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Infiltration & Ventilation |
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Thursday, 26 June 2008 |
Introduction
Air exchange in a home is an important way of reducing harmful indoor pollutants like VOCs, formaldehyde, carbon monoxide, ozone, and others. Mechanical air changes are controlled, whereas air changes from infiltration are not controlled – it is desirable to reduce infiltration and control as much of the air changes as possible. Mechanical ventilation allows for filtration and purification of the air entering the home, as well as exchanging heat with the air being vented. Introduction to Infiltration & Ventilation is discussed as a design principle.
Evaluation
Using a value of 75 cfm for mechanical ventilation in the home, the energy required to heat this fresh air is 21,300 MJ/year. With 0.25 ACH for infiltration for a tightly built home, and an additional 0.15 ACH for occupancy (entering and leaving the home), the energy required to heat infiltration air is 51,000 MJ/year. The total heating (72,300 MJ/year) is more than the conductive heat loss through the building envelope. An average home built in North America will have an additional 32,000 MJ/year of heating requirements, though it could be argued that mechanical ventilation is no longer required for these homes.
The impact on ventilation & infiltration rates on heat loss is illustrated in the following chart. It is clear that as the air change rate increases, the heating load relative to the conductive heat loss for The Living Home increases substantially.
The following chart illustrates the four design cases. Case 1 is an average home infiltration with 75 cfm of mechanical ventilation. Case 2 is the same as Case 1, but with a Heat Recovery Ventilator installed on the mechanical ventilation. Case 3 is a tightly built home with 75 cfm of mechanical ventilation. Case 4 is the same as Case 3, but with a Heat Recovery Ventilator installed on the mechanical ventilation.
The heat recovery ventilator reduces air change heating loads by roughly 10%. This results in a financial pay back of over 30 years, but an carbon reduction of 0.6 tonnes/year for an energy return of less than a year. Clearly, the greatest gains in reducing energy consumption is to minimize infiltration. The tightly built home reduces energy consumption due to air changes by over 30% for a natural gas savings of about $250/year in present (2008) value.
Recommendations
It is recommended to reduce infiltration between walls and floor, and ceiling and walls, and to give greater consideration for all penetrations (pipe and wire) through the envelope of the home. An HRV is also recommended for its environmental benefits. |
Solar Heat Gain Analysis 
