Why did my tankless hot-water flow slow again just weeks after descaling?

MGI’s recommendation: Ask for the reason the flow returned to the same problem before paying for another identical flush. Descaling may be part of the repair, but a short-lived improvement needs a documented explanation.

Descaling means removing mineral buildup. Model-specific cleaning guidance. The heat exchanger is the part that transfers burner heat to the water. Manufacturer part definition.

Keep the previous service invoice. It should help establish what was done, what the technician found, and whether flow was measured afterward. “Descaled” alone does not tell you whether the original restriction was in the heat exchanger, a filter, a fixture, or elsewhere in the water system.

Rinnai’s RSC manual identifies scale as a performance concern and includes cleaning the inlet filter after its flushing procedure. That supports checking the water path and the completed service, rather than treating every low-flow complaint as proof of fresh scale. The procedure and treatment requirements must match the actual model. Rinnai RSC manual, heat-exchanger maintenance

Without opening the heater, note whether cold-water flow has also changed and whether every hot-water fixture is affected. Save a photograph of any code, and record when the restriction returned. A recent plumbing project or water-supply interruption is useful history to share, but it does not establish the cause.

The next qualified assessment should compare the flow entering and leaving the heater, inspect the model’s water filters, and investigate the restriction that those measurements identify. Ask whether the earlier service was completed and rinsed correctly. If water quality is the concern, request a current hardness result and the manufacturer’s applicable limits, rather than assuming all Colorado homes have the same water.

A lasting plan might be a verified restriction repair, a different maintenance interval, or a suitable water-treatment proposal. The evidence should decide which. A treatment provider should explain what the proposed system addresses and who maintains it; a generic “whole-house fix” is not enough.

We would consider replacement when the findings show a damaged or unsuitable heater, or when a documented repair and ownership comparison supports it. Repeated cleaning without a diagnosis is not that comparison.

Until service, avoid taking filters or valves apart without the exact instructions and safe isolation. Send the prior invoice, the model label, the date flow improved, and the date it worsened. Ask for a root-cause visit and a before-and-after flow record.