Tightening an old house or building a tight new one means that you can’t leave ventilation to random leaks anymore. You have to control it.

Whole-house mechanical ventilation have its purpose to exchange indoor air with fresh outdoor air at a controlled rate using fans with its main purpose to improve indoor air quality.

Over the past 15-20 years houses have been build better and tighter as a result of better builders, material, energy efficiency programs and overall desire to reduce energy use.

Historically, mechanical ventilation was restricted to local-exhaust (kitchen and bath exhaust fans) for spot control of moisture and odors. Houses normally had enough natural ventilation, through leaky building enclosures, that ventilation was not necessary.

Benefits of whole-house mechanical ventilation include:

• A steady supply of outdoor air for improved indoor air quality and resident comfort
• Regulation over the amount and source of outdoor air
• Thinning of indoor contaminants, such as odors and allergens
• Helps control relative humidity and decrease moisture accumulation during the heating or temperate seasons

Different Types Of Mechanical Ventilation

There are four types of whole-house mechanical ventilation systems: exhaust-only, supply-only, balanced and Energy Recovery Ventilation (ERV)  (In colder climate it is called HRV ,Heat Recover Ventilation) In this post we talk about ERV. Each system uses a combination of fans, ducting, dampers and controls, and they each have different pros, cons, and costs accompanying them.

Exhaust-Only Ventilation

Exhaust-only ventilation commonly uses an efficient bathroom / exhaust fan to exhaust indoor air. The system exhausts air from the house while make-up air infiltrates through leaks in the building shell.

Exhaust ventilation systems are relatively simple and inexpensive to install. Typically, an exhaust ventilation system consists of a single fan connected to a centrally located, single exhaust point in the house. A better design is to connect the fan to ducts from several rooms, preferably rooms where pollutants are generated, such as bathrooms.

Exhaust ventilation systems are most appropriate for cold climates. In climates with warm humid summers, depressurization can draw moist air into building wall cavities, where it may condense and cause moisture damage.

One concern with exhaust ventilation systems is that — along with fresh air — they may draw in pollutants, including:

• Radon and molds from a crawlspace
• Dust from an attic
• Fumes from an attached garage
• Flue gases from a fireplace or fossil-fuel-fired water heater and furnace.

Exhaust ventilation systems can also contribute to higher heating and cooling costs compared with energy recovery ventilation systems because exhaust systems do not temper or remove moisture from the make-up air before it enters the house.

Pros

• Lowest installed cost

• Cons
• Contaminants may be drawn into the house from an attic, garage, crawlspace, or wall cavity.
• Potential to draw moist outdoor air into the wall cavity that could condense during the cooling season and cause moisture problems, particularly in warm, humid climates.
• Can draw pollutants into living space
• Not appropriate for hot humid climate
• Rely in part on random air leakage
• Can increase heating and cooling costs
• May require mixing of outdoor and indoor air to avoid drafts in cold weather
• Can cause backdrafting in combustion appliance
• Outdoor air may not be well-distributed.

Supply-Only Ventilation

A fan draws outdoor air into the house, which tends to pressurize the house.. Indoor air leaks out of the building through holes in the shell, bath, and range fan ducts, and intentional vents (if any exist).

Like exhaust ventilation systems, supply ventilation systems are relatively simple and inexpensive to install. A typical supply ventilation system has a fan and duct system that introduces fresh air into usually one — but preferably several — rooms that residents occupy most (e.g., bedrooms, living room). This system may include adjustable window or wall vents in other rooms.

Supply ventilation systems allow better control of the air that enters the house than exhaust ventilation systems do. By pressurizing the house, supply ventilation systems minimize outdoor pollutants in the living space and prevent backdrafting of combustion gases from fireplaces and appliances. Supply ventilation also allows outdoor air introduced into the house to be filtered to remove pollen and dust or dehumidified to provide humidity control.

Like exhaust ventilation systems, supply ventilation systems do not temper or remove moisture from the make-up air before it enters the house. Thus, they may contribute to higher heating and cooling costs compared with energy recovery ventilation systems.

Pros

• Minimizes contaminants entering through the building enclosure.
• Outdoor air is drawn from a single, known location for best air quality.
• For a CFI system, air is well-distributed and can be filtered and conditioned.
• Low installed cost, however for a CFI system, the electronically commutated motor may increase the initial cost and operating cost may be higher
• Allow better control than exhaust systems
• Minimize pollutants from outside living space
• Prevent backdrafting of combustion gases from fireplaces and appliances
• Allow filtering of pollen and dust in outdoor air
• Allow dehumidification of outdoor air
• Work well in hot or mixed climates.

Cons

• Potential to drive moist indoor air into the wall cavity that could condense and cause moisture problems during the heating season in colder climates.
• Must be designed to avoid occupant discomfort
• Will not temper or remove moisture from incoming air
• Can increase heating and cooling costs
• May require mixing of outdoor and indoor air to avoid drafts in cold weather.

Balanced ventilation

Balanced ventilation systems are a combination of exhaust and supply methods roughly providing equal indoor exhaust and outdoor supply air flows (e.g. an exhaust fan combined with a supply fan or passive inlet vents).

A balanced ventilation system usually has two fans and two duct systems. Fresh air supply and exhaust vents can be installed in every room, but a typical balanced ventilation system is designed to supply fresh air to bedrooms and living rooms where occupants spend the most time. It also exhausts air from rooms where moisture and pollutants are most often generated (kitchen, bathrooms, and perhaps the laundry room).

Some designs use a single-point exhaust with passive inlet vents. Because they directly supply outside air, balanced systems allow the use of filters to remove dust and pollen from outside air before introducing it into the house.

Balanced ventilation systems are appropriate for all climates. Because they require two duct and fan systems, however, balanced ventilation systems are usually more expensive to install and operate than supply or exhaust systems.

Like both supply and exhaust systems, balanced ventilation systems do not temper or remove moisture from the make-up air before it enters the house. Therefore, they may contribute to higher heating and cooling costs, unlike energy recovery ventilation systems.

Pros

• Appropriate for all climates

Cons

• Can cost more to install and operate than exhaust or supply systems
• Will not temper or remove moisture from incoming air
• Can increase heating and cooling costs.

Energy Recovery Ventilation System

Energy recovery ventilation systems provide a controlled way of ventilating a home while minimizing energy loss. They reduce the costs of heating ventilated air in the winter by transferring heat from the warm inside exhaust air to the fresh (but cold) outside supply air. In the summer, the inside air cools the warmer supply air to reduce cooling costs.

There are two types of energy-recovery systems: heat-recovery ventilators (HRV) and energy-recovery (or enthalpy-recovery) ventilators (ERV). Both types include a heat exchanger, one or more fans to push air through the machine, and controls. There are some small wall- or window-mounted models, but the majority are central, whole-house ventilation systems with their own duct system or shared ductwork.

The main difference between a heat-recovery and an energy-recovery ventilator is the way the heat exchanger works. With an energy-recovery ventilator, the heat exchanger transfers a certain amount of water vapor along with heat energy, while a heat-recovery ventilator only transfers heat.

Because an energy-recovery ventilator transfers some of the moisture from the exhaust air to the usually less humid incoming winter air, the humidity of the house air stays more constant.

In the summer, an energy-recovery ventilator may help to control humidity in the house by transferring some of the water vapor in the incoming air to the theoretically drier air that’s leaving the house. If you use an air conditioner, an energy-recovery ventilator generally offers better humidity control than a heat-recovery system.

You can also set up the system so that it only runs when the air conditioning system is running, or use pre-cooling coils.

Most energy recovery ventilation systems can recover about 70% to 90% of the energy in the exiting air and deliver that energy to the incoming air.

Energy recovery ventilation systems usually cost more to install than other ventilation systems. In general, simplicity is key to a cost-effective installation. The correct size duct is necessary to minimize pressure drops in the system and thus improve performance.

Pros

• Reduce heating and cooling costs
• Available as both small wall- or window-mounted models or central ventilation systems
• Cost-effective in climates with extreme winters or summers and high fuel costs.

Cons

• Can cost more to install than other ventilation systems
• May not be cost-effective in mild climates
• Require freeze and frost protection in cold climates
• Require more maintenance than other ventilation systems.