Why does my tankless water heater have low pressure?
Weak hot water pressure from a tankless heater usually points to a restriction somewhere between your incoming supply and the tap. Because tankless units pass water through a narrow heat exchanger, they can be sensitive to buildup that a tank heater might tolerate.
The cause is often a clogged inlet filter or mineral scale, both of which are maintenance items. Sometimes the issue is a partially closed valve or a fixture problem rather than the heater itself.
HeatLand has kept tankless systems running across Metro Vancouver and the Fraser Valley since 1999. Below we cover the common causes of low tankless pressure and how to restore proper flow.
Clogged Inlet Filter and Fixture Aerators
The cold-water inlet filter is the first place to check for low pressure. This small screen traps sediment, sand, and debris to protect the heat exchanger, and when it clogs, hot water flow drops noticeably. Turn off the water supply, place a towel underneath, remove the screen per your manual, and rinse it clean. This simple maintenance often restores pressure immediately. If only certain fixtures have weak hot water, the problem may be downstream, mineral deposits in faucet aerators or showerhead nozzles. Unscrew and soak them to clear buildup. Comparing hot pressure across multiple fixtures tells you whether the issue is at the heater (all hot taps weak) or at a single fixture (localized). Because Metro Vancouver water carries some sediment, especially after municipal main work, periodic filter and aerator cleaning is worthwhile. If pressure stays low after cleaning both, move on to scale and valve checks.
Scale Buildup and Isolation Valves
Mineral scale inside the heat exchanger narrows the passage water flows through, directly reducing hot water pressure and flow over time. If pressure has declined gradually and the unit hasn’t been flushed in a year or more, scale is a leading suspect. A descaling flush with a manufacturer-approved solution clears the exchanger and restores flow, and it should be part of annual maintenance. Another common and easily missed cause is a partially closed isolation valve. Tankless units have service valves on the hot and cold lines, and if one wasn’t fully reopened after maintenance, it throttles flow. Check that both isolation valves and the main shut-off are completely open. These two items, scale and valve position, account for many low-pressure complaints on otherwise healthy units. HeatLand includes descaling and valve checks in tankless maintenance and can confirm whether the exchanger or a valve is limiting your pressure.
Whole-Home Pressure and When to Call
Sometimes the heater isn’t the problem at all. If cold water pressure is also low throughout your home, the issue is upstream, a pressure-reducing valve set too low, a failing main shut-off, a partially closed meter valve, or genuinely low municipal supply. Test cold pressure at several taps; if it’s weak too, the tankless unit is only reflecting what it receives. A plumber can check and adjust your pressure-reducing valve or diagnose supply issues. Distinguishing hot-only low pressure (points to the heater or hot lines) from whole-home low pressure (points to supply or the main) is the key diagnostic step. Safety note: leave gas connections and internal heater components to a licensed technician. If cleaning the filter, aerators, and confirming valve position doesn’t help, and cold pressure is normal, book HeatLand to inspect the unit; if cold pressure is also low, a plumbing assessment of your supply is the right next step.
Quick Checklist
- Clean the cold-water inlet filter screen
- Clear aerators and showerhead nozzles
- Confirm both isolation valves are fully open
- Compare hot pressure across multiple fixtures
- Test cold pressure to rule out whole-home issues
- Schedule a descaling flush if pressure declined gradually
| Symptom | Likely Cause | Fix |
|---|---|---|
| All hot taps weak | Clogged filter or scale | Clean filter, descale |
| One fixture weak | Aerator buildup | Clean the aerator |
| Weak after service | Isolation valve not fully open | Open the valve fully |
| Hot and cold both weak | Whole-home supply issue | Plumbing assessment |
Frequently Asked Questions
How do I know if low pressure is the heater or my plumbing?
Compare hot and cold pressure at several taps. If only hot water is weak, the tankless unit, its filter, scale, or valves is likely responsible. If both hot and cold are weak throughout the home, the cause is upstream in your supply or main valves, and a plumber should assess it rather than the heater.
Can scale really reduce my water pressure?
Yes. Scale narrows the passages inside the heat exchanger, restricting how much water can flow through and lowering hot water pressure over time. If pressure dropped gradually and the unit hasn’t been flushed in a year, descaling usually restores it. HeatLand offers descaling as part of tankless maintenance.
I cleaned the filter but pressure is still low. What now?
Next, confirm both isolation valves are fully open, clear fixture aerators, and check whether cold pressure is also low. If cold is fine but hot stays weak, the exchanger likely needs descaling or inspection. If cold is also weak, your home’s supply is the issue. HeatLand can diagnose the heater side for you.
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.