3 Problems with Atmospheric Combustion Inside the Building Envelope
A lot of houses have atmospheric combustion appliances. What that means is that a lot of houses have the potential for serious health and safety problems, as well as higher energy bills.
First, let me define atmospheric combustion because, unless they're in the HVAC business or have training in how to do a home energy audit, many people don't know what it means. An atmospheric combustion appliance is one that draws air from the space around the appliance. You could, if you desired, pull the cover off and stick your finger into the blue flame. (Strangely, fewer people are drawn to do that than to stick their tongues on a frozen steel pole.)
The two atmospheric combustion appliances of most concern are furnaces and water heaters. (Fireplaces deserve their own article, so I'll leave them for later.) If your atmospheric combustion furnace or water heater is in a vented crawl space or a vented attic, it's outside the building envelope and is less likely to create health and safety problems inside the house. (Less likely but not impossible.)
Here's the basic process for furnaces and water heaters:
- Pull in air to mix with the natural gas.
- Burn the mixture of gas and air.
- Exhaust the combustion gases to the outside through the flue.
I've left out the part about why we're burning gas in the first place, which is to add heat to the home or the water in the water heater's tank. Here, my only concern is the combustion process.
An atmospheric combustion furnace pulls room air into the combustion chamber through the grill on the front (photo right). A water heater pulls air in at the bottom near the pilot light.
Problem #1 - Negative pressure increases infiltration.
When the furnace or water heater or both are running, they're pulling in room air and sending it to the outside. A basic rule of building science, and something that all home energy auditors learn, is that for every cubic foot of air that leaves the house, another cubic foot of air comes in. By running these atmospheric combustion appliances inside the conditioned space, you're increasing the infiltration that the house experiences. Your energy bills will be higher as a result, and you may find the house a bit drafty while the furnace is running.
I've written in this space before that you can't make a house too tight, but you have to pay attention to the combustion safety issues before you go air-sealing your home. You want it tight, but you don't want to increase the probability for the next problem.
Problem #2 - Negative pressure can backdraft the water heater.
See that gap between the top of the water heater and the bottom of the flue? It’s designed to draw air in to aid the natural draft (stack effect) of the water heater. Warm air rises, so as a water heater runs and the warm combustion gases rise in the flue, the opening helps allowing more warm air from the room to rise.
The problem here is that that opening also allows air to come down the flue. If the air pressure in the room is low enough relative to the air pressure where the flue terminates outside, air will come down the flue. If air is coming down the flue while the water heater is firing, combustion gases will not go up the flue. They’re coming into the room.
This is where things get interesting. Under normal operating conditions, with the combustion gases exhausting up the flue, the combustion process results mainly in water vapor and carbon dioxide. When the water heater is backdrafting, the combustion process changes. The flame may get starved for oxygen, causing incomplete combustion, which results in significantly more carbon monoxide in the combustion gases.
Carbon monoxide, needless to say, is bad. You don’t want it in the air in your house, which is exactly where it goes if the water heater is backdrafting. It can’t go up the flue.
Problem #3 - Common venting of water heaters and furnaces overrides an important safety feature of furnaces.
At the base of the flue in an atmospheric combustion furnace is a draft inducer. It’s a little fan that pulls air up through the heat exchanger. Right next to that fan is a pressure sensor that will cut off the furnace if it detects that the pressure in the flue is too high. If a squirrel or a bird builds a nest at the top of the flue, and the combustion gases can’t escape, this sensor can save your life by shutting off the furnace.
When your water heater flue is connected to the furnace flue (see second photo above), however, that squirrel nest at the top of the flue still keeps the combustion gases from going out, but the sensor may not detect a high enough pressure to shut off the furnace. The reason is that the combustion gases now have another escape path - at the top of the water heater!
If you have atmospheric combustion inside the building envelope in your home, here are some options to eliminate or reduce the likelihood of problems:
- Go with a sealed combustion furnace and direct vent, power-vented, or sealed combustion water heater if you still want to use gas inside the conditioned space.
- Create a sealed combustion closet out of the room where the atmospheric combustion appliances are. To do this, you need to isolate the room completely by air-sealing between it and the rest of the house and then bringing in combustion air.
- Convert from natural gas to electricity. We recently ran a two guest posts from David Butler advocating for this approach: Just Say No to Furnaces in High Performance Homes and Heat Pumps and Hydronics – A Great Team for High Performance Homes.
Combustion safety is a huge issue, and it's a big part of looking at the house as a system, which takes us back to Building Science 101. It's certainly possible to use natural gas safely in our homes, but we need to pay attention and do it the right way. Putting atmospheric combustion appliances inside the conditioned space is not the right way.