Author – Marge Padgitt
Negative house pressure is a common problem that can cause fireplaces and wood-burning appliances to smoke, and gas appliances to backdraft deadly Carbon Monoxide gas. This is probably the least understood house issue, but one that usually can be corrected.
All houses have a positive pressure plane, a neutral pressure plane, and a negative pressure plane. The negative plane is at the lowest portion of the house, and the positive plane is at the upper level, inside the building envelope. The planes move constantly according to pressure changes that occur when someone opens a door or window, or turns a fan or appliance on or off.
House pressure is something that can’t be seen with the eye, and the effects are obvious –once a person knows what they are looking for.
Signs of negative house pressure include stale air, mold or mildew in bathrooms, unexplained illness among family members or pets, smoking fireplaces or appliances, and condensation on windows. An inadequate amount of fresh air is getting into the house, and this can be a severe health hazard as well as a smoking fireplace problem. The EPA suggests that houses get a full six air exchanges per day for the occupants to breathe.
Tight, or well-insulated houses have a better chance of having a negative pressure problem, and larger homes with multiple stories are likely candidates, however, any house can have a negative pressure problem.
The issue is often first detected by a professional chimney sweep. This is due to the fact that smoking fireplaces are a huge annoyance, and either the sweep will find the problem during a routine inspection, or the homeowner thinks that there is a problem with the fireplace or chimney.
If, after checking for proper chimney construction, including proper height, the flue and smoke chamber sized to code, and stack effect (house taller than the chimney) the next thing to look for is negative house pressure.
There are simple techniques to check for negative pressure as well as complicated methods. First, house pressure should only be tested during cold weather in order to see what is happening when the house is closed up and the heating appliances are used. Next, place shredded lengths of tissue paper or a newspaper over the fireplace opening. Open the damper and watch what happens to the paper. If it moves inward towards the back wall of the fireplace, the draft is established and there is no negative pressure. If the paper moves outward towards the room there is negative pressure. Now open a window in the room very slowly and watch the paper, which should switch to moving inside the fireplace at some point. Measure the window opening and this is the amount of air needed for the fireplace to operate without smoking.
If interested in getting an exact measure of the problem testing can be completed using a Digital Manometer. Some professional sweeps do this testing, otherwise contact an HVAC Professional for this service.
If the house does have a negative pressure problem the next step is to correct or decrease the problem. Other things must be taken in to account are other appliances that use air for combustion and take air from the house like gas furnaces, water heaters, boilers, cooking ranges and ovens, masonry heaters, gas or wood burning stoves, etc. Also, anytime a bathroom or kitchen fan is turned on, this removes air from the home.
Signs of Negative House Pressure:
- Sick House Syndrome
- Unexplained headaches
- Unexplained flu –like symptoms that go away when leaving the house
- Pets or young children that are ill for no apparent reason
- Stale odors
- Mold or mildew in bathrooms
- Condensation on windows
- Smoking fireplaces or stoves that get worse when the gas furnace or a fan kicks on
Solutions to the problem may include the installation of a draft inducing heater which is installed on the flue collar of a stove or insert, and installation of a whole-house ventilator. A whole-house ventilator is not a Heat recovery ventilator. A WHV is installed in line with a gas furnace system and operates both actively when the furnace is on, and passively when it is off. The WHV is installed halfway between the furnace and an exterior wall with insulated duct coming from the outside wall to the unit, then pipe is attached to the supply side and return air side of the furnace. Air is heated up in the WHV box before it gets to the heat exchanger so the exchanger will not rust due to condensation.
Many professional chimney sweeps sell and install draft inducing heaters and whole house ventilators.
~ Marge Padgitt