How to tell if a house has negative air pressure?

You can tell if a house has negative air pressure by observing physical signs and doing simple tests that show air being sucked inward. Common signs include exterior doors that are unusually hard to open or slam shut by themselves, whistling or howling sounds around windows and doors as outside air is pulled through small cracks, cold drafts near outlets and baseboards even when windows are closed, fireplace smoke that comes back into the room instead of going up the chimney, and lingering cooking odors that don't exhaust well because makeup air is insufficient. You may notice that bathroom exhaust fans run loudly or backdraft dampers flap, or that your furnace or water heater shows signs of backdrafting like soot around vents, which is dangerous and can bring carbon monoxide inside. A simple tissue test can confirm negative pressure, close all windows and doors, turn on all exhaust fans and dryer, then hold a tissue near a slightly open exterior door, if the tissue is pulled inward toward the house, you have negative pressure. Another test is to hold a smoke pencil near door cracks and watch smoke being sucked in. Other clues include increased dust, high humidity in summer as humid outside air is pulled into walls causing condensation, and doors that won't stay open. If you suspect strong negative pressure, especially with fuel-burning appliances, you should hire an energy auditor or HVAC professional to measure pressure with a manometer and check for safe venting.