Can you please tell me how an air exchanger works with respect to lowering the humidity in a home? What are they usually priced at?
I enjoy your column. Thanks in advance.
It is my guess that the respondent above is referring to a Heat Recovery Ventilator (HRV) or Energy Recovery Ventilator (ERV). This is a manufactured ventilation system that has an integral heat exchanger that minimizes heat loss while bringing fresh air into a home and exhausting stale house air. There are other simpler ventilation systems available, but they often pay little or no attention to heat loss, simply air exchanges within the home.
To understand how mechanical ventilation systems reduce the humidity from our homes, we have to understand one basic scientific principle. The warmer the air, the larger the amount of moisture it can hold. The warm air that is heated in a home during the winter has the capacity to absorb a much greater amount of moisture relative to the cold air outside the home. This moisture is created in our houses from day to day living and sources such as bathing, cooking, cleaning, and human respiration. Getting rid of this warm moist air from the home and replacing it with cool, dryer air from outside will reduce the Relative Humidity in the home. In newly constructed homes, that are energy efficient, there are few areas of air leakage for this house air to escape. It is for this reason that we require ways to remove some of this moisture from our homes during the heating season.
An HRV is a mechanical device that draws outside air into a home through an insulated duct and hood and exhausts house air through another duct and hood. The HRV unit has a heat exchanger, within a cabinet, that passes the incoming outside air past the exhaust from the home, to minimize heat loss. This is done in cool winter months to partially heat the cold incoming air with the warm house air and prevent some of its heat from escaping unabated from the home. The reverse will happen if used when the central air conditioner is on in the summer. This air is circulated through ducts that are normally connected to the furnace return air ducting and bathroom and kitchen exhaust registers. These ventilators have small fans and connecting up with the furnace circulation system allows the large furnace fan to help move the air effectively through the house. This arrangement also eliminates the need for separate bathroom exhaust fans and exhausts the moisture from the bathrooms with the rest of the house air.
The modern ventilators on the market are quite energy efficient, in relation to minimizing heat loss, but may have other necessary components that reduce the energy efficiency. In very cold weather, the heat exchanger elements in ventilators often frost up and may require a defroster unit to prevent blockage. These defrosters require electricity for operation, increasing the cost of operation, somewhat. Many of these defrosters operate automatically, and most require condensate drain hoses that are often installed on the cabinets to allow melted frost to run to a drain. Even with the added energy needed to defrost these ventilators, they are much more energy conscious that direct vent systems.
Recently I have seen, in newly constructed homes, mechanical ventilation systems that are set up with a single central exhaust fan, with ducting attached to bathroom registers and furnace ducts, in a similar manner to an HRV. The main difference is that the insulated fresh air intake ducting is simply connected to the return air ducts for the furnace. This allows fresh air to be drawn in to the home when the exhaust fan turns on, but has no allowance for reducing heat loss from exhausted air or warming incoming air. The principle of air exchange from the inside to the outside of the home is the same as that with and HRV, but this is much less efficient from an energy conservation standpoint.
Either style system will operate when switches are turned on in the bathrooms, and they are often connected to a humidistat in the home, as well. When the relative humidity in the home exceeds the set level on the humidistat, the fan will turn on and exhaust the warm moist air from the home, lowering the humidity. The good new with the simpler exhaust system is that it may be easily upgradeable to an HRV. Most of the ducting is already installed in an appropriate location, and the small central exhaust fan may be replaced with the more sophisticated HRV unit. Some modifications will have to be made to the fresh air intake and exhaust pipes, but these should be minor in comparison to starting from scratch.
The cost of and HRV or ERV installation may vary with different manufacturers and the amount of duct modification required in the home. Normally the cost of installation is less that the cost of a standard furnace installation. As these units have become popular relatively recently, there are a limited number of contractors properly trained in installation and design of the systems and several units I have seen are poorly or improperly installed. Enquiring about the experience of the installers and whether they have taken the specific HRV design and installation courses and are certified by the Heating, Refrigerating and Air Conditioning Institute of Canada (HRAI) is essential. An excellent reference booklet is “Operating and Maintaining Your Heat Recovery Ventilator (HRV)”, which is available free from Natural Resources Canada, Office of Energy Efficiency (www.oee.nrcan.gc.ca).