I live in a 60-year-old bungalow which has 2 exhaust fans on the main floor that are exhausted outside. My dryer, which is located in the same room as the furnace and gas-fired hot water tank, is also vented to the outside. Shortly after moving in several years ago I caulked and sealed all the windows and doors and did several other minor repairs to improve the tightness of my house. I have my furnace fan running continuously, year round, and have a 3-inch fresh air intake that goes directly into the furnace ducting.
My problem occurs when all the windows are closed and either of the exhaust fans or the dryer is running. The furnace room get very warm and I smell exhaust. I have figured out that the smell and heat is from exhaust drawing back into the house, down the chimney, from the water heater. I thought that there may be a blockage in the chimney, but I checked and it is clear. This problem immediately goes away if I open a window in any part of the house.
I am concerned about the possible health problems with exhaust coming back into my house. What is causing this problem and how do I fix it? Should I just open a window when I turn on the exhaust fans or dryer?
As homes become more airtight, in an attempt to reduce heating bills, more problems with indoor air quality are seen. Bathroom and kitchen exhaust fans and properly vented dryers are necessary to remove excess moisture and household pollutants from homes, especially during the heating season. The problem you are experiencing is a common complaint and often is first noticed when the pilot light in the bottom of the hot water heater blows out. Most homeowners call for assistance after the pilot light is relit and extinguished a few times. You are right to be concerned about combustion products from the hot water heater getting back into the house, and I suggest immediately purchasing and installing a Carbon Monoxide detector, if you don’t already have one.
The back venting you are experiencing from your natural gas hot water heater may have a series of factors that contribute to the occurrence, but the explanation is relatively simple. By turning on your exhaust fans or dryer you are drawing out a large amount of air from the home, which is not being replaced quickly enough with makeup air from the outside. This causes the air pressure inside the basement to drop below that outside the home. The low pressure within the house will automatically draw the higher-pressure air outside in through any available opening, in an attempt to equalize this pressure differential. Since your windows and doors are well sealed, the most convenient opening is often the top of the chimney, which is basically a large hole in the top on the building. The rush of air down the chimney will counteract the natural draft from the hot water heater and blow the exhaust back into the basement.
Opening a window in the home will quickly allow enough air into the house to equalize the pressure inside and out and end the back venting. The fresh air intake in your heating system ducting may help, somewhat, but may be too indirect or too small to stop the problem. Opening the windows is a quick and easy solution to your problem but will not be desirable when it is –20 outside, in the dead of winter. I will suggest a few options for eliminating your problem.
The first thing to attempt is opening or removing the door to the furnace/laundry room or installing vents in the door or walls. This will help draw air from the entire basement into this confined space, helping to equalize the pressure when the dryer is operating. Check outside at the screen under the vent hood for your fresh-air intake to make sure it is clear of dirt and debris, which can cause it to become blocked and ineffective. If these two simple measures are not enough, a larger amount of direct fresh air may be required for the furnace room to prevent depressurization.
The quick solution is to disconnect the fresh air intake duct from the furnace plenum and allow it to enter the furnace room, directly. The hole in the furnace ducting could be easily covered with a piece of sheet metal. This duct should be insulated and extended down near the floor of the furnace room and returned up to eye level to create a loop at the bottom of the insulated duct. This loop will provide a partial “trap” that will prevent a continual rush of cold air from the outside, until it is required. However, you may find that this does not completely fix the problem, due to the small 3-inch diameter of the current fresh air intake. In this case enlarging the hole to the exterior and installing a larger 4 or 5-inch insulated duct may solve the problem. This will give the same effect as opening a window, but will provide the air directly to the furnace room, where the dryer and hot water tank are located and where the back venting is occurring.