When it comes to tankless water heater repair, few calls are more telling than a recurring overheating problem that's been quietly destroying a unit from the inside out. We recently visited a home on Lakeshore Drive North in Westford, MA, for what appeared to be a straightforward Navien water heater service call — and what we uncovered was a textbook example of how one small venting mistake can cascade into thousands of dollars in damage over time. If you're searching for tankless water heater repair near me or trying to understand why your Navien keeps acting up, this post is for you.
What We Found: Flame Rollout, a Cooked Heat Exchanger, and a Burnt Cover
The homeowner first noticed something was wrong about two years ago when he spotted a burn mark on the front cover of his Navien unit. He touched it, realized it was scorching hot, removed the cover, and watched the flame shoot out sideways — what's known in the trade as flame rollout. Not knowing this was a serious warning sign, he ordered a replacement cover, installed it himself, and hoped for the best.
Unfortunately, the root cause was never addressed. By the time our technician -site, the heat exchanger was completely failed — what we'd describe as "cooked." The surrounding seal was also leaking, the unit had been running outside of safe operating parameters, and the expansion tank was severely undercharged at just 20 psi when it should have been set at 40 to 50 psi or higher to match the incoming line pressure.
This isn't a rare situation. Our technician or five Navien heat exchanger replacements every single month. It's one of the most common water heater repair calls we run in the Westford and Chelmsford areas.
The Real Culprit: Improper Venting Pipe Size
Here's where the diagnosis got interesting — and where the real lesson for homeowners lies. Outside the house, our technician unusual: a four-inch vent pipe and a three-inch vent pipe with significant separation between them, roughly 15 feet apart. That's not a typical installation setup, so we needed to trace the pipes to understand what was going on.
What we found was that the air intake had been upsized to four-inch pipe. That might sound like a "more is better" improvement, but with a sealed combustion tankless water heater like a Navien, bigger is not better when it comes to venting. Here's why:
Navien tankless units are designed for three-inch venting under standard single-unit installations.
Four-inch venting is only appropriate in a "cascade" system — two or more units sharing a common vent.
The intake and exhaust must maintain equal pressure to allow proper combustion. Upsizing one side throws that balance off.
Additionally, having the intake on one side of the house and the exhaust 15 feet away on another side means the two openings are exposed to different wind pressures — further disrupting that critical pressure balance.
Our recommendation was clear: convert the intake back to three-inch pipe to match the exhaust side, equalizing the pressure and volume of air moving in and out of the unit. Combined with a heat exchanger replacement, this should resolve the recurring overheating issue for good.
Understanding Expansion Tanks and Why Yours Might Be the Problem
While diagnosing the Navien, our technician the expansion tank — and found it severely undercharged. The pre-charge pressure was sitting at just 20 psi. In a system where incoming water pressure was around 55 psi, this means the expansion tank was already maxed out and completely unable to absorb thermal expansion from the hot water system.
Here's a quick breakdown of how this works:
When water heats up, it expands. That expanded volume needs somewhere to go.
The expansion tank — a small tank with an air bladder inside — absorbs that expansion.
If the pre-charge air pressure in the tank is lower than your incoming water pressure, the bladder is already compressed and can't absorb anything.
That extra pressure has to go somewhere, and it puts stress on your water heater and plumbing components.
The fix is straightforward: use a bike pump or air compressor to bring the expansion tank's air pressure up to match your incoming line pressure, or simply replace the tank if it's waterlogged (which you can check by tapping the tank — it should sound hollow, not dense like it has water in it). In this case, our technician the expansion tank entirely and setting it to the correct pressure for this system.
Homeowners in Groton, Littleton, and surrounding towns with older plumbing systems or high water pressure should have their expansion tanks checked regularly — this is a simple part of gas hot water heater maintenance that often gets overlooked until something fails.
The Warranty Play: Why Checking Always Pays Off
One of the most valuable things our technician this call was immediately look for a way to get the heat exchanger covered under Navien's warranty. The Navien heat exchanger carries a ten-year warranty, and the unit appeared to be right around that threshold based on visible installation clues.
Navien's warranty process works through their online claim portal — you enter the unit's serial number, and if the unit was registered with an install date within the warranty window, the part can be covered. In this case, our technician submit the claim the same day, with the goal of getting the replacement heat exchanger shipped next-day so it could arrive by mid-week.
The part alone, if not covered under warranty, runs over $1,000. That's a significant difference, and it's exactly why we always check warranty eligibility before assuming a homeowner is on the hook for the full repair cost. As part of our water heaters services, we make this part of our standard diagnostic process — not an afterthought.
Condensate and Corrosion: Don't Use Brass Fittings on Your Furnace or Tankless Venting
One more detail from this visit worth highlighting: the homeowner had recently replaced some fittings on his condensate drain after noticing a drip. He'd used brass fittings — and they failed. Our technician we see regularly in the field: you cannot use brass fittings on condensate drains for heating equipment.
The condensate produced by high-efficiency furnaces and tankless water heaters is mildly acidic, and that acidity will corrode brass fittings over time. Always use plastic fittings on heating system condensate lines. (For AC condensate, brass is fine — that water is neutral. But for heating, stick with plastic every time.)
Key Takeaways
Flame rollout is a serious warning sign — not a quirk to work around. If you ever see flame shooting out of your water heater, shut it down and call a professional immediately.
Navien tankless units require three-inch venting for single-unit installations. Upsizing to four-inch pipe disrupts the pressure balance between intake and exhaust and can cause overheating.
Intake and exhaust vents should be close together on the same side of the house when possible. Significant separation creates unequal wind pressure between the two openings.
Check your expansion tank regularly. A waterlogged or undercharged tank can't do its job, putting unnecessary stress on your water heater and plumbing system.
Always check for warranty coverage before paying for replacement parts. A Navien heat exchanger can cost over $1,000 — but may be fully covered under the unit's ten-year warranty.
Use plastic fittings — never brass — on condensate drains for heating equipment. The acidic condensate will corrode brass fittings and cause leaks.
Heat exchanger failure on Navien units is common but often preventable with proper installation and routine maintenance.
Frequently Asked Questions
What is a hybrid water heater, and is it different from a tankless unit?
A hybrid water heater (also called a heat pump water heater) uses electricity and ambient air heat to warm water, combining a traditional storage tank with heat pump technology. It's different from a tankless water heater like a Navien, which heats water on demand using gas combustion with no storage tank. Both have their advantages depending on your home's energy setup, hot water demand, and available space. If you're unsure which is right for your home, our team can walk you through the options.
What causes a Navien tankless water heater to overheat?
Overheating in a Navien unit can be caused by several factors: improper venting (wrong pipe size or pressure imbalance between intake and exhaust), scale buildup inside the heat exchanger from hard water, a failed or undercharged expansion tank, or in rare cases, extremely extended runtimes. In the Westford case described in this post, the likely cause was the four-inch intake creating a pressure imbalance — leading to incomplete combustion, excess heat, and eventual heat exchanger failure.
How much does a tankless water heater heat exchanger replacement cost?
If the unit is out of warranty, the heat exchanger part alone typically costs over $1,000, plus labor for the replacement. If the unit is within Navien's ten-year heat exchanger warranty window, the part may be covered — significantly reducing your out-of-pocket cost to labor only. That's why it's always worth having a technician check warranty eligibility before proceeding with any major water heater repair.
What is proper gas hot water heater maintenance for a tankless unit?
Routine maintenance for a tankless water heater should include: annual descaling/flushing of the heat exchanger (especially in areas with hard water), checking and correcting expansion tank pre-charge pressure, inspecting the venting system for blockages or improper connections, verifying inlet filter screens are clear, and confirming the unit is registered with the manufacturer for warranty purposes. Staying on top of these items can significantly extend the life of your unit.
How do I know if my expansion tank needs to be replaced?
Tap the side of the expansion tank. If it sounds hollow throughout, it still has air in the bladder and may just need the pre-charge pressure adjusted. If it sounds dense or full of water all the way through, the bladder has likely failed and the tank needs to be replaced. You can also check the pre-charge pressure using a standard tire gauge on the Schrader valve at the top of the tank — it should match your incoming water pressure (typically 50–80 psi for most homes).
Need Tankless Water Heater Repair in Westford or the Surrounding Area?
If you're dealing with a Navien error code, recurring overheating, or just want a professional eye on your tankless system before something fails, Jeneral Plumbing is here to help. We serve homeowners throughout Westford, Chelmsford, Groton, and the surrounding Middlesex County communities with honest diagnostics, warranty-first thinking, and repairs done right the first time.
Give us a call at (978) 882-8699 — we'd love to help you get your hot water back on track.