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Bosch Tankless Pressure Drop Off Explanation


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I recently inspected a home with a Bosch AQ125BNG tankless water heater. I turned the shower faucet all the way on to hot and after a few seconds the pressure dropped off significantly....about to the point where it would be hard to get the shampoo out of your hair.

The owner of the home explained that this was a characteristic of this particular water heater. He explained that as the demand for hot water increases, the flow drops...by design as it attempts to regulate how much hot water it can deliver at a given pressure.

I have never observed this type of drastic drop off in flow while inspecting homes with tankless units.

I'd be more inclined to think that this might have something to do with debris obstructed the flow, or something to do with a temperature/pressure balancing faucet component.

Anyone else ever observe such a drop off in flow, and know what the cause was? Does the home owner's explanation make sense?

I wish I could return to the home and troubleshoot this a bit. I did not observe this at any of the other hot water delivery points, but then again, I was not in the troubleshooting mode, and at the time of the inspection, the client told me that he probably would not keep this unit, so I didn't spend much time trying figure this out.


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Mike - I browsed a few plumber forums and self-help forums, and there weren't a lot of positive comments about Bosch customer service, so I came here.

Bill - could you explain a bit more about "if there is a single fixture operating, there is a problem." Only a single fixture was running when this happened. It went from full flow with great pressure, to about less than 60%.


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That unit should be able to supply a single fixture at full flow easily, so it shouldn't have to throttle back as you experienced. There are a lot of moving parts and chips in those things, and a lot to go wrong, but it sounds like it worked at the other fixtures.

We had a problem similar to what you describe on a recent remodel with major replumbing and a water heater change-out. I don't have the full details, but one of the existing shower valves didn't like it. This did not have to do with debris, it was related to the internal pressure balancing device and was a known issue to the plumber.

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