11/13/2014

How Does a High-Efficiency HRV Increase Comfort in the Home During the Cold Winter Months?

With the winter weather starting and the temperatures lowering, heating season has begun in many climates across the US and Canada.  Many inhabitants are accustomed to blasting the heat to combat the cold frigid air infiltrating a leaky home.  In a leaky home, during a 0° C/32° F cold winter day, incoming air enters in the building through cracks and openings at the same cold outside temperature. Therefore, an inhabitant needs to operate heating equipment frequently to warm spaces to a comfortable room temperature. The unconditioned, uncontrolled outside air can produce uncomfortable drafts and variations in temperature throughout the building depending on the sources of leaks/infiltration.

Unlike a leaky house with constant drafts and high-heating loads, an energy-efficient home with good air sealing, insulation and heat recovery doesn’t experience these uncomfortable drafts and heating loads during the cold winter months.  The Heat Recovery Ventilator (HRV) brings in continuous fresh filtered incoming outdoor air to pre-heat and mechanically ventilate it throughout the house within several degrees of the room temperature. Therefore, heating loads can be greatly reduced and uncomfortable cold drafts eliminated.

Why efficiency of the HRV matters for comfort?

Many of the heat exchangers found in the US and Canada have efficiencies in the 60%-70% range.

Assuming optimal performance of 70% efficiency and a room temperature of 20° C/68° F, the make-up air temperature coming through the home is a brisk 14°C/57°F (see chart below).  This is a 6° C/11° F differential from the 20°C/68°F existing room temperature. There is a continuous stream of uncomfortable 57°F air being supplied to the home before the home is provided with additional heat.  Therefore, one will have to increase the heating load more to bring the rooms to a comfortable temperature in comparison to utilizing a greater efficiency heat recovery ventilation system.

With a high-efficiency HRV in the 85%-95% efficiency range, the cold outside temperature entering the house is preheated through the heat exchanger within a few degrees from room temperature (see chart below).  This eliminates cold, uncomfortable air being spread throughout the living spaces as well as reduces the heating load because of the smaller differential between the air coming in and comfortable room temperature.

HRV

60% Efficient HRV

(32°F + (68°F – 32°F)*(0.60) or 0°C + (20°C-0°C)*(0.60) =12°C/54°F

70% Efficient HRV

(32°F + (68°F – 32°F)*(0.70) or 0°C + (20°C-0°C)*(0.70) =14°C/57°F

85% Efficient HRV

(32°F + (68°F – 32°F)*(0.85) or 0°C + (20°C-0°C)*(0.85) =17°C/63°F

95% Efficient HRV

(32°F + (68°F – 32°F)*(0.95) or 0°C + (20°C-0°C)*(0.95) =19°C/66°F

What are other important factors that impact a HRV system’s efficiency?

Choosing an HRV unit and piecing together unspecified ductwork and air distribution components sometimes lead to decreased heat recovery efficiency of the HRV system. The less efficient the HRV system, the more uncomfortable the continuous below room temperature incoming air will be for the inhabitants.   This is the reason we recommend having a complete HRV system designed and then after the system is installed, have it commissioned to ensure the system is working properly.