At some point in time man gained control over fire. Fire was very useful: it could be used to cook food, provide light, and its heat allowed us to survive in hostile environments.
Thousands of years later, some things haven’t changed much. Whether we have a natural gas or propane furnace or (in some parts of the country) an oil fired boiler, most of us still rely on fire in some form to heat our homes and our water. Many of us use open flames for cooking as well: the gas cooktop is a favorite of many culinary experts.
However, we’ve come to a point in history when using fire in our homes is no longer always the most cost-effective, sustainable, or safe option. While we should do everything we can to make fire-burning homes safer and more efficient, we should remember that there are costs to doing so—costs that we could eliminate entirely by getting rid of “fire” from our homes altogether.
The technology for heating homes and water with fire is pretty basic. In a forced-air system, you burn natural gas or propane in a furnace to heat a metal box. Air is then circulated around the hot box; once the air is warm, it’s forced into the house by a fan. If your house uses radiators, a boiler uses fire to heat water, which is then circulated by pumps to heat your home.
For heating water, the process exactly the same as making hot water for a cup of tea or coffee. Fire warms up a tank full of cold water. You open the tap and the water flows into your house to provide your nice warm shower. Tankless water heaters work the same way, they just have a big enough flame that they can heat the water on the fly as it goes through the pipes. That’s how we provide those endless showers your teenagers are so fond of.
All of this is fine, as long as the fire stays where it should be and the smoke leaves the house safely. Modern gas appliances have come along way in this regard. These devices operate automatically, and as a result, the manufacturers have installed several safety features to protect consumers. In a water heater, a small pilot light burns and heats a device called a thermocouple. If the thermocouple is not hot (if the pilot is blown out) the main gas valve won’t open. This prevents large amounts of unburned fuel from spilling out of the device and creating a potentially explosive situation. Furnaces and boilers have similar safety circuits and more, to ensure the device will not fill your home with unburnt gas while you’re sleeping or away. Making sure the “fire” stays where it should and only operates as intended is a key safety feature of these devices.
The other side of safety is what happens to the fuel after you burn it. We all know that when you burn something, you get smoke. Combustion devices (those that rely on fire) produce smoke as well. Even though today’s appliances are very good at burning most of their fuel, physics dictates that they will never be 100 percent efficient and that there will always byproducts of combustion. Most of these byproducts are not visible to the human eye. These toxic byproducts must be expelled or vented from the home.
Another requirement for fire to burn is oxygen. We all know what happens when you put a cup over a candle--the lack of oxygen snuffs out the fire. This means combustion devices in your home need both a place to expel the burnt fuel and a place to draw oxygen from. For the most part, people give little thought to this process. The appliance is installed, fired up, and it provides service for many years, often decades.
All is well until a home performance contractor comes along and changes things.
If you’re trying to make a home as efficient as possible, obviously sealing up all the holes to the outside is a great place to start. This has been a basic premise of energy efficiency upgrades for years. We now have devices to quantify how tight or leaky your home is (e.g., the blower door). This means we can do a really good job of sealing all the holes. The problem is, what happens to the combustion devices as we make homes tighter and tighter to save energy?
Think of your house as a box with a bunch of holes in one side. Your combustion device uses air from inside the box for combustion and expels the combustion byproducts via a vent (flue or chimney) through the roof. New air for combustion is drawn in through the holes.
Now consider what happens if we seal up all the holes and make the box more airtight. Now the combustion device may not have enough oxygen to burn properly, which can create deadly byproducts, the most serious being carbon monoxide. Now let’s add a big fan to the box and suck air out of it. (There are several devices in your home that do this: a dryer, a range hood, bathroom fans, and your furnace, potentially). If the fan is too strong, the combustion byproducts don’t flow up the chimney and out of the home. Instead they are “back drafted”, or sucked back into the home, where you get to breathe them.
So how do we make sure we are not causing unintended consequences by sealing up all the holes? Enter combustion safety testing, often known as CAZ (combustion area zone) testing.
CAZ testing is an important part of any home performance job where combustion appliances are used. The basic process is to identify where the devices are, how they interact with the conditioned (occupied) space, and to insure the devices are working as intended. CAZ testing requires a highly trained technician using very precise (expensive) equipment to determine pressure changes within the home. CAZ testing makes sure the fans are not sucking all the air out of the house and pulling the flue gasses back in. It will also identify other hazards such as gas leaks and potentially carbon monoxide. In general CAZ testing is a good idea and should be performed any time we make changes to the “envelope” or box of a home. This includes changing out windows, adding new fans, replacing furnaces, adding insulation, and so on. In most cases today, CAZ testing only happens when home performance contractors are part of the process and are making changes to the building envelope.
As you might imagine, CAZ testing is expensive. For most houses it takes about an hour to do all of the testing. The equipment is super sensitive and needs to be frequently calibrated, and you need a well-trained technician to operate it. This all costs money.
Many rebate programs require CAZ testing before rebates can be disbursed. This is a good idea, as the goal is always health and safety first. But as you might imagine, CAZ testing, and verification that it has been done correctly, add significant costs to any rebate program. There are many things you can simplify in a rebate program to reduce cost, but reducing or getting rid of CAZ testing is not one of them. This means we spend millions of dollars sending highly trained individuals into homes to verify that contractors have performed CAZ tests and that their numbers are correct. As I said, this is a good idea, as many CAZ issues cannot be seen and must be a measured.
Even with all the time and money spent there are still some problems with CAZ testing that have not received a great deal of attention. First off, CAZ testing is very technical. There are many variables to consider when testing pressure balances in homes. Simple things like how windy it is outside or the outdoor temperature can have drastic effects on your results. There are other variables, such as the assumptions the manufacturer made when making the testing devices and the sensors themselves. This level of complexity makes repeatability a real concern in CAZ testing and verification.
In fact, it’s not uncommon to have different results on different days in the same house. I witnessed this first hand when I attended a combustions safety training several years ago. The training facility rented a vacant house to teach a real-world class on CAZ testing. There were two classes the same day. In the morning class the water heater failed it’s CAZ test. The draft test indicated combustion byproducts were not being drawn up the vent (flue) and spent gasses were “spilling” back into the house. Later in the day, the second class when through the exact same testing process in the same house and it passed with flying colors. Factor in the different types of measurement devices and you can see that we’re putting a lot of faith into a process with many variables. To make things worse, the utilities use a different standard than home performance contractors (NGAT vs. BPI). This means a home performance contractor might fail a job, requiring them to call to the gas provider. Then the utility sends out one of their technicians to confirm the issues, and the house passes, because they’re using a different set of standards. All of this is expensive, and the cost is eventually passed on to the consumer.
There are solutions. Newer gas appliances can be rated as “sealed combustion”. This means the air for combustion and the flue gas are completely sealed, so all of the combustion air and venting is outside the house. With sealed combustion, pressure changes to the “box” or building envelope do not affect the combustion process. Naturally these devices are more expensive and require additional labor to install, so not all new appliances being sold are sealed combustion. Worse yet, it’s possible to install a sealed combustion appliance and not take full advantage of the sealed combustion chamber. Many furnace installers install sealed combustion furnaces, due to their higher efficiency, but only run the vent pipe to the outside as it is cheaper and easier. This means pressure differences in the home can still affect the combustion process, as the combustion air is still being pulled from inside the house.
Ultimately, the real solution is to eliminate combustion from our homes completely. There is no need to have open flame (fire) for space heating or making hot water. There are a number alternative solutions, one of the most prominent being heat pumps. Heat pumps can do the job of space heating or making hot water by moving heat from one place to another using refrigerants and electricity in place of combustion. This is not some newfangled miracle technology--in fact, it has been in use since the late 1800s. Heat pumps are everywhere: cars, homes, offices, stores. Some common examples are your refrigerator, your air conditioning system, and the air conditioning unit your car. The elimination of combustion appliances makes all-electric homes inherently safer than homes using fire (combustion) appliances.
There are many reasons to adopt heat pumps and all-electric homes as the path to the future. We should recognize that combustion appliances in our homes are fueled by natural gas, propane, or fuel oil, all of which are derived from fossil fuels. Electricity is often produced from fossil fuels as well, but it can be produced from a variety of carbon neutral sources, including wind, solar, geothermal, and more. This means that a switch to an all-electric future is a significant first step to reducing our reliance on fossil fuels. The home of tomorrow will likely be all electric, with a mix of heat pumps, solar panels, and storage. The storage could be in the form of batteries, or by using water heaters to store excess solar production. All of these technologies exist today. None of this is rocket science nor does it require some new miracle materials science revolution.
When we think about the costs of switching to an all-electric future, we need to make sure we remember that the current way of doing things has costs too. The combustion safety issues and required costs of CAZ testing is a big one that’s often overlooked. We’re spending millions of dollars today to ensure that homes are safe and that CAZ safety issues are not a factor once energy retrofits are completed. CAZ testing often produces inconsistent results and is difficult to repeat. This means we are putting a tremendous amount of time and money into CAZ testing to protect consumers, with questionable results.
On the other hand, if we were to remove the combustion appliances altogether, there would be no need for CAZ testing, and all of the cost and confusion over the results would simply go away. Homeowners would be safer, utilities would save money by not requiring combustion safety testing, and the planet would benefit from switching away from fossil fuels as our primary fuel. It’s time to embrace the future and stop relying on “fire” as the go to energy solution in our homes.
Efficiency First California
This blog originally appeared on www.efficiencyfirstca.org.
Image from iStock.