Today, we revisit one of my favorite topics, Passive House Mechanical Ventilation.
First of all, we need the ventilation to be balanced. What does that mean? Simply, the amount of air leaving the home should equal the amount of air entering the home. Most homes experience unbalanced ventilation, usually this is resulted from removing large amount of indoor air without a good pathway to replace it. This often occurs in the kitchen. For the last decade, with the glorification of celebrity chefs and popularity of cooking shows, many people want to cook in a chef’s kitchen (real home cooks usually just have nice reliable equipment). This means home kitchens are equipped with commercial appliances. One of these must have appliances is a shiny commercial range hood. The suction power of range hood varies from 100 CFMs to 1600 CFMs. At some point, these hoods can suck up objects or small animals ( I kinda want to see a cat… :). The problem is, when you remove so much air from the kitchen, you create a lot of negative pressure that sucks something else into that space. Providing clean replacement air is the smart thing to do. When you don’t, that “something else” is most likely something harmful. i.e. combustion gases from water heater, furnace and other gas appliances, fumes from garage, outside air that flows through moldy cracks in your walls, etc. you get the idea. Additionally, more air infiltration into the home in many regions also means more moisture which can cause mold issues and at the very least, add to the latent load that the air-conditioner must work harder to remove. In a tight home, the backdrafting from gas appliances problem can be even worse. Now that I’ve scared people from breathing. We know the reasons why ventilation should be balanced.
We move on to mechanical. It would seem kind of funny that a Passive House is so NOT passive in ventilation. A certified Passive House has an air-infiltration rate of 0.6 ACH 50 or lower. The home is so tight that it would be nuts to not mechanically ventilate. Mainstream home building is following this trend too. The 2012 IRC mandates that any new home with a blower door test result of less than 5 ACH 50 must have a whole house ventilation system. Given that 2012 IRC also requires any home built in Zone 3- Zone 8 to be tighter than 3 ACH 50, every new home built in a jurisdiction that adopted 2012 IRC will have to have a whole house mechanical ventilator. I guess we all need to buy stocks in ERV/HRV companies! This trend toward tighter homes is really a good thing, gone are the days that builders can tell you, “you don’t want too tight of a house, the house needs to breath.” The truth is, the house does not need to breath, you do! So, the age of “build it tight ventilate it right” is finally here. A tighter home brings all sorts of benefits. See http://oikos.com/esb/37/bldtight.html Mechanical ventilation is also beneficial in many ways, better indoor air quality, comfort, reduced risks of mold and increased durability of home.
In the Arlington Passivhaus, we are doing our best to tighten up the home to our 0.6 ACh 50 goal as you saw from the post yesterday. In terms of mechanical ventilation, we are using an Energy Recovery Ventilator, which not only ventilates, it also recovers the temperature or energy from the outgoing treated air and it dehumidifies.
We fired it up for the first time today to balance the intake and exhaust flow rates. Here are some pictures of Michael Bonsby HVAC folks balancing flow rates throughout the house.
This is used to measure overall supply and exhaust.
Duct tape is used to temporarily reduce air flow to the correct amount, then a piece of metal is cut to the same size to be inserted and permanently fixed in the duct.