What Is an Energy Recovery Ventilator (ERV), and Does a Florida Home Need One?

What an ERV is, and what is inside the cabinet

The Environmental Protection Agency (EPA) says outdoor air can enter a building by infiltration, natural ventilation and mechanical ventilation.

A Department of Energy guide for existing homes, published in 2014, says most ERVs are rectangular cabinets that hold fans, filters and heat exchange media. The heat exchange media is the part where the two airstreams cross, and where heat and moisture pass between them. Four ducts usually connect to the cabinet. One brings outdoor air in, one sends exhaust outdoors, one supplies the fresh air to the house, and one pulls stale air from the house.

The guide calls ERVs balanced systems, which means they exhaust air and supply outdoor air at the same time. It says these systems do not put a large positive or negative pressure on the house. A Department of Energy education page says both kinds of recovery ventilator include a heat exchanger, one or more fans and controls.

ERV versus HRV: heat only, or heat and moisture

There are two kinds of recovery ventilator. A heat recovery ventilator, or HRV, moves only sensible heat, meaning temperature, between the airstreams. An ERV also moves moisture, which the guide calls latent heat.

The 2014 guide gives one question for choosing between them. Are indoor moisture levels usually more comfortable than outdoor levels? If so, it says, an ERV may be the more practical choice.

The guide says that question is easiest to answer in hot, humid climates, and it names Florida. Indoor air in a comfortable Florida home is relatively cool and dry, while outdoor air is quite humid. The Building America Solution Center, an online Department of Energy reference, likewise says that in humid climates the moisture transfer of an ERV can help reduce the moisture a ventilation system brings in.

What an ERV does with Florida's humid air

The National Weather Service office for Tampa Bay says the rainy season typically brings an increase in humidity, with persistent dew point values above 70°F. The Weather Service glossary defines the dew point as the temperature to which air must be cooled to reach saturation, assuming air pressure and moisture content stay constant. It says a higher dew point indicates more moisture in the air.

The 2014 guide says outdoor air passing through an ERV in Florida is somewhat cooled and dried, but warns that running one still raises indoor humidity, only by less than an HRV would.

The same guide says air conditioners often cannot remove all the extra moisture when humid air is brought in. Why a house feels humid with the AC running explains how an air conditioner removes moisture. The Solution Center adds that supplemental dehumidification is often necessary for homes in humid climates. It says high indoor humidity can have a negative effect on thermal comfort and can contribute to problems with biological growth. Our reading: an ERV can help reduce the moisture fresh air brings in, but it does not replace a plan for indoor humidity. Whole-home dehumidifiers in Florida covers that plan.

The guide's warning about moisture freezing in the heat exchanger is written for cold weather.

Does a Florida home need one?

Florida Solar Energy Center researchers wrote that whole-house mechanical ventilation is a critical part of good indoor air quality in tight homes. They also noted that enthalpy recovery is advocated in efficient home design to reduce risk.

The same paper says builders, contractors and homeowners may see best practice as a risk, for example if it is used when outdoor conditions may hurt comfort and energy use. The researchers studied a control strategy that shifts ventilation from hours with large indoor-to-outdoor temperature and moisture differences to hours when those differences are smaller.

The 2014 guide says recovery ventilators are typically the most expensive of the ventilation systems the guide compares, and can be more involved to install. It says heat recovery savings are larger in colder climates. In milder climates, it says, the unit's electricity cost can be greater than the heating or cooling energy it saves. The Solution Center checklist says equipment should be chosen for the target airflow and the climate. This page does not answer the question for any one home.

How an ERV should connect to the ducts

Some manufacturers' literature shows an ERV tied into the ducts used for heating and cooling, the 2014 guide says. Its authors found that this setup does not work well. When the central air handler fan is not running, the fresh air takes the path of least resistance, and very little reaches the living space.

The guide says running the air handler whenever the ERV runs can solve that, but raises electricity use dramatically and is not recommended. It recommends that at least one side of the ERV have its own duct. A fully separate duct system may distribute fresh air best, it says, but that is often impractical in an existing home. The Solution Center checklist says an ERV connected to the supply side of the central heating and cooling system should have a damper, a flap that stops air flowing backward through the unit when the ventilator is off.

The guide also says the outdoor air intake should draw clean air. It should not draw from an attic, basement, garage or crawlspace, and should sit away from garages, parking areas and exhaust outlets. Many units make a fair amount of noise, so the guide says to keep that in mind before placing one near bedrooms.

Bathrooms, controls and upkeep

The 2014 guide says many manufacturers suggest the ERV can replace bathroom exhaust fans by pulling air from each bathroom. Its authors heard occupants complain that the bathroom never dries out. With an ERV, it says, much of that moisture is recaptured and kept in the home. It recommends keeping separate bathroom fans that send moisture straight outdoors.

Most ERVs are available with programmable timers, the guide says. Some controls use humidity or carbon dioxide sensors to speed the fans up at higher levels, which the guide says usually indicate higher occupancy.

The guide says some higher-performing ERVs need their filters, heat exchange media and outdoor duct openings checked or cleaned quarterly. Without that upkeep, it warns, airflow drops over time to a small fraction of the design amount. The Solution Center checklist says the filter should be easy for occupants to reach. It also says operation and maintenance steps, such as how and when to change the filter and clean the intake screen, should be reviewed with the homeowner. It says the ventilation rate should be measured to make sure the fan provides the minimum rate. Filter types for the main system are covered in HVAC air filters.

Questions to ask before buying one: Is the house tight enough to need planned fresh air? How will humidity be handled? Where will the intake and the unit go? Will bathroom fans stay? Who will maintain it, and how will airflow be measured?