In any home, uncontrolled air leakage is a fickle ventilator. The only way to ensure adequate ventilation is to install some type of automatically controlled ventilation system. As you'll see, you have quite a few choices.
Ventilation Standards
Exhaust fans in the kitchen and bathrooms are standard equipment in new homes. They provide spot ventilation to expel moisture and odors from limited areas. With spot ventilation, people sense an obvious problem and then manually flip a switch to solve the problem.The controlled ventilation described in this article is intended to maintain overall indoor air quality. It differs from spot ventilation in three ways. It affects the entire living space. It provides makeup air from outside. And, it's controlled automatically. It's called controlled ventilation to distinguish it from the more limited spot ventilation.
According to standards published by the American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE), houses should have a controlled ventilation rate of 15 cubic feet per minute (cfm) per person. So, a household of four would require 60 cfm. Some building codes and utility programs also use 0.35 air changes per hour (ach) as a ventilation target. (To quickly estimate the air flow in cfm needed to meet the 0.35 ach requirement, divide the floor area in square feet by 20.) Remember these ventilation targets are for controlled ventilation, not spot ventilation.
Basic Functions
Ventilation systems are more than exhaust fans. They serve three important functions:- Expel stale air containing water vapor, carbon dioxide, airborne chemicals and other pollutants.
- Draw in outside air, which presumably contains fewer pollutants and less water vapor.
- Distribute the outside air throughout the house.
- Control system operation automatically.
Equipment Options
The equipment that performs these four basic functions comes in all shapes, size and costs. Here are six sample systems:- Surface-mounted Fan
- Remote-mounted In-line Fan
- Remote-mounted Multiport Fan
- Balanced Ventilator
- Air-to-Air Heat Exchanger
- Exhaust Air Heat Pump
Exhaust Only vs. Balanced
In simpler systems, the main component is an exhaust fan that places the building under a slight negative pressure. This draws outside air into the house through passive fresh air inlets. Because they are simpler, negative pressure systems are generally less expensive. Plus they help prevent water vapor from migrating into building cavities, such as walls and attics, where the vapor could condense and cause problems.Unfortunately, fireplaces, wood stoves and gas-burning appliances were not designed to operate in a negative pressure environment. Under some conditions, even a slight negative pressure could cause flue gases, including carbon monoxide, to spill into the living space. You can either leave them out or install only sealed combustion appliances that draw air from outside.
Other systems strive for a balanced flow by using two fans: one for exhaust and another for fresh air. In theory, backdrafting shouldn't be an issue. However, experience with forced-air heating systems shows that balanced air flow can be difficult to achieve. Detailed duct design and careful installation are needed. Once installed the system must be adjusted. Even then the carefully balanced air flow can be thrown off by someone closing a door between a supply and a pickup.
Heat Recovery or Not
Controlled ventilation systems collect the outgoing air into a single duct so it's possible to capture heat from that air with a heat-recovery ventilator (HRV). The most common type of HRV is an air-to-air heat exchanger. It transfers heat from the outgoing air stream to the incoming one. In these systems, air flow is balanced. Another type of HRV is an exhaust air heat pump. Commonly used in Sweden, it transfers heat from the outgoing air into the domestic water tank. The compressor is about the size of a window air conditioner.High Quality Fans
Controlled ventilation systems operate many hours every day. Some never turn off. You want a durable, high-quality fan intended for continuous operation. Most high-quality fans use permanent split capacitor motors. Because this fan will run thousands of hours per year, look for a fan motor with low electrical consumption.Noise prevents many people from operating fans. Surface-mounted fans should have a noise rating of 2.0 sones or less. A few manufacturers make fans with noise ratings less than 1.0 sone. Low sone ratings are less important for remote-mounted fans. However, you should use sound absorbing fan mounting and duct connections to prevent sound transmission into living areas.
Control Options
Here are a few control options that would work with most types of ventilation systems:- Twenty-four hour timers allow the occupants to set certain times for ventilation. Set the timer to run the fan at least eight hours per day.
- Twist timers, also called interval timers, allow occupants to engage the fan whenever it's needed. Twist timers can be set up to 60 minutes and are generally located in bathrooms, utility rooms or kitchens.
- Speed controllers allow the fan to operate at low speed for background ventilation with a manual high-speed boost.
- Indoor air quality sensors activate a fan when they detect carbon monoxide, formaldehyde or other pollutants. This is an option in a couple of the more sophisticated controls.
- Dehumidistats engage the fan on rising humidity. They work well when relative humidity accurately indicates the need for ventilation. By setting the dial at 40, you are telling the dehumidistat to operate the fan whenever the humidity is 40 percent or higher. Unfortunately, relative humidity isn't always a reliable indicator. Climates with low humidity might never reach 40 percent, so the fan would never turn on. In wet climates the fan might never turn off. Dehumidistats aren't used as much as they once were because of this problem.
- Continuous operation simplifies the controls, but you should at least install an on/off switch. It's a good idea to locate the switch out of the way to reduce the chance that someone will accidentally flip it off.
Furnace Integration
It's tempting to combine a controlled ventilation system with existing forced-air heating and cooling ductwork. However, running the furnace blower causes three problems.First, virtually all duct systems in new and existing homes have significant air leakage. Second, homes with forced air systems frequently have air pressure differences around the house that increase building air leakage. Third, typical furnace blowers are turned by large, inefficient motors. Running the typical single-speed blower an additional eight hours per day could easily burn more than 2,000 kWh per year. Some new air handlers reduce energy use with multispeed controls or more efficient motors.
Before using heating and cooling ducts for fresh-air distribution, these issues need to be resolved.
Occupant Information
Controlled ventilation systems are new to most home buyers. It's your job to educate the occupants.- Label all components, including the fan, controls and ducts.
- Write a brief description of the system that explains the principles of operation, control strategy and maintenance. Attach product literature for the components used.
- Show the occupants the location of each component and how to operate the system.
Surface-mounted Fan
The simplest controlled ventilation system uses a quiet, high-quality surface-mounted fan. Fresh air enters through passive vents located in window sashes or outside walls. Surface-mounted fans provide good ventilation for smaller areas. Large houses may need more than one.
- Noise Rating: 2.0 sones or less
- Locations: central hallway or bathroom
- Air Flow Capacity: 80-400 cfm
- Heat Recovery: none
- House Pressure: negative
- Makeup Air: passive inlets
- Multispeed Operation: no
- Equipment Cost: $100 - 150
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Remote-mounted In-line Fan
Remote-mounted fans can pick up stale air from a single point. Or, they can be attached to a branched duct system with picks ups in two or three locations. This makes them a good choice for large houses. If properly rated, the fan could be attached to a range hood.
- Noise Rating: not applicable
- Locations: basement, attic or crawlspace
- Air Flow Capacity: 80-400 cfm
- Heat Recovery: none
- House Pressure: negative
- Makeup Air: passive inlets
- Multispeed Operation: optional
- Equipment Cost: $150 - 250
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Remote-mounted Multiport Fan
Large houses and several multifamily units can be ventilated by a single multiport fan. Some units can accept a duct from the range hood. Most operate at two or more speeds. Several manufacturers sell complete kits with all the ducts and accessories. These may cost a bit more, but the kits simplify installation.
- Noise Rating: not applicable
- Locations: basement, attic or crawlspace
- Air Flow Capacity: 100-400 cfm
- Heat Recovery: none
- House Pressure: negative
- Makeup Air: passive inlets
- Multispeed Operation: optional
- Equipment Cost: $200 - 700
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Balanced Ventilator
If you want balanced operation, without the extra expense of heat recovery, these would be a good choice. Only one or two manufacturers make balanced ventilators without heat recovery.
- Noise Rating: not applicable
- Locations: basement, attic or crawlspace
- Air Flow Capacity: 100-400 cfm
- Heat Recovery: none
- House Pressure: balanced
- Makeup Air: ducted
- Multispeed Operation: optional
- Equipment Cost: $400 - 800
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Air-to-Air Heat Exchanger
This type of heat-recovery ventilator provides balanced air flow and recovers up to 85 percent of the heat from outgoing air. By warming the incoming air, AAHXs provide greater comfort in cold climates than other types of ventilation systems. Units can be sized for any home and small commercial buildings.
- Noise Rating: not applicable
- Locations: basement, inside utility or any tempered space
- Air Flow Capacity: 150-1200 cfm
- Heat Recovery: 60 - 85% recovery efficiency
- House Pressure: balanced
- Makeup Air: ducted
- Multispeed Operation: standard on many units
- Equipment Cost: $800 - 2,000
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Exhaust Air Heat Pump
By employing a heat pump unit about the size of a window air conditioner, an exhaust air heat pump (EAHP) offers exceptional heat recovery efficiency. It can also provide most of the hot water needed by an average family. While the exhaust fan is controlled by timers, heat recovery engages only when hot water is needed. That means ventilation sometimes occurs without heat recovery. The operating characteristics of an EAHP lead to greater air flow than required for a typical small house.
- Noise Level: similar to refrigerator
- Locations: basement, inside utility or any tempered space
- Air Flow Capacity: 100-200 cfm
- Heat Recovery: 200 - 300% efficiency
- House Pressure: negative
- Makeup Air: passive inlets
- Multispeed Operation: no
- Equipment Cost: $1,000 - 3,000
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