The unseen effects of air movement throughout your home
In part one, I discussed the implications of sealing up your home to the point where running exhaust fans and mechanical ventilation will put your home in a negative air pressure state. This is something you do not want to encourage in the cold seasons. This negative air pressure condition will cause cold air to be drawn through small cracks and crevices throughout the building envelope. This cold infiltrated air if allowed to breach the vapour barriers will cause a condensation and frosting/melting condition within the walls and ceiling, until they manifest into a larger problem, such as mould and mildew. Other implications of a home or building in a negative air pressure condition is that it will be drawing air through areas that may contain harmful gases and pollutants. The lower levels of the home such as basements already have an influence on air infiltration due to the “stack effect” created by the warm air leaving the top of the building and cooler heavier air continuously trying to get in through the lower areas. Some of this infiltrated air will be drawn up through cracks and unsealed areas from under the concrete basement floor. This encourages damp air with musty odours and radon gas. It is interesting to note here, that the concern about radon gas entering your home seems to coincide with the advent of the high-efficiency furnace and a more airtight draft-free building. As high-efficiency furnaces were becoming standard equipment in a new home, about the mid-90s, it was required that the furnace can be integrated with the stove fan which would exhaust on average, three times more air than the average bathroom fan. Combining the furnace fan with the stove fan would help pull in fresh air through the furnace air intake system, which would help to replace the air that was being vented by the stove fan. This ventilating system works okay as long as you are running the stove fan or the furnace fan.
We are now using a new and better system, that is installed and coupled with the furnace air handling system. It is for an air to air heat exchanger to be installed with the furnace ventilation system. This is commonly called an HRV. A heat recovery ventilator draws indoor warm moist air, usually off the ceiling and as it exits the building, it passes its heat through a heat exchanger which warms up the incoming outside fresh air. This tempered air can be either piped into the furnace air system or simply distributed through venting into selected areas within the home. This system works quite well, and it can be set up to operating on its own without the aid of the furnace. This system does several things, it keeps the air moving within the building and helps to eliminate cold spots or dead air spaces. It keeps the air fresh and helps to reduce high levels of radon gas buildup. It helps to balance the air pressure with the outside to maintain the indoor air in a balanced condition, which helps prevent air infiltration through cracks and crevices.
So, here we are 25 years later, and what have we done. We have sealed up our homes and made them more energy efficient, so we believe. But how much energy are we actually saving? Our furnaces are definitely more efficient and using less gas, which is a good thing. But any savings that we may realize on our gas bill, we are now spending on our electric bill by running fans, dehumidifiers, humidifiers HRV’s and the like. Not to mention that when one of these systems fail, there is a considerable cost to have them fixed or replaced. Let me mention here that with the old 80% efficient furnace and hot water tank, all we needed was a combustion air vent. This combustion air vent was tucked away in the furnace room in the basement. It was always there and free, it allowed fresh air for the furnace combustion, and for us, and it kept our air balanced within the home all the time. It did not cost us anything. Now we have shut that off, our furnaces and hot water tanks are taking the air they require from outside. Now we are adding another hole in the wall at a large expense to bring fresh air back into the house.