flex duct often sags, which creates additional friction loss and reduces flow rates. The sags may also be spots where condensation collects in the exhaust duct. Flex ducts should be supported every three feet, approximately.

Some experts recommend the cold-side ducts slope slightly down, toward outdoors, from the HRV so any condensation will drain outside.

The warm-side ducts are usually uninsulated rigid metal. Combustible ducts on an HRV system are allowed under some circumstances, but most are metal.

Balancing dampers are usually provided on warm-side ducts, close to the HRV. These are needed to balance the system from time to time, so we don't over-pressurize or depressurize the home.

Flow Measuring Stations Flow measuring stations, flow stations, or flow collars may be installed on the warm-side ductwork to allow balancing of the system. These collars are typically installed at least twelve inches away from the balancing dampers. They can usually be identified by the two pins protruding from the collar. These pins are connected to the gauges. Where flow collars are provided, there is usually one on the fresh air intake and one on the exhaust duct. Balancing should be checked and adjusted annually.

The exhaust air from the house can be taken from

  • Kitchens and bathrooms, sources of high humidity and pollution. If exhaust air is drawn from the kitchen, it should not be close to the stove. The grease may clog the system. An exhaust grille in the kitchen should have a grease filter, even if it is well away from the cooktop. There may be a separate range hood vent.
  • Any other rooms in the home.
  • The return air plenum of a forced-air heating system.

The fresh air supply coming from the HRV into the home can go

  • through dedicated ducts to various rooms of the house. In this arrangement, the supply registers are usually at the ceiling level, or on walls within twelve inches of the ceiling.
  • into the cold air return plenum for the furnace.
If the exhaust air is also drawn from the return plenum, the supply connection should be at least three feet downstream of the exhaust connection. The fresh air connection to the plenum may be a direct or indirect connection. In an indirect connection, an open T may be provided on the top of the fresh air duct, near the return plenum. Alternatively, the fresh air duct may also terminate four to twelve inches in front of a grille on the side of the return plenum. Indirect connections should not be made inside a furnace room. The furnace may draw room air in through the opening in the return duct, depressurizing the furnace room. This may lead to backdraft of the furnace.

Outdoor terminations for exhaust and fresh air intakes

The fresh air intake should be

  • located six feet away from the exhaust;
  • 18 inches above grade;
  • 40 inches away from the corners of buildings, to avoid turbulent air;
  • three feet from gas meters;
  • well away from driveways and garages (to avoid bringing car exhaust fumes into the house); and
  • three feet from clothes dryer vents, exhaust fan vents, boilers vents, furnace vents, water heater vents, and fuel oil fill and vent lines.
The fresh air intake should not be
  • in crawlspaces or attics, or
  • in areas where snow may accumulate.

The fresh air intake should be arranged so that rain, snow and wind won't enter the duct system or the home. Screening should be provided to keep out animals, birds and insects.

Exhaust terminations should be away from fresh air intakes and away from attics, garages and crawlspaces. The exhaust outlet should be a minimum of four to eight inches above grade, and should be protected from rain and snow with a hood similar to the fresh air intake. Screening may or may not be provided on the exhaust, but it should have a damper that opens readily when the system is operating and closes tightly when the system is at rest.

In some jurisdictions, the fresh air intake and exhaust outlets must be labeled on the building exterior.

Controls HRVs are controlled a number of ways:

  • Thermostat
  • Humidistat
  • Dehumidistat
  • Manual switches
  • Timers
  • Fan speed controls
A manual switch near the center of the house and labeled Ventilation Fan is a common way to control the system. In modern installations, where the HRV is interconnected with furnace ductwork, the furnace fan is often interlocked with the HRV, so that when the HRV fan is operating, the furnace fan operates as well. Some older systems were not set up this way. Sometimes the ventilation fan would be labeled and an adjacent switch labeled Circulation Fan would control the furnace fan. Many new systems have the HRV and furnace operate continuously at low speed. The Ventilation Fan switch operates both fans at high speed.

HRVs use some electricity to move 70 to 200 cfm air. Furnace fans move considerably more air, and consume substantial amounts of electricity, especially if they run continuously to support a ventilation system. There are modern, highly efficient motors (ECM motors, for example) that use less electricity. These are rare in residential applications.

Many ventilation systems depend on separate exhaust fans for bathroom and kitchen areas. These fans can be controlled manually, with dehumidistats, or with timers.

The HRV can also be controlled a number of ways:

  • Manual operation, as we have discussed above.
  • Automatic operation. These may use timers to operate the HRV a specified number of hours per day at various times, or dehumidistats that activate the system when humidity levels rise.
  • Continuous operation. Some HRVs are set to operate continuously at low speed, and move to high speed in response to a manual switch or a dehumidistat.

Sophisticated control systems may include occupancy sensors that detect people in a room and operate the system only when the room is occupied.

Air Filters Most HRVs have integral air filters that require regular inspection, cleaning or replacement. These are similar to furnace filters and are usually accessed by opening a cover on the HRV itself.

Condensate Drains

Many HRVs have condensate pans and drainpipes. The drain pipe is typically half an inch in diameter and includes a two-inch trap. The trap is often created by simply looping the pipe.

The drain should slope to carry water by gravity to a safe location. An outdoor termination may be susceptible to freezing. An air gap is required where the drain dumps into a plumbing fixture or drain. In many jurisdictions, hard connections to plumbing stacks, for example, are not allowed.

Some HRVs, including energy recovery wheels or ERVs, allow some moisture to move from the exhaust air into the supply air. Manufacturers argue that this improves the quality of the fresh, indoor air, and maintain the level of humidity can be controlled. These systems typically do not have condensate drains since a good deal of the moisture is transferred into the incoming fresh air.

Defrost Control Very cold fresh air coming in can cause condensation and frost in the heat exchanger. This can ice up the unit. Defrost cycles are often automatic, coming on for a few minutes every hour when the temperature is below a given set point (e.g., 20F), There are four ways the defrost is typically arranged: 1. The incoming supply air is preheated with an electric duct heater. 2. The exhaust air leaving the HRV is recirculated through the fresh air inlet. The HRV sees no fresh air during this cycle. 3. The exhaust fan stops and the fresh air intake is blocked with the damper. Warm house air is drawn through the fresh air side of the heat exchanger. 4. The fresh air fan is shut off and the exhaust fan continues to move warm air through the HRV.

Regular maintenance includes

  • clean or replace air filters monthly.
  • clean fresh air inlet and exhaust air hoods.
  • clean the heat exchanger core
  • clean the condensate drain.
  • lubricate the fan motors as necessary.
  • clean the ductwork.
  • clean the dehumidistat (if used).
  • check and balance flow rates.

Summary It's probably fair to say that house ventilation systems are evolving. Many people believe the heat recovery ventilator is the best approach to solving indoor air quality problems, controlling house moisture and minimizing heat loss.

  • Others feel these systems are too expensive to install and operate.
  • There are many examples of poorly installed and operating systems.
  • An unbalanced ventilation system can be worse than no ventilation system.
  • Ventilation systems are not sophisticated and do not respond automatically to changing conditions in houses.
  • Many homeowners do not understand the purpose and operation of ventilation systems, and often shut the system down or operate it without proper maintenance.

While the HRV does capture some of the heat that would otherwise be exhausted, there is a price to pay. In addition to the original installation costs, there is the electrical cost to operate the fans in the HRV, and often a larger cost to operate the furnace fan.

Some maintain that houses are already too complicated for the average homeowner to maintain, and devices such as HRVs are more trouble than worth. We believe we will see more changes in house ventilation and we hope solutions become simpler, less expensive and more user-friendly.

Read More
expert -
Sears Technician
April 26, 2007