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3 Problems with Atmospheric Combustion Inside the Building Envelope

Atmospheric combustion appliances inside the building envelope can create health & safety problems.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:

  1. Pull in air to mix with the natural gas.
  2. Burn the mixture of gas and air.
  3. 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 waterAtmospheric combustion appliances use room air, which can create negative pressure. 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.

atmospheric combustion water heater health safety carbon monoxide flue gapSee 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.

atmospheric combustion furnace water heating common venting carbon monoxide draft inducer

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!

The Solutions

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.

All one needs to do is to Google “CO Accidents” and we see first hand the dangers of atmospheric combustion appliances in living spaces with humans (and pets for that matter).  Here in the DFW area we recently had a family trying to keep warm by burning their charcoal grill inside the house. All present went to the hospital unconscious and the father of the family had 2 heart attacks on the way to the hospital triggered by CO poisoning. Obviously this is an extreme situation and not quite the same as what we’re talking about here, but the premise is the same. CO is bad. It kills. Slowly.

How about we all go all-electric and be safe?

Cold Interior Walls, Useless Insulation, and Building Science

It seems so simple. There’s outside, and there’s inside. The walls, ceilings, and floors that separate the cold outside from the warm inside should be the only surfaces that rob heat from your home. Interior walls have conditioned space on both sides, so they shouldn’t be a problem, right?

Wrong! I’ve seen and worked on several houses that lose heat through their interior walls. They’re usually older houses that have been modified from the original, but I’ve seen these problems in new houses, too.

In such houses, the problem results from the top of the walls being open to the attic. You can go into the attic and look down into the interior walls and see the drywall. That means that cold attic air gets down into those cavities.

With cold air inside the wall and warm air on each side in the house, heat moves through the drywall, which has a low R-value, and warms up the air in the cavity. The warm air then rises into the attic (a process called the stack effect). As that warmed air leaves the interior wall, cold attic air moves into the cavity to take its place, resulting in more heat loss. This process continues as long as the attic stays cold and the house is warm.

To fix the problem, you have to stop air from moving into or out of the interior wall cavities. One way to do this is to cover the opening with a rigid material and use caulk or spray foam to seal the edges.

Another way is to stuff a piece of fiberglass insulation into the gap at the top of the wall and spray foam over it. It’s important to note that the fiberglass by itself is not sufficient because fiberglass does not stop air movement. (That’s why it’s used in filters, duh!)

Another problem I see in attics is attic with two ceilings and insulation in the wrong place. A lot of old houses with high ceilings have had lower ceilings put in below the original ceiling. In that case, insulation on top of the original ceiling fails to keep the house warm. Again, it’s a matter of understanding where the attic air is going. These houses have two ceilings: the original ceiling and the current ceiling, which is lower.

The blown fiberglass insulation is almost always on top of the original ceiling because that’s the ceiling you see from the attic. In the course of modifying the houses, however, the workers opened many pathways for air to move from the attic to the space between the original ceiling and the current ceiling.

The result is that the air under the insulation (between the two ceilings) is at the same temperature as the attic air. That cold air is separated from the house only by a layer of drywall. As that air warms up from heat moving through the drywall, it moves through the openings into the attic and is replaced by more cold air. The insulation is useless because it’s in the wrong place!

If you want to understand politics, you often have to follow the money. If you want to understand heat loss in your home, you often have to follow the air.

Dubious ‘energy audit’ phone solicitations anger local residents offering “free” energy audits…

The calls are not from electric companies, even though that’s what solicitors say If you use electricity in your home — and who doesn’t? — there’s a telephone solicitor who has your number.

Where did they get it? Who knows? But the fact of the matter is, they are NOT calls from your local utility company and they are NOT whom they claim to be.

Who gives away a free energy audit unsolicited? Only contractors who have your pocketbook in mind. Did you know there are contractors who will sell what they CALL energy audits for as little as $99? Some will even do them for free. Just about everyone offering LOW COST energy audits does it as a loss leader or a lead source generator because they’re looking to sell you THEIR specific solution (i.e. insulation, windows, A/C equipment, etc…).

It’s like a free exam at a chiropractor. They give something away to get the opportunity to sell their core product. Pro Energy Consultants believes it is extremely important to be completely objective and extremely thorough. People spend more money upfront so they can have the peace of mind that they aren’t being sold some kind of repair. We actually have had many homeowners who have had a “free” audit and then ended up calling us.

The bottom line is, you get what you pay for. Even if it’s free…