Part 4: Time to Talk about Residential Ventilation – Selecting the System (cont.)

There are a lot of choices when it comes to installing a whole building ventilation system.  The system has to be quiet or people won’t use it.  It must be serviceable for both the motor and, if applicable, the filters.  And it must be energy efficient.

QUIET FANS:  That car alarm that doesn’t shut off, the woodpecker hammering on the top of the chimney, the jackhammer on the street – none of us do well with annoying noise.  The ventilation system in the house has to be virtually invisible to the ear.  It has to perform the task of changing the air as quietly as your breathing, at least as quietly as the refrigerator.  A quiet refrigerator in a quiet kitchen is the definition of 1 sone of sound level.  ASHRAE 62.2-2010 requires that the whole building fan operate at 1.0 sone.  If it isn’t quiet, the occupant of the home will seek it out and shut it off.   If it’s not running it’s no use at all!

Courtesy of Panasonic

EXHAUST-ONLY:  You might choose an exhaust-only system for the simplicity and the relatively low cost.  It can be as basic as a centrally located bath fan, venting to the outside, relying on leakage into the house for make-up air, hard wired to run 24/7 or perhaps with a control to operate part of every hour.  The primary drawbacks are that there may be parts of the house that don’t get a lot of fresh air and that these systems rely on putting the house under negative pressure, randomly sucking air in through the building system.  In the scheme of things, these are relatively minor drawbacks.  Operating cost is surprisingly small.  The energy efficient bath fans that  are available can draw less than 20 watts – less than the light bulb in the refrigerator, typically less than $20 per year.  The annual conditioned air cost is also quite small – roughly equivalent to the airflow volume times $1. about $60 per year for a 60 cfm fan.  Total cost for a year to breath – about twenty-two cents per day.

SUPPLY-ONLY:  The supply-only systems are harder to find.  They work in the opposite direction than the exhaust-only, pressurizing the house and allowing the air to leak out through the building shell.  They can be as simple as adding a pipe to the outside connected to the return side of a central air handler blower.  Whenever the air handler turns on, fresh air is drawn in and distributed around the house.  A control can be added to the air handler so that it operates periodically just for ventilation.  The benefit of this approach is that the air can be fully distributed.  The down-side is that the air handler blower uses a lot more electricity to operate than a small bath fan.  Another down-side is that the house is pressurized and warm, moist air can be forced into the building shell components which is not a great idea in a heating dominated climate like Maine or Ohio.  If the air handler blower is efficient, the operating cost can be reasonable.

BALANCED WITH HEAT OR ENERGY RECOVERY:  These devices are designed to be balanced.  They should draw into the house the same amount of air they blow out of the house.  Ideally they should have their own, dedicated ducting system, drawing the air out of the bathrooms and putting the fresh air back into the bedrooms or other highly occupied spaces.  If the  house is particularly tight, these systems are the way to go.  They don’t rely on leaks or holes to let the air in or out.

Courtesy of Venmar

An ERV exchanges both heat and moisture.  So in a heating dominated climate when the air outside is cold and dry relative to the inside, more moisture will be retained in the house because it will be brought back in by humidifying the incoming air stream.  For that reason, an HRV may be better in a heating dominated climate.  In a cooling dominated climate, the outside air is hot and humid.  An ERV will reject some of that humidity by adding it to the cooler and dryer outgoing air stream.  So an ERV has some advantages in a cooling dominated climate.  Go to the HVI web site and check out the Product directory.  The power consumption and the efficiency of these units is listed there.  Compare the “Sensible Recovery Efficiency” numbers to compare units.

Even though HRVs and ERVs save energy by preheating or precooling the ventilation air, they commonly use more energy because of less than optimum efficiency motors.  That problem is exacerbated when they are connected to the air handler for distribution.


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