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Can an inlet-filter cleaning and descale bring back tankless heater flow?
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Our gas tankless water heater is about 8 years old, and lately the hot-water flow has dropped while the temperature sometimes swings during a shower. We’re in a hard-water area, and I don’t know when the inlet filter was last cleaned or when the heater was last descaled. There aren’t any error codes showing.
Before paying for professional service or considering replacement, I’d like to know which homeowner checks are reasonably safe. I can also check the showerhead aerators and confirm the supply valves are fully open, since the restriction may not be in the heater itself. If the inlet filter is dirty and the unit is scaled, can cleaning and a proper manufacturer-approved descale usually restore performance enough to be worthwhile for water and energy efficiency?
For anyone familiar with these systems, what warning signs would suggest a failing flow sensor, heat exchanger, or another fault instead of routine maintenance?
2 replies
The temperature swings are useful diagnostically, but first separate a heater restriction from a fixture restriction. Check hot flow at more than one outlet, with the cold supply pressure unchanged:
- Weak hot flow at every fixture points toward the heater’s inlet screen, service valve, partially closed valve, or restricted supply piping.
- Normal cold flow but weak hot flow only when the heater is firing, especially with temperature hunting, warrants the manufacturer’s service-manual checks for the flow sensor and heat exchanger.
Cleaning the inlet filter is usually a reasonable maintenance step if the manual permits homeowner access. Descaling can restore heat-transfer performance and reduce cycling, but it won’t repair a failing sensor, a blocked internal passage, or a gas/combustion fault. Shut off gas and water as specified, isolate power, and don’t open the gas train or combustion compartment. If the filter is clean and the restriction remains, or if flow readings are unstable, professional diagnosis is safer than repeated chemical flushing.
The comparison across several outlets makes sense, but what’s the safest way for a homeowner to measure it without opening the heater? Would timing how long each hot tap takes to fill a known-size container, with the showerhead or aerator removed where practical, give a useful gallons-per-minute comparison against the cold side?
I’m especially wondering what result should trigger a service call before trying a descale. For example, if hot flow is consistently much lower than cold flow at multiple fixtures, or if the measured flow changes noticeably while the heater is firing, would that point strongly enough to a sensor or internal restriction that chemical flushing shouldn’t be the first step?
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