Tankless water heaters are reliable, but when something goes wrong, you'll know it immediately. Here are the most common issues we're called out to fix across Ontario — and what they usually mean:

No Hot Water at All

If the unit isn't producing any hot water, the most common causes are a gas supply interruption, a failed igniter or flame sensor, a tripped high-limit switch, or a blocked venting pipe. Error codes on the unit's display will often point directly to the cause. Our technicians carry the most commonly needed parts for Rinnai, Navien, Rheem, and Bradford White units to resolve most no-hot-water calls on the first visit.

Water Is Lukewarm or Inconsistent Temperature

Temperature fluctuations or lukewarm water are frequently caused by scale buildup in the heat exchanger (especially in GTA homes with harder water), a partially blocked inlet filter, a failing modulating gas valve, or an undersized unit struggling to meet peak demand. Descaling and filter cleaning often restore full performance without any parts replacement.

Unit Won't Ignite

Ignition failures are one of the most common tankless repair calls we receive. Causes include a faulty igniter, a dirty flame rod, a gas pressure issue, or a blocked air intake. Most ignition-related error codes are displayed on the unit's panel. These repairs are typically straightforward for a licensed technician with the right parts on hand.

Error Code Displayed on the Unit

Modern tankless heaters display alphanumeric error codes that identify the fault. If your unit is showing a code, see our error code reference tables below — and call us with the code so our dispatcher can ensure the technician brings the right parts.

Water Leaking From or Around the Unit

Leaks around a tankless unit can come from loose water connections, a failing pressure relief valve, a cracked heat exchanger (in older or severely scaled units), or condensate drainage issues on condensing models. Any water leak near a gas appliance should be treated as urgent — shut off the water supply to the unit and call us immediately.

Low Water Pressure or Flow at Fixtures

If water pressure has dropped across your hot water fixtures, a clogged inlet filter screen is the most common culprit — and one of the easiest fixes. Scale buildup in internal passages can also restrict flow over time. This is resolved with a filter cleaning and descaling service.

Unit Keeps Shutting Off Mid-Use

Nuisance shutdowns mid-shower or mid-use are often caused by overheating due to scale buildup, a faulty thermistor reading incorrect temperatures, venting restrictions limiting combustion air, or a flow sensor that isn't detecting water correctly. These issues worsen over time and should be addressed promptly.

Unusual Noises (Rumbling, Banging, or Whistling)

Rumbling or kettling sounds usually indicate significant scale buildup in the heat exchanger. A high-pitched whistle can point to a partially restricted gas valve or a flow issue. Banging on startup can be caused by delayed ignition. None of these should be ignored — they typically indicate a repair is needed before a complete failure occurs.

If your unit is displaying an error code, use these tables to identify the fault. Then call us at 1-888-675-5907 — share the code with our dispatcher so we can send the right technician with the right parts.

Rinnai Error Codes — Common Fault Reference

Navien Error Codes — Common Fault Reference

Don't see your error code above? Call us at 1-888-675-5907 — our dispatch team can look up any code for Rinnai, Navien, Rheem, Bradford White, A.O. Smith, and other brands we service.

While you wait for our technician to arrive, here are a few things you can safely check yourself:

1. Note the error code. Write down the exact code or combination of codes displayed on your unit's panel. Take a photo with your phone. This helps our dispatcher and technician prepare.
2. Check your gas supply. Confirm your gas is on by checking another gas appliance (stove, furnace). If no gas appliances are working, contact Enbridge Gas at 1-877-362-7434.
3. Check the power supply. Gas tankless units need electricity to run. Check that the unit is plugged in and that the circuit breaker hasn't tripped.
4. Check the isolation valves. Make sure the cold water inlet and hot water outlet valves on the unit are fully open.
5. Try resetting the unit. Many tankless heaters have a reset button or can be reset by powering off at the breaker for 30 seconds and powering back on. This clears some temporary faults. If the error code returns immediately, the fault is real and a technician is needed.
6. Check the vent termination (winter months). In Ontario winters, the exterior vent cap can become blocked by ice, snow, or debris. If you can safely access it, check that the vent opening is clear. A blocked vent will cause the unit to shut down on a safety fault.
7. If you smell gas — leave the building immediately. Do not operate any electrical switches. Call Enbridge Gas emergency line at 1-866-763-5427 from outside, then call us.

Our licensed technicians are trained and equipped to diagnose and repair all major tankless water heater brands. Here's what our service covers:

Our technician will always give you an honest assessment. Here's the general guidance we use to help Ontario homeowners make the right call:

If replacement is the right call, we carry Rinnai and Navien units and can complete a same-day or next-day swap in most areas — including handling your old unit removal and any available rebates.

We dispatch 24/7 to all of the following areas. If you don't see your city, call us — our service area is extensive and we'll confirm coverage immediately.

When your hot water is out, you need someone who shows up fast, knows what they're doing, and fixes it right the first time. Here's why thousands of Ontario homeowners call us:

• 24/7 availability — no after-hours surcharge for emergency calls: We're available nights, weekends, and holidays. Hot water emergencies don't wait for business hours and neither do we.
• Licensed gas technicians on every call: All repairs involving gas components are performed by G1 or G2 licensed technicians — not generalist handymen or uncertified subcontractors.
• Stocked service vehicles: Our technicians carry the most commonly needed parts for Rinnai, Navien, Rheem, and Bradford White units — resolving most repairs on the first visit without a second trip.
• Transparent, upfront pricing: We quote the repair cost before any work begins. No surprise invoices.
• Warranty on all repair work: Our labour is warranted in writing. If the same issue recurs, we come back.
• Repair or replace — honest advice: We'll tell you honestly if a repair makes financial sense or if replacement is the smarter investment. We don't push replacements when a repair will do the job.
• Same-day replacement available: If your unit needs to be replaced, we carry Rinnai and Navien units and can often complete a same-day swap — including rebate assessment and old unit removal.

Most tankless water heater breakdowns are preventable with annual maintenance. Our maintenance plan keeps your unit running at peak efficiency, extends its lifespan, and keeps your warranty valid — all for a predictable annual fee.

Each maintenance visit includes:

• Full heat exchanger descaling (flushing) to remove mineral scale buildup
• Cold water inlet filter cleaning
• Venting inspection — intake and exhaust terminations checked for blockages or deterioration
• Gas pressure check and burner inspection
• Combustion analysis to verify safe and efficient operation
• Thermistor and sensor check
• Error code diagnostic scan
• Written service report with any recommendations

Plan members also receive priority dispatch on repair calls — meaning you go to the front of the line if an emergency occurs. Call 1-888-675-5907 to ask about current maintenance plan pricing for your unit.

How much does tankless water heater repair cost in Ontario?

Repair costs vary based on the fault and parts required. A diagnostic call and minor repair (such as filter cleaning or a sensor replacement) is typically in the $150–$350 range. More involved repairs involving gas valves, ignition systems, or descaling service typically run $300–$600. We provide a written quote before beginning any work, so there are no surprises.

How quickly can you get to me for an emergency repair?

Response times vary by location and technician availability, but we prioritize emergency calls and dispatch as quickly as possible — in most GTA and surrounding area cases, we aim to have a technician at your door within a few hours of your call. Call us at 1-888-675-5907 and our dispatcher will give you an accurate ETA.

My unit is still under manufacturer warranty — can you still repair it?

Yes. Manufacturer warranties cover parts; they do not typically cover labour costs. We can perform warranty-related repairs using approved parts. If your unit is still under warranty, let our dispatcher know the model, serial number, and purchase date when you call — we'll advise you on how to proceed to preserve your warranty coverage.

My tankless heater is making a rumbling sound but still producing hot water. Is it urgent?

Yes — rumbling or kettling sounds are a reliable sign of significant scale buildup in the heat exchanger. The unit may still function for now, but scale insulates the heat exchanger, forces it to work harder, drives up gas consumption, and eventually causes overheating damage. Book a descaling service before the problem escalates into a full breakdown or heat exchanger failure.

Can I reset my tankless heater myself?

Yes — a power cycle reset (turning off the circuit breaker for 30 seconds, then back on) is safe to attempt. If the unit resets and works normally, monitor it closely. If the same error code returns immediately or within a short time, the fault is real and a technician is needed. Do not attempt to disassemble the unit or bypass error codes yourself — gas appliances must only be serviced by licensed technicians.

Don't spend another cold morning waiting. Constant Home Comfort dispatches licensed HVAC technicians across Ontario around the clock — no after-hours fees, no waiting until Monday, no runaround.

Call 1-888-675-5907 now for 24/7 emergency tankless water heater repair. Serving Toronto, the GTA, Hamilton, Ottawa, London, Waterloo, Barrie, and all of Ontario.