Considerations When Adding Exhausting Equipment to an Existing Home
I was recently contacted by a friend who wanted to add a kitchen range exhaust hood to their 100-year-old home. They were looking for insight on how the fan might be added, where the ducts could be located, sizing of the system, and any other concerns that might need to be considered. One of my first questions was what equipment was presently used for space heating and water heating. A critical question that needs to be considered before adding any type of exhaust system. Why? Health and safety of the occupants.
As it turns out, this home had both an atmospherically vented, natural gas boiler and water heater. This type of equipment has the potential to “spill” or backdraft combustion exhaust into the home should the home become negatively pressurized. How do we know if the equipment is drafting correctly? What happens if a new exhaust fan is added? How do we know if there is carbon monoxide or combustible gas present? The answer to all these questions, we test!
The testing includes combustion appliance zone (CAZ) testing, monitoring ambient and flue CO levels, and a blower door test. The process of performing both CAZ and blower door testing is outlined by both Building Performance Institute (BPI) and the Residential Energy Network (RESNET). This article will follow the standards of ANSI/BPI 1200, Standard Practices for Basic Analysis of Buildings, the RESNET standards are similar.
We begin the process by calibrating monitoring and testing equipment in fresh air, outdoors. The four pieces of equipment that require fresh air calibration are wearable CO monitors, explosive gas monitors (lower explosive limit or LEL monitors), combustible gas leak detector, and a combustion gas analyzer. (My wearable CO monitor checks for both CO and lower explosive limits). For my initial trip into the home, the CO and LEL detector is clipped to my clothing.
Beginning the Investigation and Safety Check
I start the investigation by having a conversation with the homeowner and performing a visual inspection of all interior spaces. As I move around the home, I occasionally look at my CO and LEL monitors to make sure I’m not registering any low-level CO or combustible gases. The monitor will alert me through an audible alarm should levels become unsafe, but lower levels of either are unwanted in indoor air. I then move the investigation outside, to inspect exterior conditions, taking note of any gas supply equipment, either natural gas or propane (fuel oil could also be present, but my market is seeing less and less of that fuel source).
Once I’ve established the condition of the home, its equipment, and made sure the environment is safe for both the occupants and me, I will begin CAZ testing.
CAZ Testing
CAZ, or Combustion Appliance Zone testing begins with the identification of all CAZ zones within the home. A combustion appliance zone is defined as: “The room and enclosed air volume that contains a combustion appliance. This may include, but is not limited to, a mechanical room, mechanical closet, or main body of the house”. In the case of the home that I am performing testing, the CAZ is the entire basement area. Both the natural gas boiler and water heater are located in the basement, which is an open space. The space’s boundary extends to a door at the top of the stairs connecting the main level to the basement. There is also a gas stove in the kitchen, a secondary CAZ location. Sometimes the space heating and water heating equipment is found in separate areas within the home, resulting in multiple CAZ locations.
Once I’ve established the CAZ zones, I move onto leak testing the gas lines that supply gas to every appliance in the home. For this I use the combustible gas leak detector which was previously calibrated or “zeroed” in outdoor ambient air. I move the wand of the detector along all gas piping at a rate of 1-inch per second, checking completely around all joints in the piping. Because natural gas is lighter than air, we run the detector along the topside of the fuel line when that fuel source is present. Propane is heavier than air, so we test below the fuel line when that fuel source is present. If a leak is found, soapy water is used to verify the leak. The leak is marked or tagged, and a recommendation is given to the homeowner suggesting a repair be completed. There was not a gas leak detected at this home.
The next step is to inspect the combustion venting equipment for proper installation and operation. We are looking for proper vent piping material, correct vent size, correct slope (1/4” per foot upwards from the appliance), and any other hazards that may affect proper combustion. No issues with venting were present in this home.
We determined the combustion appliance zone earlier, now we need to determine if there is adequate combustion air available for the appliances in the CAZ. For this, we need to add up the BTU rating of each appliance within the CAZ, then calculate the volume of the CAZ. The formula for proper combustion air is 50 cubic feet per 1,000 BTU of listed appliance input. If we find there is not enough air for combustion, we recommend installing a combustion air duct to the outside or connecting the CAZ to additional volume inside the home.
After that work is completed, we can then move onto testing the appliances for spillage. First, we disable all combustion appliances and turn off all exhaust fans, forced air fans and other ventilation equipment. There can be no active fires in any wood burning appliances, all ashes should be removed or covered so as to not be dispersed into the home during either worst-case depressurization testing or the blower door test. Close the woodburning appliance damper and any doors for the appliance.
All windows and exterior doors are closed and locked. Any room that has an exhaust fan or return air duct from a forced air furnace, the door to that room is open. All other interior doors are closed. Any door leading to the combustion appliance zone is also closed.
Measuring worst case depressurization is conducted using a manometer. The manometer used for blower door testing works, but I often use a single channel manometer for this purpose, either Minneapolis Blower Door’s DG-8 or Retrotec’s Solo works well. The unit needs to be baselined with reference to the outside, so we have to find a location where a tube can get to the outdoors. Sometimes finding a hole to the outside is difficult, I’ve fished the tubing alongside a dryer vent or placed it alongside a loose-fitting door or window. Once the tube is outside and connected to the manometer, we can baseline or zero the difference between inside and out. The newer two channel manometers used for blower door testing have built-in baseline features, if using the single channel models, the baseline will need to be visually determined. Once the baseline is accounted for, we can now proceed with the worst-case depressurization.
We begin by starting all exhaust equipment, including the dryer, (be sure to clean the lint filter before starting) and record the value shown on the manometer. Next, we open the door leading into the CAZ and record the new value. A third check is done by starting the fan on the forced air furnace (if there is one) and recording that reading with the door to the CAZ both open and closed. This gives us up to four different options for setting up the home for worst case depressurization. Whatever condition produces the most depressurization within the home is how we set the home for the spillage test. This home measured a -3.3 Pascals during worst case depressurization.
Testing for spillage starts with the smallest BTU rated appliance within the CAZ, often this is the water heater. The water heater is started, and spillage is checked after 2 minutes of operation (considered a “warm vent” appliance). If the appliance is a cold vent, such as a furnace or boiler tested during the summer months, the test happens after 5 minutes of operation. Spillage is determined by the use of either a smoke generator or a mirror (I prefer to use smoke over a mirror). If smoke does not get sucked out the exhaust flue and instead moves into the CAZ space, we have spillage. A mirror will fog up with moisture should the appliance spill.
If spillage occurs, there is a procedure for determining if the home can be set up in a way to eliminate the hazard. The procedure is outlined in the BPI document titled “Combustion Appliance Safety Inspection for Vented Appliances”. Often an HVAC professional is called in to correct the condition. The home I was working on did not spill.
Next up we test the appliance(s) for proper combustion using a combustion analyzer. This is a tool that HVAC professionals and some energy auditors and people performing building investigations will have. It measures the conditions of the flue gas to determine if the appliance is operating correctly. Complete combustion will contain carbon dioxide, water vapor, and possibly a small amount of carbon monoxide. Incomplete combustion will contain higher levels of carbon monoxide.
A combustion analyzer can measure both ambient CO, sometimes called “as measured”, and what is called “air free” or “excess CO”, which is a calculation involving carbon monoxide and oxygen or carbon dioxide readings. The combustion analyzer tests air free CO number, which needs to be under a specific level depending on the appliance, for instance, a natural gas or propane water heater must have air free CO readings of under 200 ppm. This home did not have any issues with excessive CO in the exhaust gases.
We’ve determined that the atmospherically vented gas appliances are drafting and operating correctly within the home under the conditions present. There were two exhausting appliances, a bath fan and dryer that produced the worst-case depressurization. What will happen when another exhaust appliance is installed? To determine this, we need to perform a blower door test. I’m not going to get into how to perform a blower door test in this post, there are several articles on this blog discussing blower door testing, my favorite is: Why You Need Blower Door Testing – Northern Built
What I am going to discuss is the test results for this particular home. The home is fairly leaky, 7.68 ACH50, 1856 CFM50. I perform most of my blower door tests using automation software, the test result shown in the graph below is called a multi-point test. The test collects 100 samples at each specific pressure, starting at 50 Pascals and ending at 15 Pascals. These results are graphed, the graph is called a building leakage curve. This curve estimates the leakage rate at various house pressures. So, if I have a total exhaust rate of the existing equipment plus the new fan for our tested home of 350 cfm, the house pressure will be approximately -4 Pascals. (We can determine the exhaust rates by the nameplate rating or by testing the exhaust fan using a flow hood.) The lower depressurization found in this home should not affect the draft of the atmospherically vented appliances, but we are close, so I will be back to verify there isn’t a problem after the new exhausting equipment has been installed.
Draft through the exhaust vent is affected by the chimney or flue height, the pitch of a vent connecting the appliance to a chimney, the number of elbows in the exhaust vent, and the temperature of the flue (chimney located outside the building envelope often have colder flues which can affect draft). The pressure limit for most homes with atmospherically vented equipment will be 5 Pascals for gas appliances and 4 Pascals for oil fired appliances.
Had this home been tighter, we may have experienced a higher negative pressure during the worst-case depressurization testing, which probably would have led to the exhaust spilling back into the home. Had that been the case, we may have planned for a combustion air supply duct to be installed within the combustion appliance zone at the same time as the kitchen exhaust range hood install. Other options would be to replace the atmospherically vented appliances with sealed combustion or electric appliances.
In my opinion, not a lot of contractors are thinking about the effects a renovation can have on combustion appliances. Adding a mechanical ventilation system, and even window or siding replacement can tighten up a home enough where the risk of carbon monoxide poisoning becomes a danger. We only know if we test, or if the residents are injured or killed by CO, I can guarantee that testing is less costly. BPI and RESNET are good places to find certified professionals, and we can always use more people trained in combustion appliance safety testing.
This post first appeared on the Green Building Advisor website.
As it turns out, this home had both an atmospherically vented, natural gas boiler and water heater. This type of equipment has the potential to “spill” or backdraft combustion exhaust into the home should the home become negatively pressurized. How do we know if the equipment is drafting correctly? What happens if a new exhaust fan is added? How do we know if there is carbon monoxide or combustible gas present? The answer to all these questions, we test!
The testing includes combustion appliance zone (CAZ) testing, monitoring ambient and flue CO levels, and a blower door test. The process of performing both CAZ and blower door testing is outlined by both Building Performance Institute (BPI) and the Residential Energy Network (RESNET). This article will follow the standards of ANSI/BPI 1200, Standard Practices for Basic Analysis of Buildings, the RESNET standards are similar.
We begin the process by calibrating monitoring and testing equipment in fresh air, outdoors. The four pieces of equipment that require fresh air calibration are wearable CO monitors, explosive gas monitors (lower explosive limit or LEL monitors), combustible gas leak detector, and a combustion gas analyzer. (My wearable CO monitor checks for both CO and lower explosive limits). For my initial trip into the home, the CO and LEL detector is clipped to my clothing.
Beginning the Investigation and Safety Check
I start the investigation by having a conversation with the homeowner and performing a visual inspection of all interior spaces. As I move around the home, I occasionally look at my CO and LEL monitors to make sure I’m not registering any low-level CO or combustible gases. The monitor will alert me through an audible alarm should levels become unsafe, but lower levels of either are unwanted in indoor air. I then move the investigation outside, to inspect exterior conditions, taking note of any gas supply equipment, either natural gas or propane (fuel oil could also be present, but my market is seeing less and less of that fuel source).
Once I’ve established the condition of the home, its equipment, and made sure the environment is safe for both the occupants and me, I will begin CAZ testing.
CAZ Testing
CAZ, or Combustion Appliance Zone testing begins with the identification of all CAZ zones within the home. A combustion appliance zone is defined as: “The room and enclosed air volume that contains a combustion appliance. This may include, but is not limited to, a mechanical room, mechanical closet, or main body of the house”. In the case of the home that I am performing testing, the CAZ is the entire basement area. Both the natural gas boiler and water heater are located in the basement, which is an open space. The space’s boundary extends to a door at the top of the stairs connecting the main level to the basement. There is also a gas stove in the kitchen, a secondary CAZ location. Sometimes the space heating and water heating equipment is found in separate areas within the home, resulting in multiple CAZ locations.
Once I’ve established the CAZ zones, I move onto leak testing the gas lines that supply gas to every appliance in the home. For this I use the combustible gas leak detector which was previously calibrated or “zeroed” in outdoor ambient air. I move the wand of the detector along all gas piping at a rate of 1-inch per second, checking completely around all joints in the piping. Because natural gas is lighter than air, we run the detector along the topside of the fuel line when that fuel source is present. Propane is heavier than air, so we test below the fuel line when that fuel source is present. If a leak is found, soapy water is used to verify the leak. The leak is marked or tagged, and a recommendation is given to the homeowner suggesting a repair be completed. There was not a gas leak detected at this home.
The next step is to inspect the combustion venting equipment for proper installation and operation. We are looking for proper vent piping material, correct vent size, correct slope (1/4” per foot upwards from the appliance), and any other hazards that may affect proper combustion. No issues with venting were present in this home.
We determined the combustion appliance zone earlier, now we need to determine if there is adequate combustion air available for the appliances in the CAZ. For this, we need to add up the BTU rating of each appliance within the CAZ, then calculate the volume of the CAZ. The formula for proper combustion air is 50 cubic feet per 1,000 BTU of listed appliance input. If we find there is not enough air for combustion, we recommend installing a combustion air duct to the outside or connecting the CAZ to additional volume inside the home.
After that work is completed, we can then move onto testing the appliances for spillage. First, we disable all combustion appliances and turn off all exhaust fans, forced air fans and other ventilation equipment. There can be no active fires in any wood burning appliances, all ashes should be removed or covered so as to not be dispersed into the home during either worst-case depressurization testing or the blower door test. Close the woodburning appliance damper and any doors for the appliance.
All windows and exterior doors are closed and locked. Any room that has an exhaust fan or return air duct from a forced air furnace, the door to that room is open. All other interior doors are closed. Any door leading to the combustion appliance zone is also closed.
Measuring worst case depressurization is conducted using a manometer. The manometer used for blower door testing works, but I often use a single channel manometer for this purpose, either Minneapolis Blower Door’s DG-8 or Retrotec’s Solo works well. The unit needs to be baselined with reference to the outside, so we have to find a location where a tube can get to the outdoors. Sometimes finding a hole to the outside is difficult, I’ve fished the tubing alongside a dryer vent or placed it alongside a loose-fitting door or window. Once the tube is outside and connected to the manometer, we can baseline or zero the difference between inside and out. The newer two channel manometers used for blower door testing have built-in baseline features, if using the single channel models, the baseline will need to be visually determined. Once the baseline is accounted for, we can now proceed with the worst-case depressurization.
We begin by starting all exhaust equipment, including the dryer, (be sure to clean the lint filter before starting) and record the value shown on the manometer. Next, we open the door leading into the CAZ and record the new value. A third check is done by starting the fan on the forced air furnace (if there is one) and recording that reading with the door to the CAZ both open and closed. This gives us up to four different options for setting up the home for worst case depressurization. Whatever condition produces the most depressurization within the home is how we set the home for the spillage test. This home measured a -3.3 Pascals during worst case depressurization.
Testing for spillage starts with the smallest BTU rated appliance within the CAZ, often this is the water heater. The water heater is started, and spillage is checked after 2 minutes of operation (considered a “warm vent” appliance). If the appliance is a cold vent, such as a furnace or boiler tested during the summer months, the test happens after 5 minutes of operation. Spillage is determined by the use of either a smoke generator or a mirror (I prefer to use smoke over a mirror). If smoke does not get sucked out the exhaust flue and instead moves into the CAZ space, we have spillage. A mirror will fog up with moisture should the appliance spill.
If spillage occurs, there is a procedure for determining if the home can be set up in a way to eliminate the hazard. The procedure is outlined in the BPI document titled “Combustion Appliance Safety Inspection for Vented Appliances”. Often an HVAC professional is called in to correct the condition. The home I was working on did not spill.
Next up we test the appliance(s) for proper combustion using a combustion analyzer. This is a tool that HVAC professionals and some energy auditors and people performing building investigations will have. It measures the conditions of the flue gas to determine if the appliance is operating correctly. Complete combustion will contain carbon dioxide, water vapor, and possibly a small amount of carbon monoxide. Incomplete combustion will contain higher levels of carbon monoxide.
A combustion analyzer can measure both ambient CO, sometimes called “as measured”, and what is called “air free” or “excess CO”, which is a calculation involving carbon monoxide and oxygen or carbon dioxide readings. The combustion analyzer tests air free CO number, which needs to be under a specific level depending on the appliance, for instance, a natural gas or propane water heater must have air free CO readings of under 200 ppm. This home did not have any issues with excessive CO in the exhaust gases.
We’ve determined that the atmospherically vented gas appliances are drafting and operating correctly within the home under the conditions present. There were two exhausting appliances, a bath fan and dryer that produced the worst-case depressurization. What will happen when another exhaust appliance is installed? To determine this, we need to perform a blower door test. I’m not going to get into how to perform a blower door test in this post, there are several articles on this blog discussing blower door testing, my favorite is: Why You Need Blower Door Testing – Northern Built
What I am going to discuss is the test results for this particular home. The home is fairly leaky, 7.68 ACH50, 1856 CFM50. I perform most of my blower door tests using automation software, the test result shown in the graph below is called a multi-point test. The test collects 100 samples at each specific pressure, starting at 50 Pascals and ending at 15 Pascals. These results are graphed, the graph is called a building leakage curve. This curve estimates the leakage rate at various house pressures. So, if I have a total exhaust rate of the existing equipment plus the new fan for our tested home of 350 cfm, the house pressure will be approximately -4 Pascals. (We can determine the exhaust rates by the nameplate rating or by testing the exhaust fan using a flow hood.) The lower depressurization found in this home should not affect the draft of the atmospherically vented appliances, but we are close, so I will be back to verify there isn’t a problem after the new exhausting equipment has been installed.
Draft through the exhaust vent is affected by the chimney or flue height, the pitch of a vent connecting the appliance to a chimney, the number of elbows in the exhaust vent, and the temperature of the flue (chimney located outside the building envelope often have colder flues which can affect draft). The pressure limit for most homes with atmospherically vented equipment will be 5 Pascals for gas appliances and 4 Pascals for oil fired appliances.
Had this home been tighter, we may have experienced a higher negative pressure during the worst-case depressurization testing, which probably would have led to the exhaust spilling back into the home. Had that been the case, we may have planned for a combustion air supply duct to be installed within the combustion appliance zone at the same time as the kitchen exhaust range hood install. Other options would be to replace the atmospherically vented appliances with sealed combustion or electric appliances.
In my opinion, not a lot of contractors are thinking about the effects a renovation can have on combustion appliances. Adding a mechanical ventilation system, and even window or siding replacement can tighten up a home enough where the risk of carbon monoxide poisoning becomes a danger. We only know if we test, or if the residents are injured or killed by CO, I can guarantee that testing is less costly. BPI and RESNET are good places to find certified professionals, and we can always use more people trained in combustion appliance safety testing.
This post first appeared on the Green Building Advisor website.