Apply for Financing
AC Repair & Heating Repair


Some heating systems, such as gas furnaces, produce byproducts of combustions that people refer to as flue gases. The flue gases normally flow out of the house and disperse into the atmosphere. However, sometimes problems arise, and the flue gases spill back into the house.

Potential Causes

Anything that interferes with the flow of flue gases out of the house can cause spillage. The following are some potential causes.

Improper Vent Installation

The furnace vent is responsible for getting flue gases safely out of the house. The vent’s size and design determine how efficiently it does its job. For example, the vent relies on gravity to get the gases out of the house. For this to happen, the vent shouldn’t dip or sag. For example, a vent with a long horizontal section might not do its job efficiently.

Chimney-Vent Mismatch

If a vent empties into a chimney, then it should have enough draft to carry the flue gases the rest of the way out. Say you have a boiler system for hot water and heating the house that vents out through a chimney. If you install a high-efficiency furnace that vents via the side of the house, the water heater might lack the draft to carry the gases out of the house.

Vent Blockages

Vent problems are common causes of flue gas spillage. For example, when something blocks the flow of gases out of the vents, the gases might spill back into the house. Accumulated snow, pest (such as bees or birds) infestation, and leaves are some things that can block the vent.

Building Depressurization

Your furnace’s vent relies on atmospheric pressure to push the flue gases out of the house. Thus, the flue gases might spill back into the area around the furnace since no pressure is available to push out the gases. Such a situation might arise if other appliances or installations pull gases away from the furnace. Appliances that might do that include:

  • Fireplaces
  • Fans
  • Cooking appliances, such as a range hood
  • Clothes dryers

These are just a few examples. Anything that depressurizes the building or the area around the furnace can cause flue gas spillage.

Inadequate Air Supply

Improper air circulation, which leads to inadequate combustion air, can also lead to flue gasses spillage. Consider an example where you enclose your furnace in a small room. The setup will starve the furnace of air. The flue gases might circulate in the room alongside the little fresh air in it.

Start-Up Issues

Lastly, flue gas spillage does not always mean that something is wrong with your heating or venting setup. For example, many heating systems vent flue gases back into the house in the first few minutes after startup.

The anomaly happens because when you start the heater, the vent is not hot enough to start a draft to drive up the gases. However, the situation usually resolves itself after the vent warms up. You only have to worry if the situation doesn’t rectify itself.

Potential Dangers

Flue gases contain a mixture of different compounds, many of which are dangerous for your health. One of them is carbon monoxide (CO), which can cause organ damage. Extended exposure to CO can be fatal. Other compounds to worry about include nitrogen dioxide, acidic water vapors, and particulate matter that can clog your respiratory system.

You should watch out for flue gas spillage and fix the issue as soon as possible. Get a professional heating technician to design and install your heating system. Maintain and service the system as necessary. Lastly, contact Carrano Air HVAC Contractors, Inc, immediately if you suspect flue gas spillage. We will diagnose and fix the anomaly as soon as possible.

Let's Get Started

Select your Finance Assistance


Residential Financing


Commercial Financing

This field is for validation purposes and should be left unchanged.