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No testing is needed to find a bypassed furnace door switch. All you have to do is pay attention to the installation. Many technicians will spot the poor gas line connection in this illustration but overlook the bypassed door switch because they see it so often, or the previous technician did a great job hiding it.

Three Reasons Why Bypassing a Furnace Door Switch is a Bad Idea

May 15, 2023
Dangers are backdrafting, return grill bypass and electrocution.
NCI/David Richardson
No testing is needed to find a bypassed furnace door switch. All you have to do is pay attention to the installation. Many technicians will spot the poor gas line connection in this illustration but overlook the bypassed door switch because they see it so often, or the previous technician did a great job hiding it.
No testing is needed to find a bypassed furnace door switch. All you have to do is pay attention to the installation. Many technicians will spot the poor gas line connection in this illustration but overlook the bypassed door switch because they see it so often, or the previous technician did a great job hiding it.

Summer maintenance season is here, and everyone is ready to forget about heating equipment. Unfortunately, one of the “repairs” you might have made this winter could come back to haunt you. 

If you’re an HVAC technician who works on gas furnaces, you’ve probably encountered a bad furnace door switch. Most of us would admit we had to bypass a few to regain the heat. But is this a wise choice? What could happen if you leave the furnace in this condition? Let’s look at three reasons bypassing a furnace door switch is a bad idea.

Purpose and Function of a Furnace Door Switch

What is the purpose of a furnace door switch? If you pay attention to the labels on a gas furnace blower panel, you’ll notice one that says, “Danger!” with a skull and crossbones. This warning label alerts you to the potential hazards of carbon monoxide (CO) poisoning. 

In the last sentence on this label, there’s an important statement. It reads, “Carbon monoxide emissions can be (re)circulated throughout the structure if the furnace or air handler is operating in any mode.” So, what does that have to do with the furnace door switch? 

The door switch prevents the furnace blower from circulating CO (or anything else near the furnace) in case the blower panel is off. It turns the power off to the furnace controls because we sometimes forget to put the panel back on, or it falls off during operation. However, if you bypass the door switch, the blower operation continues. That situation could trigger a deadly chain of events. 

Three critical issues from a failed or bypassed furnace door switch include:

  • Backdrafting
  • Return Grill Bypass
  • Electrocution.

Let’s investigate more on each of these issues.

Backdrafting

One main issue of having a bypassed door switch is depressurization. It doesn’t take much airflow to change the pressure in a confined space (utility room, closet, mechanical room). When the blower panel is off and the blower continues operating, the furnace pulls return air from the confined space. Remember, air takes the path of least resistance. 

For example, let’s say you have an 80% gas furnace that is common vented with a natural draft water heater. Both are in a mechanical room. A previous technician bypassed the furnace door switch, and the blower door panel subsequently fell off. If the blower circulates 900 cfm (cubic feet per minute) of airflow, it will pull most of the air from the mechanical room. 

What do you think will happen to the water heater flue gases (located in the same room) when the blower turns on? If you said backdraft, you’re right. Because of the negative pressure in the room, flue gases reverse direction and spill from the drafthood until the blower turns off. What’s scary is this can happen in either heating or cooling mode.

Return Grille Bypass (or Worse)

What if the furnace is in a crawlspace or attic? These areas could have additional fuel-fired equipment in them and could have the same problems. It’s easy to assume these areas are “outdoors.” However, testing reveals they’re usually more connected to the inside of a building than we imagine. 

Unfiltered air that enters the duct system is another issue to consider. If the blower door falls off and the door switch fails, 100% of the system’s return air pulls from the attic or crawlspace. There’s some nasty air no one should breathe found in attics and crawlspaces. 

Up to this point, I haven’t mentioned garage installations. These are especially concerning because of auto exhaust and gasoline fumes. If the concentrations are high enough, auto exhaust has excessive CO levels, and gasoline fumes are flammable. Please pay special attention to garage locations.

Electrocution (or Fewer Fingers)

Unfortunately, some air filters are inside the gas furnace. I know it’s a poor location and shouldn’t happen, but it does. If you bypass the door switch, all electrical components stay energized with the blower door panel off.

For safety’s sake, the door switch turns off the power if your customer decides to get into the equipment. An unsuspecting homeowner could get injured when they change their air filter. Electrocution or the blower turning on are serious risks with a bypassed door switch. 

Weigh the Consequences

Best practices call for replacing a faulty door switch. This is the right way to do it. Unfortunately, it rarely happens. I hate to say that, but many technicians just don’t do it. The most common excuses are that there are many switch types, which would mean a second trip to the job site. Regardless of how you rationalize it, there’s no excuse for potentially putting someone in danger. 

If you bypass a switch and have no intention of replacing it, you are responsible if something happens to the homeowner because the blower and furnace continue to run. You must take every step to ensure the blower door won’t accidentally come off, even if a bus hits the furnace. Don’t forget that there may also be a newer, less experienced technician who services the furnace after you. Is it worth endangering them? 

In the end, weigh the consequences of your actions. If you bypass the door switch, can you assure your homeowner’s safety if the blower door panel falls off? Did you unintentionally compromise their well-being because of shortcuts you took to get the heat back on? Your answer should guide your next steps. That second trip doesn’t sound so bad now, does it?

David Richardson serves the HVAC industry as Director of Training for the National Comfort Institute, Inc. (NCI). NCI specializes in training to improve, measure, and verify HVAC and Building Performance. If you’re an HVAC contractor or technician interested in learning more about combustion testing, contact David at ncilink.com/ContactMe. NCI’s website, www.nationalcomfortinstitute.com, is full of free information to help you improve your professionalism and strengthen your company.
About the Author

David Richardson | Director, technical curriculum

David Richardson serves the HVAC industry as director of technical curriculum at National Comfort Institute, Inc. (NCI), Avon, Ohio. NCI specializes in training that focuses on improving, measuring, and verifying HVAC and Building Performance.