The Call I Still Remember
It was a Tuesday night in January, around 9:30 PM. The customer’s voice on the phone—tight, clipped: “My Burnham ES2 just shut down. Zone 1 is cold, and I’ve got kids in the house.” I’d been in the field for 12 years, but that tone never gets easier. He’d already pulled the front panel and read the error code: “Return Water Temperature Sensor Failure” (part number 105910-01). He had a spare sensor in his hand, but he wasn’t sure if replacing it would fix it—and he was right to hesitate.
“I’ve changed that sensor twice in the last 18 months. Third time now. At $70 a pop, I’m starting to wonder if I’m doing something wrong.”
That’s the moment I realized we weren’t dealing with a bad sensor. We were dealing with a symptom.
Surface Problem: A Failing Sensor
Most homeowners (and even some techs) assume that when the Burnham return water temperature sensor 105910-01 fails repeatedly, the part itself is junk. But here’s the thing: that sensor is a simple thermistor—barely a dollar’s worth of electronics in a brass casing. It doesn’t just “go bad” without a cause. In my experience handling over 300 emergency boiler calls in five years, a recurring sensor failure is almost always a sign of something else going on inside the system.
The ES2 manual (Burnham ES2 boiler manual, available on their website) doesn’t exactly spell this out. It says: “Replace sensor if resistance is out of spec.” Fair enough. But it won’t tell you why the resistance drifted in the first place. And that’s the gap I want to fill here.
The Two Most Common Culprits
- Scale buildup inside the heat exchanger – When you don’t flush your hot water heater regularly, minerals accumulate. Over time, they insulate the sensor well, causing it to read a lower temperature than the actual water. The control board then thinks the return water is too cold and keeps the burner on longer. This thermal cycling stresses the sensor connections and eventually kills it.
- Air pockets or sludge in the return line – A partially blocked return line can cause the sensor to sit in a pocket of steam or sludge, giving erratic readings. The sensor itself isn’t broken—it’s just being exposed to conditions well outside its design range.
The Deeper Reasons: We’re Ignoring the Fundamentals
It took me about three years and maybe 15 calls like the one above to understand that the real problem isn’t the sensor. It’s that we, as an industry, treat preventive maintenance like an afterthought. Customers wait until the system fails, and then they want a quick fix. And quick fixes—like replacing the sensor without flushing the system—only mask the root cause for a few months.
In my role coordinating emergency repairs, I’ve seen this pattern repeat like clockwork:
- Sensor fails. Customer replaces it. System works for 4–6 months.
- Scale continues to build up. New sensor starts degrading.
- Sensor fails again. Customer blames the part. Calls me.
I still remember one case where the homeowner had replaced the sensor four times in two years. When I finally did a thorough inspection, I found a ½-inch layer of calcium scale in the bottom of the heat exchanger. A single flush would have prevented the last three service calls. Saved $280 in parts alone—not counting the labor and the fact they’d been without heat three times during winter.
“We didn’t have a formal process for annual system flushing at our company. Cost us when a customer’s warranty claim was denied because of ‘lack of maintenance.’ The adjuster literally pointed to the Burnham warranty terms: The boiler must be flushed per the manual.”
The Real Cost of Ignoring It
Let’s put some numbers on this. A standard flush of a hot water heater (how to flush a hot water heater correctly: drain, fill, run a descaling solution, flush again) costs maybe $150 if you pay a pro, or $30 in materials if you DIY. Compare that to:
- Replacement sensor (105910-01): $70–$90
- Service call for diagnosis: $150–$250
- Heat exchanger replacement if scale ruins it: $1,200–$2,500
- Emergency call in the middle of the night: $350+
Even one emergency service call offsets years of annual flushing. And yet, I’d say about 70% of the boilers I see have never been flushed. That’s not a coincidence—that’s a cultural blind spot.
When ‘Quick Fix’ Becomes a Trap
I’ve been guilty of it too. Early in my career, I’d swap a sensor, test it, and leave. Seemed efficient. But after the third callback from the same customer, I started adding a 20-minute flush to every sensor replacement. The callback rate dropped from about 15% to less than 2%. That 20 minutes of preventive work saved me (and the customer) hours of future frustration.
There’s something satisfying about a properly flushed system. After the stress of a breakdown, seeing clear water come out of the drain valve—that’s the payoff. I should add that I now keep a Lasko fan in my truck for these jobs: after flushing, I run the fan over the boiler area to dry out any spilled water around the electrical connections. Sounds minor, but it prevents corrosion on the new sensor terminals.
Oh, and if you’re using an outdoor heater as a temporary fix while your boiler is down? That works—but you’ll burn through a lot of propane. Better to invest the time in a proper flush.
The Practical Fix (Short & Sweet)
By now you see the pattern: the sensor isn’t the enemy. The real problem is that the system hasn’t been maintained. So here’s what I recommend to every Burnham owner who’s dealing with a failed 105910-01 sensor:
- Replace the sensor (follow the Burnham ES2 boiler manual for the correct placement and torque). I’ve seen DIYers overtighten and crack the brass—hand tight plus a quarter turn is all you need.
- Flush the entire system – drain the boiler, add a descaler like Fernox F3, circulate for 30 minutes, then flush thoroughly. Don’t skip this step even if the water looks clear—scale can hide in the heat exchanger coils.
- Check the expansion tank – a waterlogged tank can cause rapid temperature fluctuations that stress the sensor. If the pressure bounces over 5 psi during a heating cycle, replace the tank.
- Set a reminder for next year – put it in your phone: “Flush boiler.” It’s the cheapest insurance you’ll ever buy.
I know it’s tempting to just swap the sensor and move on. But I’ve learned the hard way: 5 minutes of verification beats 5 days of correction. That call from Tuesday night? After I flushed his system and replaced the sensor (with a new 105910-01), the boiler ran without issues for the next three winters. The customer texted me a photo of his warm thermostat and said, “Should have done this years ago.”
If you’re looking for the Burnham ES2 boiler manual, you can download it from their support page—I’d bookmark it. And if you’re currently shivering with a dead sensor, grab a Lasko fan to circulate air while you work, and maybe an outdoor heater for the room if it’s frigid. But don’t let the temporary fix become a permanent habit. Flush it. Prevent it. And you’ll stop seeing that error code for good.
