Weak hot water flow or a tankless unit faulting for no clear reason is often nothing more than a clogged inlet filter screen.
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Quick answer
The inlet filter is a small mesh screen at the cold water inlet of a tankless water heater, catching sediment and debris before it can reach the flow sensor and heat exchanger. It fails, or rather clogs, gradually as trapped sediment accumulates and restricts flow, eventually triggering low-flow faults or reduced hot water pressure at fixtures. A licensed CANRO technician checks the filter's condition as routine service, cleaning or replacing it before buildup becomes severe.
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A tankless water heater's heat exchanger is built with narrow internal passages to maximize heat transfer efficiency, and those narrow passages are exactly the kind of feature that sediment and debris can clog if nothing catches it first. The inlet filter, a fine mesh screen positioned right where cold water enters the unit, is that first line of defence, trapping grit, mineral flakes, and any debris carried in the supply line before it ever reaches the flow sensor or the heat exchanger itself.
Unlike most components on this list, the inlet filter isn't really a part that fails in the traditional sense -- it's designed to gradually accumulate debris and needs periodic cleaning as a normal part of maintenance, similar in principle to a furnace filter, though on a much smaller scale.
We remove and inspect the filter screen directly, since visible clogging is usually obvious once it's out, and we compare flow before and after cleaning to confirm the filter was actually the cause rather than a coincidental factor. We also check whether debris is coming from a specific source, such as recent plumbing work, nearby municipal main repairs that can stir up sediment, or a breaking-down component elsewhere in the system such as a dip tube on an adjacent storage tank, since a filter that clogs unusually fast points to something upstream worth investigating.
| Symptom | Likely cause | Next step |
|---|---|---|
| Weak hot water flow only | Clogged inlet filter restricting flow | Remove, clean, and reinstall filter |
| Low-flow fault with multiple fixtures open | Filter restriction preventing minimum flow threshold | Clean filter, retest flow rate |
| Filter clogs again quickly after cleaning | Ongoing sediment source upstream | Investigate supply line or nearby plumbing work |
In the vast majority of cases, the inlet filter simply needs to be removed, rinsed clean of trapped debris, and reinstalled, which fully restores normal flow without needing a replacement part at all. Replacement only becomes necessary if the mesh screen itself is torn or damaged, which is uncommon but can happen if the filter has been handled roughly during a previous service or if debris has been unusually abrasive. Because this is one of the few components on a tankless unit designed to need periodic attention as normal maintenance rather than as a sign of a fault, we typically check and clean it during every annual service regardless of whether a flow complaint prompted the visit.
Richmond's pace of ongoing construction across its townhome, condo, and infill developments means water main work and nearby excavation happen fairly regularly across the city, and that kind of nearby disturbance can stir up sediment in supply lines that ends up trapped at a tankless unit's inlet filter sooner than it would in a quieter, more established neighbourhood. On brand-new installations specifically, we also commonly find inlet filters doing exactly the job they're meant to during the first several months after a tankless unit goes in, catching residual construction debris from new supply piping before it has a chance to reach the heat exchanger, which is a normal part of a new system settling in rather than a sign of a problem. Given how much of the city's mechanical equipment sits in easily accessed ground-level utility closets, checking and cleaning this filter is typically a quick part of any Richmond service visit.
Safety note
The inlet filter itself is a simple water-side screen, but it's part of a natural-gas-fired tankless water heater, and any work that requires opening the unit's casing should be done by a Technical Safety BC licensed gas fitter -- CANRO holds gas licence LGA0043870 and boiler licence LBP0046450. If you smell gas near the unit, leave the home and call FortisBC at 1-800-663-9911.
This is a common sign of a clogged inlet filter restricting water into the tankless unit, and it's usually a quick, low-cost fix once a technician cleans or replaces the screen.
It's typically checked and cleaned during annual maintenance, though homes with older supply piping or recent nearby construction may need it checked more often.
This requires opening the casing of a gas-fired appliance, so it should be done by a licensed technician rather than as a homeowner task.
Frequent reclogging usually points to an ongoing source of sediment upstream, such as aging supply piping or nearby construction activity, which is worth investigating rather than just cleaning the filter repeatedly.
It's primarily a performance issue rather than a safety hazard on its own, though restricted flow can also trigger nuisance low-flow faults that shut down hot water delivery.
Yes, every tankless unit relies on this filter to protect its heat exchanger, so periodic cleaning remains part of normal maintenance regardless of how new the unit is.
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