Hey there! If you’re a furnace tech, HVAC shop owner, or just someone who’s been stuck trying to figure out why their furnace’s draft inducer’s acting up, you know furnace accessories make or break how well the whole system runs. I’ve been selling furnace accessories for over 8 years now, and let me tell you—one question I get way more than any other is “How do I actually test these parts to make sure they don’t quit on me mid-job?” No more guessing games, let’s break this down like we talk in the shop—no stuffy engineering jargon, just real, usable steps I’ve tested myself a hundred times. Furnace Accessories
First off, let’s get one thing straight: testing furnace accessories isn’t just for when something’s broken. You should be doing pre-install checks too, right? I’ve had a brand-new draft motor come in from a batch that was dead out the box—stuff happens. So before you even slap it into a furnace, run through these quick tests. Let’s start with the most common accessories people mess up on: draft inducers, limit switches, flame sensors, and pressure switches. Those are the big four that cause 90% of furnace headaches, so we’ll focus there.
First up: draft inducer motors. These little guys pull exhaust fumes out of the heat exchanger, so if they’re not working right, you’re looking at carbon monoxide backup—super serious stuff. How do I test one? I don’t just plug it in and hope it spins. Here’s my go-to method. First, a visual check: look for any frayed wires, burnt spots on the housing, or bent fan blades. If the blades are all warped, even if the motor runs, it’ll be lopsided and create too much draft, which kills efficiency. Next, the voltage test—easy with a multimeter, everyone should have one of these. Turn the power off to the furnace first, duh, safety first. Disconnect the wires going to the inducer motor. Set your multimeter to AC voltage (most residential furnaces use 120V or 240V, but always check the part label). Touch the multimeter probes to the motor’s wire terminals, then turn the power back on just long enough to read the voltage. If it’s not matching what’s listed on the motor (say it says 120V and you get 100V), the motor’s not getting enough power, and that’ll cause it to underperform. Then, continuity test to check if the windings are shot. Switch your multimeter to continuity mode (beeps if there’s a closed circuit). Touch the probes to the motor’s two terminals—if it doesn’t beep, the windings are burned out, toss that motor. Last, spin the fan blade by hand—if it’s stiff or grinds, the bearings are bad, even if it runs. I’ve sent so many return requests for motors that passed the voltage test but had bad bearings—this quick spin test catches that.
Next: flame sensors. These little rods are what tell the furnace “hey, there’s fire, keep the gas on”—if they’re dirty or broken, the furnace will shut off mid-cycle, leaving your customer’s house cold. Testing a flame sensor is simple, no fancy tools needed. First, pull the sensor out (it’s usually a ¼” hex head bolt on the burner assembly). Look at the end—if there’s a thick crust of black gunk on it, that’s the issue. Wait, but don’t just clean it and call it good—test it first. Here’s how: after pulling it, turn the furnace’s power on, hold the sensor close to the burner (where it would pick up flame), and use a multimeter set to microamps. Touch one probe to the sensor, the other to a ground on the furnace frame (like a bolt). For a working sensor, you should get a reading between 3 and 10 microamps. If it’s zero, the sensor is dead, no signal getting sent. If it’s less than 3, it’s dirty—sand the end with fine steel wool (never use sandpaper, it leaves residue) and test again. I always tell guys: if the reading’s still low after cleaning, replace it—don’t waste time. I’ve seen a tech scrub a sensor three times when he just needed a new one, and he lost a service call because it took too long.
Limit switches—these are the safety thing that turns the blower off if the furnace gets too hot, so they’re non-negotiable. If a limit switch fails, it can cause overheating, which is a fire hazard. How to test these? Again, safety first: turn power off. Limit switches are usually near the heat exchanger, with two wires. First, test continuity when it’s cold. Set your multimeter to continuity mode, touch probes to the two switch terminals. It should beep—if it doesn’t, the switch is already open (bad). Then, to test if it trips at the right temp: the limit switch has a little lever you can actuate. Gently push the lever to trip it—if after pushing it, the multimeter stops beeping, that means it works (it should open when tripped). Wait, but how do you know it trips at the right temp? If you’ve got a heat gun, warm the switch up (don’t melt the plastic!)—when it reaches its rated temp (check the label, usually 150-200°F), it should trip and stop continuity. If it doesn’t trip when it’s supposed to, that’s a bad switch—you can’t adjust these, they’re preset. I’ve had customers try to “tweak” a limit switch before, which is a terrible idea—just replace it.
Pressure switches—oh man, these ones get so many calls about. They make sure the draft inducer is working before the furnace starts, so if the exhaust isn’t pulling right, the switch stays open and the furnace won’t light. Testing a pressure switch is a little trickier, but I do it all the time. First, locate the two tubes connected to the pressure switch—one is a “static” tube, one is a “draft” tube. Make sure those tubes aren’t clogged with dust or spider webs—spiders love to build nests in those tiny tubes, and that’s the number one cause of pressure switch issues. Clean the tubes out with a small brush (even a pipe cleaner works) first. Then, test continuity. With the furnace power off, disconnect the wires to the pressure switch, set multimeter to continuity. Touch probes to the switch’s terminals—what you get depends on the type. For a “normally closed” pressure switch (most common), it should beep when the inducer isn’t running. Now, run the inducer manually (you can jump the wires at the inducer to turn it on without the furnace) — when the draft is created, the switch should trip and stop beeping. If it doesn’t, the switch is bad, or the tubes are still clogged. Another quick test: if you have a spare pressure switch, swap it in and see if the furnace works. I keep a few spares of the most common pressure switch models on hand for exactly this—no need to overcomplicate it with fancy tools.
Wait, let’s not forget smaller accessories—like furnace filters, blower belts, and ignition transformers. Those are the ones people overlook, but they’re just as important. For filters: test fit. I’ve had filters that are labeled “1-inch” but are slightly too big and warp, or too small and leak dust. When you slide it into the slot, it should fit snug, no gaps around the edges—if there are gaps, it’s useless, get the right size. For blower belts: stretch test. Pull on the belt—if it stretches more than ½ inch when you push it, it’s too loose. If it’s cracked or frayed, replace it. I always tell guys: change the belt when you change the filter, that’s a easy combo to remember. For ignition transformers: test output voltage. Disconnect the wires, set multimeter to AC voltage, touch probes to the transformer’s secondary terminals (the ones that go to the igniter). It should match the label—usually 12,000V for spark igniters. If it’s way lower, the transformer is dead, and your igniter won’t spark.
Now, a few pro tips I’ve picked up over the years. First, always reference the part number. A lot of guys grab a “similar” accessory and it doesn’t fit, so testing won’t help if it’s the wrong part. I’ve had people come to me with a part they bought elsewhere, saying it’s broken, and it turns out it’s for a 80% efficiency furnace, not their 95%. Second, document your tests. I keep a little notebook for my own orders—if a batch of 10 draft motors had two bad ones, I note that so I can check the next shipment extra careful. Third, test accessories individually before installing them into a full furnace. If you test the limit switch and it’s bad before you mount it, you don’t have to take the whole furnace apart to fix it—save yourself the hassle.
Look, I get it, testing accessories can seem like a chore, but it’s way cheaper than dealing with a service call where a part failed mid-install, or worse, a carbon monoxide scare from a bad draft inducer. At the end of the day, a furnace system is only as good as its parts, so making sure each accessory works before you put it to use is just good business—for you and your customers.
If you’re tired of guessing if your furnace accessories are gonna work, or if you need reliable, tested parts that pass my own shop’s checks before they leave here, hit us up to talk about bulk orders or specific part needs. We’ve got all the accessories I mentioned—draft inducers, flame sensors, limit switches, pressure switches, and more—at prices that won’t break the bank. No hidden fees, no parts that die out the box, just stuff that works when you need it.
Oh, and one last thing: never skip safety. Always turn off power to the furnace before testing any electrical parts—you don’t want to get shocked. And if you’re not comfortable working with electrical components, call a pro, but if you are, these steps work every time.
Resistance Furnaces Reference
ASHRAE Handbook—HVAC Systems and Equipment, American Society of Heating, Refrigerating and Air-Conditioning Engineers
Furnace Installation and Service Manual, National Comfort Institute
HVAC Troubleshooting Guide, United Association of Journeymen and Apprentices of the Plumbing and Pipe Fitting Industry
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