Why is my tankless water heater not producing hot water?
Turning on the tap and getting only cold water from a tankless system is frustrating, especially during a damp Metro Vancouver winter. Unlike a storage tank, a tankless unit heats water on demand, so a single interrupted component can stop hot water entirely.
The good news is that many causes are simple: a closed gas valve, a tripped breaker, or a clogged inlet screen. Others, like ignition faults or a failed flow sensor, need a certified technician.
At HeatLand, we’ve served Metro Vancouver and the Fraser Valley since 1999. Below we walk through the most likely reasons your tankless heater isn’t producing hot water and how to tell when it’s time to stop DIY and call for help.
Check the Basics: Gas, Power, and Water Flow
Start with the three inputs every tankless heater needs: gas, electricity, and adequate water flow. Confirm the gas shut-off valve at the unit is fully open and that other gas appliances in your home are working; if your furnace or stove is also out, the issue is your gas supply, not the heater. Next, check that the unit has power. Most tankless models need electricity for ignition and controls, so a tripped breaker or unplugged unit means no hot water at all. Finally, verify water is actually flowing. Tankless units require a minimum flow rate to fire, so a barely-open tap or a partially closed main valve can prevent ignition. Open a hot tap fully and listen for the burner to engage. If all three inputs are present and the unit still won’t heat, note any error code on the display and move to component-level troubleshooting. Never bypass a gas valve or safety device to force operation.
Clogged Inlet Filter and Scale Buildup
Metro Vancouver water is relatively soft, but sediment, sand, and mineral scale still accumulate over time, especially in older homes or after municipal water work. Most tankless units have a cold-water inlet filter that traps debris; when it clogs, flow drops below the minimum needed to fire the burner. Turn off the water supply, place a towel underneath, and remove the inlet screen per your manual to rinse it clean. Internal scale on the heat exchanger can also reduce efficiency and eventually trigger lockouts. Annual descaling (flushing with a manufacturer-approved solution) keeps the exchanger clear and extends the unit’s life. If you’re not comfortable isolating the water lines or handling the descaling pump, a HeatLand technician can perform a full flush and inspection. If you notice discoloured water or persistent low flow after cleaning, there may be a deeper plumbing or exchanger issue worth a professional look.
Ignition and Flame Failure
If gas, power, and flow are all confirmed but the burner never lights, you likely have an ignition or flame-sensing fault. Common culprits include a dirty flame sensor, a failed igniter, a blocked venting path, or air in the gas line after recent service. Many units display a specific error code for ignition failure, so record it before resetting. A single reset is fine, but repeated lockouts signal a real problem that needs diagnosis. Blocked intake or exhaust vents, common after storms or from bird nests, can also prevent safe combustion and trigger a shutdown. Safety note: if you smell gas at any point, do not attempt a reset. Leave the home, avoid switches and open flames, and call your gas utility, then a licensed HVAC company. HeatLand’s TSBC-certified gas fitters diagnose ignition faults safely and carry parts for all major tankless brands we service.
Quick Checklist
- Confirm the gas shut-off valve is fully open
- Check the unit’s breaker and power connection
- Open a hot tap fully to ensure minimum flow
- Clean the cold-water inlet filter screen
- Note any error code before resetting
- Verify intake and exhaust vents are clear
| Symptom | Likely Cause | Action |
|---|---|---|
| No hot water anywhere | Gas or power supply lost | Check valve and breaker |
| Weak or no flow | Clogged inlet filter | Clean the inlet screen |
| Unit clicks but won’t light | Ignition or flame fault | Record code, call technician |
| Repeated shutdowns | Blocked venting or scale | Book professional service |
Frequently Asked Questions
Why is only some water cold and not all of it?
If some taps run hot and others cold, the problem is likely plumbing (a crossed connection or a fixture issue) rather than the tankless unit itself. When every hot tap runs cold, the heater has lost gas, power, or flow, or has locked out. Isolating which fixtures are affected helps a technician pinpoint the cause faster.
Can I reset my tankless water heater myself?
Yes, a single reset using the control panel or power cycle is safe when no gas odour is present. However, if the unit locks out again quickly, stop resetting and book service. Repeated resets can mask a real fault like ignition failure or venting blockage that needs a licensed technician to diagnose safely.
How quickly can HeatLand restore my hot water?
HeatLand serves Metro Vancouver and the Fraser Valley with emergency service for no-hot-water situations. Many ignition, filter, or sensor issues are diagnosed and repaired in a single visit because we carry common parts for all major tankless brands. Call us to describe the symptom and any error code so we arrive prepared.
Reviewed by the HeatLand Heating & Cooling team — licensed HVAC contractor serving Metro Vancouver & the Fraser Valley since 1999. This article is for general guidance only. If you smell gas, suspect a carbon monoxide leak, or are unsure about any electrical or gas component, stop troubleshooting immediately and call a licensed technician or 911.