HRV / ERV Guide

  1. Why do I need one of these things?

    When we heat or cool houses, we actually are heating or cooling (Conditioning) the air in these houses.  Once this air has been conditioned to be comfortable for our bodies, we want to keep it in the house.  Insulation does this for us.  Some types of insulation are prone to allowing air to pass through them freely.  These products may or may not require installation of an HRV, it depends on how well the trades airsealed the house.  The Insulation Man, LLC uses all types of insulation products, and we select which product to use in each application based on its ability to prevent the air from traveling through it.  This means we must make provisions for some fresh air to enter the home in a controlled way.  An HRV or ERV is the best way to bring fresh air into the house every day in a controlled process. 

  2. Why not just open a window?

    First, air from an ERV that enters a home on a day when it is 20°F outside will enter the house at about 55°F also; the incoming air will have about 30 to 40% relative humidity.  This air will enter at a very slow rate throughout the day.  In typical winter conditions, opening a window will result in exterior temperature air that is very dry entering the house.  Dry, cold air will enter at whatever speed the wind is blowing for as long as the window is open.  Opening the window is not comfortable, so people tend not to do it. 

    Second, opening the window relies on a pressure differential creating ventilation.  This process works easily in the winter, but during the times of the year when interior and exterior temperatures are nearly equal, or during times of the year when there is no wind, opening a window will not necessarily result in ventilation happening without a fan.  If you put a window fan in each window and blow them all into the house, then they will not work, you cannot stuff more air in the house simply by running a motor, somewhere it has to leak out.  Usually during the summer opening the windows brings in too much humidity, which makes the house less comfortable, so the occupants tend to not open the windows.

  3. What is the difference between an ERV and an HRV?

    HRV stands for Heat Recovery Ventilator.  These units are designed to transfer heat only from the stale air being exhausted from the house into the fresh air being brought into the house.  This type of unit will tend to dry the air in a house throughout the winter, and bring humid exterior air into the house in the summer. 

    ERV stands for Energy Recovery Ventilator.  These units are designed to transfer both heat and moisture from one air stream to the other.  They transfer the same amount of heat, and they transfer moisture from the higher humidity air into the lower humidity air.  In winter and ERV will help a house maintain a constant relative humidity by recycling moisture from the exhaust air into the fresh air stream.  In summer, an ERV will dry the incoming air.  This helps an air conditioning system work more efficiently, since it is not trying to dehumidify exterior air. 

  4. Do I need a separate duct system for the ERV? 
    An ERV requires two 6" round, insulated ducts to outside, one for the exhaust air, the other for the fresh air.  These ducts should be located at least 6 feet apart, and the fresh air intake must be located in a place where it will get fresh air.  This means not near a driveway, furnace flue, fuel tank, garbage can, etc. 

    If your home is equipped with a ducted heating system it is possible to utilize that system for the ERV.  Normally however, some additional ductwork is necessary. 

    If your home only has air conditioning ducts, then the ERV can utilize these ducts, but most of the time this will not result in good distribution of the fresh air. 

    If a totally separate duct system for the ERV is planned, then the plan is to remove air from all moisture sources, and bring fresh air into spaces where cool air is not objectionable. In winter, air from the ERV will be near 55°F.  It enters the house at a slow speed.  It needs some time to dilute with the heated air in the house before the occupants feel it.  Normally the best place to introduce the air is high in the wall in any room or common area, best place is the bedrooms.  Remove air from each bathroom, the laundry room, and the kitchen (never use a HRV or ERV as a kitchen range exhaust!).  Try to balance the number of exhaust and supply ducts.  In a typical 2000 to 3000 square foot house, 3 exhaust and 3 supply ducts are normally used, and they are all located in bedrooms or bathrooms.  Usually, there will be a short (2 to 3 ft) piece of 6" flex duct coming off the ERV connected to some 6"round steel duct.  This will lead to 4" round or 5" oval branch lines that go to the bedrooms or bathrooms. 

  5. How is the ERV controlled?

    There are several different electronic central controls.  These controls allow the ERV to run either at high or low speed, for specific durations of time.  Some types allow the ERV to be used as an air filter as well as a ventilator.  Some controls will react to humidity. 

    There are secondary wall switches that are installed in the same locations as the exhaust ducts.  Activating these switches will cause the ERV to run on high speed for 20 minutes, and then resume the control pattern established by the central control. 

    All the controls are wired to the ERV using 18-3 thermostat wire. 

    The ERV will require an 110v outlet near where it is installed. 

  6. Does an ERV or HRV require maintenance?

    Yes, it is necessary to remove the heat recovery core and filters from both types of units at least once a year.  Clean them with mild detergent and water, allow them to dry, and put them back in the unit. 

This document is intended as a simplified guide to give potential clients some idea of how an HRV or ERV works and is used.  It is not intended as an installation guide.  Installation guides are available from the product manufacturers, or from Dundon Insulation, Inc. for a small fee. 

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