How to Repair a Tankless Water Heater: 12 Common Problems And How to Fix Them
Did you know that over 90% of homeowners who switch to tankless water heaters experience an issue within the first five years? It’s not because tankless water heaters are unreliable. In fact, they’re more energy-efficient and longer-lasting than traditional water tanks. But they’re also more complex. So if you’re staring at your faucet wondering why the hot water isn’t flowing like it used to – keep reading.
This guide digs deep into how to repair a tankless water heater, with no fluff, no over-promising, just real problems and real fixes.
Problem#1: No Hot Water at All – Not Even a Lukewarm Trickle
If you turn the tap expecting a hot shower and get nothing but cold disappointment, it’s probably the most frustrating thing about owning a tankless water heater. No hot water is one of the most common issues, and it doesn’t always mean the whole unit is dead.
Signs:
The signs are pretty obvious: you turn on the hot water, and nothing happens. Maybe there’s a slight clicking sound from the unit, or maybe there’s dead silence. If you’ve got a digital display, it might be showing an error code, or it might be as blank as your face when the cold water hits. In any case, it’s not working.
How to repair the issue:
Now, let’s talk about how to repair a tankless water heater with this issue. Start by checking the obvious stuff. Is there power to the unit? Has the circuit breaker tripped? Is the gas line open? Yes, those things sound basic, but they’re often overlooked. Once you rule those out, clean the water inlet filter and flush the system. Mineral deposits can block heat exchangers and sensors, essentially shutting everything down.
If that doesn’t solve it, the flow sensor could be stuck. Take it out and clean it thoroughly. And if it still won’t fire up, the ignition pack or control board might be to blame. At that point, it might be time to call a plumbing pro in Miami – unless you enjoy crawling around with a multimeter on your weekend off.
Problem#2: Hot and Cold Water Swings During Use – Temperature Goes From Sauna to Ice Bath
This problem usually sneaks up on you. One minute you’re enjoying a nice hot shower, and the next minute it’s like you’ve been dropped into a glacier-fed river. It’s not just uncomfortable, it’s downright aggravating. These fluctuations in water temperature are one of the more confusing problems because they feel random. You might even think it’s a plumbing issue elsewhere in your house. But in most cases, it’s your tankless water heater being finicky.
Signs:
The signs? The temperature shifts unpredictably. You’re not adjusting the handle, no one else is turning on water in another part of the house, and yet you’re swinging between hot and cold. This is often due to a low flow rate, which causes the unit to shut off and then reignite. Other times, it’s mineral scale choking up the heat exchanger, causing heat output to sputter. Or your water heater might be undersized for your household, constantly fighting to keep up.
How to repair the issue:
Here’s how to repair a tankless water heater in this case: flush the system to remove mineral build-up. Use white vinegar or a descaling solution and run it through the heater for at least 45 minutes. Then, check the flow rate. Make sure it meets the minimum required by your heater’s specifications. You might also want to invest in a larger unit if you’re consistently maxing out its capacity. And check that mixing valves aren’t messing with the temperature regulation. It’s tedious, yes—but it beats jumping between heat stroke and hypothermia.
Problem#3: Burner Ignites and Then Shuts Off Instantly – Heater Won’t Stay Lit
This one’s a head-scratcher.
Signs:
You hear the familiar click-click-whoosh of the burner firing up, and then, bam – it shuts off. It’s like the heater is teasing you. You may get a brief burst of hot water before it goes cold, or maybe it doesn’t even make it that far. If this is what’s happening, you’re dealing with a unit that can’t stay lit. And that usually comes down to flame sensors, gas pressure, or airflow problems.
How to repair the issue:
If you’re trying to understand how to repair a tankless water heater with this issue, start by inspecting the flame sensor. These sensors can get dirty or corroded, causing the system to think there’s no flame when there is. Clean it gently with fine-grit sandpaper and reinstall it.
Still cutting out? Your gas pressure might be inconsistent or too low. Check the supply line and regulator. For gas-powered units, inadequate venting or blocked air intake can also cause the system to shut off for safety. Make sure nothing is blocking the exhaust or intake pipes. Rodents love to build nests in weird places. You’ve been warned.
Problem#4: Error Code Showing on Display – That Blinking Code Isn’t Just a Decoration
If your tankless water heater has a digital display, it’s trying to talk to you. And no, the error code isn’t random gibberish – it actually tells you something useful.
Signs:
When your unit flashes an error code, it’s basically sending an SOS. You might notice the code after the hot water cuts out or the burner refuses to fire. Other times, you won’t get hot water at all.
How to repair the issue:
To figure out how to repair a tankless water heater that’s throwing codes, look up the error in your manual. Each code corresponds to a specific issue—like ignition failure, combustion issues, or overheating. Follow the troubleshooting steps one by one. Reset the unit, clean filters, check gas and water pressure, inspect venting. Error codes are annoying, sure, but they’re also your best bet at solving the issue without taking the whole thing apart.
Problem#5: Low Water Pressure From Hot Taps Only – The Flow Is Weak
Low water pressure from your hot taps, but not your cold ones, is a dead giveaway that your tankless unit is involved.
Signs:
You might notice it in the shower first, where the water barely dribbles even though the cold is flowing strongly. Sinks are next, and it spreads like a slow plague. This isn’t about your main water line-it’s internal.
How to repair the issue:
The likely culprit is a clogged water filter or heat exchanger packed with scale. Over time, minerals from hard water build up inside the system and restrict flow. The more gunk inside, the less pressure you get. Cleaning out the inlet filter is step one. Then flush the whole unit.
A vinegar or descaling solution can do wonders, but you need to let it run for a while – 30 to 60 minutes usually. Once clean, recheck your water pressure. That’s how to repair a tankless water heater when the hot water pressure vanishes.
Problem#6: Strange Noises During Operation – Banging, Sizzling, And Clicking
If your tankless water heater starts making sounds like it’s haunted, it’s not trying to scare you; it’s trying to get your attention.
Signs:
You might hear popping, crackling, banging, or sizzling sounds. These aren’t normal. What you’re likely dealing with is mineral build-up. As scale accumulates, it causes water to overheat in small pockets, making those weird noises as the water flashes to steam.
How to repair the issue:
To fix this, flush the system. Yes, again. This is one of the top reasons how to repair a tankless water heater even matters in the first place. Regular descaling isn’t optional, it’s essential. Also, check the venting system. Improper venting can cause back pressure, which sometimes makes bizarre noises. Tighten everything down and clear any blockages. If it sounds like it’s about to explode, don’t panic. It’s just calcium being dramatic.
Problem#7: Unit Overheats And Shuts Off Suddenly
If your heater shuts down during a shower and the water goes ice-cold, it’s probably overheating.
Signs:
Signs include a sudden cutoff of hot water, error codes related to temperature limits, or a display that looks like it just gave up.
What’s happening here is the internal temperature hit the max threshold, and the system is protecting itself.
How to repair the issue:
Here are the steps on how to repair a tankless water heater that keeps overheating:
- Clean the heat exchanger, as restricted flow causes overheating.
- Check for blocked airflow or restricted venting. Also, the temperature sensor might be faulty.
- Replace it if cleaning doesn’t help.
- Finally, lower the thermostat setting – some units are simply set too high out of the box.
- Start there before you start yanking wires.
Problem#8: Water Smells Like Rotten Eggs – Yep, It’s the Heater
Signs:
When your hot water smells like sulfur or rotten eggs, don’t blame your pipes just yet. That smell is often caused by bacterial growth inside the water heater or, more rarely, in the plumbing. You’ll only notice it when using hot water, which is your cue.
How to repair the issue:
To handle this, you’ll want to flush the system with a hydrogen peroxide solution. Fill the unit, let it sit for several hours, and then flush it with clean water. That’s how to repair a tankless water heater that stinks. Also, consider adding a sediment filter or a water treatment system to prevent this from recurring.
Problem#9: Cold Water Sandwich Effect – First Hot, Then Cold, Then Hot Again
Whew! This one’s troublesome issue.
Signs:
If you get a quick burst of hot water followed by a cold blast, then hot again, congratulations – you’ve been “sandwiched.” This often happens when multiple users are drawing hot water one after the other. It’s caused by the water sitting in the pipe between uses, followed by a delay in reheating.
How to repair the issue:
Unfortunately, there’s no one-size-fits-all fix. But how to repair a tankless water heater in this situation starts with adjusting your usage habits. Add a small buffer tank or recirculation pump to reduce the gap. These tools keep hot water in the pipe longer, minimizing the sandwich effect.
Problem#10: Water Heater Is Leaking – Drips or Streams Around the Unit
A leaking tankless unit is serious business.
Signs:
You might spot moisture near the bottom, droplets on connections, or full-on puddles. Internal seals may have worn down, pipes could be cracked, or connectors might have failed.
How to repair the issue:
How to repair a tankless water heater that leaks? First, shut it off immediately. Water and electronics don’t mix. Then, check all connections, especially the inlet/outlet joints and relief valve. Replace any worn washers or faulty valves. If the internal heat exchanger is leaking, the repair might not be worth it. Time for a replacement.
Final Thoughts
Need professional help? Sunny Bliss offers fast, affordable plumbing and tankless water heater repair services all across Miami, Florida. We’re not here to sell you what you don’t need – we’re here to fix what’s broken. Give us a call at 305-990-1399, and let’s get your hot water running again.