ERV and HRV controls confuse people because the equipment looks like an air handler and sits near one. These units are ventilators: they exhaust stale indoor air, bring in an equal volume of outdoor air, and pass the two streams through a core that transfers heat between them. An energy recovery ventilator also transfers some moisture. Neither cools, heats or dehumidifies the way conditioning equipment does, and expecting them to produces disappointment.
What the core actually accomplishes
Bringing outdoor air into a tight house is necessary for indoor air quality, and doing it through an open window wastes whatever energy went into conditioning the air that leaves. The recovery core recaptures a large share of that energy by putting the two airstreams in thermal contact without mixing them. In winter, outgoing warm air preheats incoming cold air. In summer the direction reverses. An energy recovery core additionally moves water vapor between the streams, which moderates how much humidity the incoming air adds or removes. The effect is a reduced penalty for ventilating, not conditioning in its own right. Recovery is never total, so incoming air still arrives closer to outdoor temperature than to indoor temperature on an extreme day.
The difference between the two types
A heat recovery ventilator transfers sensible heat only. An energy recovery ventilator transfers both heat and moisture. Which suits a given house depends on climate and on the moisture balance inside it. In a very cold dry climate, retaining indoor moisture during winter can be helpful. In a hot humid climate, limiting how much outdoor moisture rides in with the ventilation air matters more. The choice is a design decision made with the climate and the house in mind, and neither type substitutes for a dehumidifier when the house genuinely has a moisture problem.
How the controls usually work
A main wall control sets the ventilation rate or schedule, often with modes for continuous low-rate operation and intermittent higher rates. Boost switches in bathrooms or a kitchen let occupants run the unit at high speed for a set period, replacing a conventional exhaust fan in some designs. Many installations interlock the ventilator with the furnace blower so the fresh air distributes through the existing ductwork, which requires a control connection to the air handler. In cold climates the unit also runs a defrost cycle, temporarily reducing or reversing airflow so the core does not ice up, and that behavior is normal.
Why balancing is not optional
These units are designed to move equal volumes in and out. If the supply exceeds the exhaust, the house is pressurized and conditioned air is pushed into wall cavities where it can condense. If the exhaust exceeds the supply, the house is depressurized, which draws outdoor air through every leak and, in homes with naturally drafting combustion appliances, risks pulling combustion products down a flue. Balancing is done at commissioning with instruments, using balancing dampers in the ductwork, and it is a step that gets skipped depressingly often. If nobody measured airflow during your installation, it probably is not balanced.
Ducting and maintenance details that determine performance
Outdoor intake and exhaust terminations have to be separated enough that the unit does not simply recycle its own exhaust, and the intake should be away from driveways, dryer vents and combustion appliance terminations. Ducting between the unit and the terminations needs insulation and a vapor barrier in cold climates, because those ducts carry outdoor-temperature air through conditioned space. Filters on both airstreams need regular cleaning or replacement, and the core itself needs periodic washing according to the manual. A neglected ERV moves a fraction of its rated airflow and provides very little of the ventilation you think you have. Condensate is the other maintenance item people forget. Many units produce water at the core and route it to a drain, and a blocked drain can back up into the cabinet or onto the floor around it. Check the drain when you clean the filters and the two jobs become one visit.
What to check before you act
- Find out whether airflow was actually measured and balanced at commissioning.
- Clean or replace filters on both airstreams on the manufacturer’s schedule.
- Check that intake and exhaust terminations are clear and well separated.
- Do not expect the unit to solve a humidity problem on its own.
Common questions
Will an ERV reduce my summer humidity?
It reduces how much moisture the ventilation air adds, but it is not a dehumidifier and cannot lower indoor humidity on its own.
Why does my unit periodically change airflow in winter?
That is normally a defrost cycle protecting the core from icing, and it is designed behavior rather than a fault.
The practical takeaway
Judge a ventilator by whether it delivers balanced, measured airflow and keeps its filters clean, not by whether the house feels cooler. Ask whether commissioning included actual measurements, keep the terminations clear, and treat humidity control as a separate system with its own equipment.
References and further reading
- Department of Energy: home energy and ventilation guidance
- ENERGY STAR: certified ventilation and home equipment information
- CPSC: carbon monoxide risks tied to house pressure and combustion appliances
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
