What Is Negative Air Pressure?

The difference in the air pressure in your house and the outdoors can be negative or positive.  Neither is particularly beneficial.  Let’s discuss negative air pressure as it relates to your home and specifically to your fireplace.  Keep reading to see why this subject is important.

WHAT IS IT AND HOW DOES IT OCCUR?

Stack effect diagram

Stack effect diagram

Negative air pressure can occur when the pressure inside a house is less than the barometric pressure outside.

We know that warm air rises in a home.  As this happens, a slight positive pressure is created in the upper levels of a house.  This can lead to air being forced out of the house.  Negative pressure can develop in lower levels.  Often there is no easy way for replacement air to re-enter the home.  When the home tries to pull air in, instead of pushing it out, the result is negative air pressure.

In serious situations, negative air pressure can actually out-muscle the draft in a chimney and sucks that “make-up” air down the chimney.  Needless to say, the problem with this is the difficulty of getting a fire established in your fireplace, stove or insert and the smoke filling the house!  With any volume of outside air being pulled down your chimney, it will be difficult to get any sort of draft established.

Negative air pressure will increase your energy and heating costs, too.  It draws cold air in, through the paths of least resistance.

Bathroom exhaust fans, clothes dryers, kitchen exhaust hoods and central vacuum cleaners add to negative air pressure issues when they’re being used.  They are removing air from inside the house while they operate.

WHAT CAN BE DONE?

As we mentioned, neither negative or positive air pressure is good for your house.  The exterior walls, or the building envelope, allows air to leak to and from your home.  Depending on the outdoor temperature and humidity levels, the air leakage will vary seasonally and will have different effects on the comfort inside the house.  It’s worth noting that negative air pressure is much less common with older homes than it is in newer houses.

Most of us are very conscious about reducing air infiltrating our homes.  We know the importance of sealing and weatherstripping doors and windows, insulating sidewalls and the ceiling, insulating basement walls and the sill box area.  But, with today’s construction materials and building techniques, homes are now being built exceptionally tight.  The goal, of course, is to keep interior and exterior environments as isolated as possible, regardless of the season.  In other words, keep the outdoor temperatures and humidity out, while maintaining a comfortable, controlled, conditioned space, in.   In newer homes, pressure concerns and outside air exchange can normally be addressed with an outside air make-up system installed by a knowledgeable HVAC contractor.

An outdoor air source, supplied directly to the inside of the fireplace, is standard equipment on many new models today.  And if a unit doesn’t come standard with an outdoor air kit, spend the few extra dollars for it.   One will be offered as an optional accessory to the fireplace.  It’s required by code in most areas.

WIND GUSTS

Don’t get wind gusts confused with negative air pressure.  A high gust of wind from a certain direction may cause a periodic downdraft.  But a negative air pressure scenario will draw a constant stream of spillage into the room.

GAS FIREPLACES AND NEGATIVE AIR PRESSURE

Direct vented (DV) gas fireplaces, stoves and inserts do not have a problem with negative air pressure.  DV equipment uses a sealed combustion system.  Unlike a wood fire that often uses room air for combustion purposes, a direct vented gas unit, uses 100% outside air for combustion to occur.  This is done through a co-axial vent pipe.  That is, a pipe within another pipe.  The larger outer pipe brings outside air into the firebox for combustion and the smaller inner pipe removes the by-products from combustion.  The only room air that interacts with a DV gas model is the air that’s drawn into the convection air chamber.  This room air is heated and returned back to the room via a blower fan.

AN EASY TEST

If you have noticed back-drafting of the chimney in your wood burning fireplace, here’s a quick and simple way to test the pressure.  While the chimney and fireplace are cold, attempt to get a fire burning with a little kindling and a few wads of newspaper.  If you notice the smoke is backing up into the room, simply open a window or door several inches and see what effect it has on the fire and draft.  By opening a window or door, you are simply supplying plenty of “make-up” air to the house and the smoke should be easily pulled up the chimney.

Obviously, you cannot leave the windows open during the heating season, but it’s a good first-step to determine precisely what the problem is.  There are other tests that can be used to determine how to effectively correct and balance the air supply through proper ventilation.  Equipment that measures the difference in air pressure is called a differential pressure sensor or a manometer.  Remember that good diagnostic testing will consider exhaust fans, clothes dryers, kitchen exhaust hoods and the force of the wind on the upwind or downwind side of the house.

Your WE LOVE FIRE dealer and/or a knowledgeable HVAC contractor is the best place to continue the evaluation process.

Below is a list of fireplace and grill stores that service Virginia (VA)

Manassas, VA

Winston's Chimney Service
11301 Industrial Road
Manassas, VA 20109

Mechanicsville, VA

Hearth & Home Shoppe
8151 Mechanicsville Tpk
Mechanicsville, VA 23111

Hearth & Home Shoppe LogoVisit Store Page

Roanoke, VA

Dixie Building Products Inc.
3342 Melrose Ave NW
Roanoke, VA 24017


For more information, contact your local fireplace and grill store..

2 Responses

  1. […] https://welovefire.com/fireplaces/what-is-negative-air-pressure/https://www.healthlinkbc.ca/healthlinkbc-files/indoor-air-qualityhttp://www.pureairplus.com/iaq.html […]
  2. […] call your WE LOVE FIRE expert. There might be a serious problem with the chimney flue. It could be a negative air pressure situation. Or, it could be an operator error. Regardless, it’s a serious situation, and you may […]
  3. We have more awareness about radon. Which is a deadly gas that sips through the ground, through your basement floor and in to you basement or whole house if it sits on a slap. My question is which chimney/fireplace system generates less or no suction ie negative pressure inside the house that it will not further affect radon suction in to the house via negative pressure. Specially for homes with a radon mitigation system. Thank you in advance for your knowledge and reply.
  4. I really have a question not a comment as such other than I'm happy there is someone out there that tries to figure this stuff out. I had to install a radon removal system in my slab on grade building . ( its a garage conversion to a living space) unfortunately I was not able to get to every location where there was an airspace between the slab floor and below so I'm pulling some air (I don't know how much) from the interior spaces , down thru the floor -under the slab and outside with the radon fan. If I'm careful opening the stove door I can usually not get smoke roll back into my room ( I'm sure it helps that the windows and doors are not super tight or efficient as I'm getting outside air in that way. My space is not super tight like current new home construction). Even when there doesn't appear to be smoke in the house at all could the negative pressure I'm creating be pulling dangerous gas from the stove into my space? or is the air being circulated thru the radon pull cleaning my airspace? What do you think is probably happening and what would you recommend I do since the radon system is here to stay? I can remove the wood stove of course but would like to find a solution that allows for the ambiance of the fire. I"m currently installing a direct vent rannai propane wall heater to cover most of the heating requirements . Thankyou for reading and hopefully sharing your insights. Sincerely Jay
  5. […] air pressure. A high gust of wind from a certain direction may cause a periodic downdraft. But a negative air pressure condition will draw a constant stream of cold outside air spilling into the […]

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