In Ontario, sadly, the residential building code still allows for cheap and infective ‘exhaust only’ ventilation systems and as Joe Lstiburek says, these systems suck – literally and figuratively.
The sign of a cheap home: The photo above shows the wall in a typical living/dining room with artwork, a thermostat, a light switch and “FAN SWITCH”. Though declining in popularity since 2012, the Ontario Building Code section 9.32.3.4 still allows suck only systems as long as they are labeled “VENTILATION FAN”. This switch controls a bath exhaust fan one floor above. Make sense?
As the number of quality homes increase in the marketplace and get more air tight, “Suck only” ventilation systems are not being installed as often, but being replaced with systems that “recover” heat or reject it depending on the season.
Home with exhaust only ventilation systems e.g. bath fans, range hoods – exhaust only ventilation is about as effective as the old “Shut the Front door!” technique.
When compared to “Exhaust only ventilation” systems, ERVs and HRVs save energy by recovering conditioned air temperature.
Once properly adjusted or commissioned, the best ventilation systems don’t affect the pressure inside the house and the debate turns to Heat Recovery Ventilators (HRV) versus Enthalpic Recovery Ventilators (ERV).
Both ERVs and HRVs recover heat (sensible heat recovery), but what separates ERVs from the more common HRV is the fact that only ERVs maintain humidity levels (latent recovery). Retaining humidity levels indoors at the ideal range of 40-50% RH in cold climates is difficult if a ventilation system needs to run continuously and doesn’t have latent recovery like the ERV does.
- This trusty old Health Canada graph shows just how beneficial indoor humidity is toward human health with the optimum zone of humidity between 40% and 50% Relative Humidity.
Moist air has more energy than the same volume of dry air – so it only makes sense in super-efficient homes to try to keep that “energy” or moisture inside – or outside – depending on the season:
With an Enthalpic Recovery Ventilator, both the sensible heat and the latent heat is recovered from the air which tends to yield more savings, greater health and better comfort. Net result – humidity stays indoors.
In summer time, the ERV works to keep the humidity OUT of the house which helps the Air Conditioner keep the air cooler with less work than say an exhaust only system.
This energy/moisture is what building scientists call latent heat. Latent heat is the energy stored in water dissolved in the air and by keeping it indoors, we not only save more money in conditioning – read heating or cooling - but we also stay more comfortable and healthy.
The evidence so far suggests that people living in air-tight homes situated in cold climates tend to dry-out their homes if running an HRV, however homes with ERVs and poor performing windows tend to get condensation forming at the window’s edge.
When it comes to the “heat exchanger”, residential ERVs come in two varieties– either a sealed system that works using a semi permeable membrane or a permeable filter like wheel. The sealed system works much like most HRVs using plate heat exchangers that are numerous and closely spaced but the great advantage with this system is the air tight separation between the stale air and fresh air. The membrane in this case is permeable to humidity. The later system with a media wheel rotates through the alternating air streams of fresh and stale air and though these systems work well, there is always some cross contamination between streams.
To minimise the cross contamination through ERVs, at the very least ensure that the unit you select is listed in HVI’s List of Certified Products.