How to Prevent Negative Pressure in Your Home
We have posted several articles on negative pressure in the home and its consequences, the most alarming consequence being how it can suction dangerous gasses from the garage and gas-fired appliances into your living space (check out our post here with eye-opening video link of a model house!). To make it all clear, we wanted to summarize what happens unknowingly when certain exhaust appliances are operated, and how to avoid this dangerous situation.
Many people have gas or electric dryers in rooms within their home. When running a load of laundry, the dryer will suck about 200 cubic feet per minute (cfm) from the conditioned air in your home, and exhaust it outdoors through the vent. (see our post on heat pump dryers and how this type of dryer eliminates this problem).
The loss of 200 cfm of air during a 45 minute-1 hour drying cycle not only wastes money, it creates negative pressure in your home.
If you close the laundry room door while operating the dryer, as many people do for noise abatement, most of the negative pressure is confined to the laundry room. However, this means that:
the dryer is being starved for fresh air, which causes a longer drying cycle, and
the laundry room is pulling all kinds of contaminants through cracks and crevices from other places in order to feed the dryer. Where are many dryers located? In the garage or next to the garage, or in the basement, which means that you are pulling air from a space that has exhaust fumes and/or mold contamination to dry your freshly-washed clothes. Yuck!
If your laundry room also contains your gas-fired furnace or hot-water heater, it’s likely that the exhaust fumes from the heater are backdrafting to fill the air need for the dryer. This means that combustion gasses are being sucked into the laundry room and dryer over your freshly-washed clothes. Danger AND yuck!
If you don’t close the laundry room door, the dryer is pulling that 200 cfm of air from the rest of your home. Add to that your kitchen exhaust range, which is usually 300 cfm or more. Since the average home is fairly leaky, where is that air coming from?
The attached garage: yuck!
Down the chimney past a leaky damper: yuck!
From the dusty attic through your unsealed attic door and can lights: yuck!
From the neighbor’s stinky apartment: yuck!
From the pollen-laden or pollution-laden outdoor air through unsealed holes in the wall around your pipes, outlets and switches on exterior walls and other penetrations: yuck!
Kitchen exhaust hoods are required and dryers are a normal part of life in most places, so it’s best to control where the make-up air comes from: through a filtered intake!
The best solution for prevention of negative air pressure in our opinion is installation of a Heat Recovery Ventilator (HRV) or Energy Recovery Ventilator (ERV). These devices can recover the heat or cool in a normal exhaust (like a bathroom vent), and will supply the fresh air needed so that pressure in the home remains close to neutral. However, it’s not usually recommended to connect the dryer or kitchen exhaust to your HRV or ERV. Why? These are typically “dirty” exhausts that would quickly plug up a heat exchanger with lint (in the case of the dryer) or grease (in the case of the kitchen). (Sources: greenbuildingadvisor.com here and here).
In the first part of this Home Diagnosis video, the host shows a kitchen exhaust hood with remote fan and duct silencer (brilliant devices to minimize noise in the kitchen) and preheated and filtered make up air. Even if you don’t have a preheater/cooler, just having make up air on demand in conjunction with the range hood is a great idea, not to mention it satisfies the International Residential Code (IRC) requirement that “Exhaust hood systems capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be mechanically or naturally provided with makeup air at a rate approximately equal to the exhaust air rate.” (IRC code M1503.4)
The same should go for the dryer. In order not to pull a suction on the rest of the house, the HRV/ERV should supply fresh air to the laundry room.
In mild climates where air conditioning or heating is not needed for much of the year, or the homeowner simply likes the idea of opening windows for fresh air, an HRV or ERV doesn’t make much sense. Thankfully there are ways to get that “makeup air” so that your home air pressure will stay balanced no matter what appliances/vents are running. Here are some of them:
Window screen filters are a great option to get that fresh makeup air by opening a window and passing it through a nanofilter to remove the smallest particles of pollution.
In-Door Return Air Pathways by Tamarack Technology bridge the inevitable: closed interior doors. Simply install these in the bottom of your hollow-core or solid wood interior doors (door must be 1-3/8” thick to fit) so that you can have privacy AND ventilation at the same time.
One last source of negative pressure in the home is power attic roof vents. Most people think that using a power attic roof vent will cool the attic by sucking out hot attic air, and drawing cooler air in from outside. The attic does end up being cooler, however, this is not because of outside air being drawn into the attic; the vent is actually so powerful that it is sucking the air-conditioned air from your house up through large and small crevices, into the attic! (bobvila.com) If you have your gas water heater or furnace in the attic, it can also be backdrafting. In most cases, unobstructed natural vents at the soffit and ridge, combined with a good layer of attic floor insulation, will be better than power attic roof vents.Just like your budget, don’t let your house go “into the negative” zone when it comes to air pressure, and get that fresh air coming in when you need it. Balanced or Make Up Air Systems (MUAs) not only avoid pulling all kinds of pollutants into your air space, they also dilute higher CO2 and VOC levels so that you’re feeling comfortable and energized (check out our post on CO2 here). It’s a decision that can improve your home air quality with only a medium amount of effort and money!