NPE-240A2 won’t exceed 1.4 gpm output

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Sly S.

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Was tired of running out of hot water so I installed a new NPE-240A2 (w/ a Watts OneFlow filter in series prior to the Navien to help keep the unit clean). Original intent was to keep the nearly new 75 gal tank heater to use as a pre-warmer in the coldest months, but bypass and drain it most of the time. When completed and placed into operation though, the output pressure was terrible. Re-plumbed the system to have the tankless backfill the tanked heater to provide an endless supply of hot water. Less efficient, but was willing to eat that cost. Initially, seemed to be working. However, over the last few weeks it’s become apparent this just masked the issue. If more than one shower head is used, pressure still drops excessively.

Got a manometer today and tried testing the gas flow to see if that could be the issue. I can’t even perform the test because when I open multiple high-flow faucets they just trickle and the tankless simply will not increase flow rate beyond 1.4 gpm, no matter how high the demand on the system. I haven’t found any answers online so I’m hopeful you fellas (or ladies) have ideas.
 

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Fitter30

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Look at page 117 pressure drop dhw side. You need to install some gauges entering after filter and after heater. Then you can diagnose what is going on.
 

Fitter30

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Supposed to be a 9 gpm filter, but I’ll try and report back. Thx.
9 gpm is when new at what pressure? That's why gauges are important just and have you physically checked gpm? Know container size and a stop watch. Fill a bucket compare with reading. 1.4 gpm = 179.2. Oz per minute
12.2 oz per tenth of a second
 

Breplum

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Was tired of running out of hot water so I installed a new NPE-240A2 (w/ a Watts OneFlow filter in series prior to the Navien to help keep the unit clean). Original intent was to keep the nearly new 75 gal tank heater to use as a pre-warmer in the coldest months, but bypass and drain it most of the time. When completed and placed into operation though, the output pressure was terrible. Re-plumbed the system to have the tankless backfill the tanked heater to provide an endless supply of hot water. Less efficient, but was willing to eat that cost. Initially, seemed to be working. However, over the last few weeks it’s become apparent this just masked the issue. If more than one shower head is used, pressure still drops excessively.

Got a manometer today and tried testing the gas flow to see if that could be the issue. I can’t even perform the test because when I open multiple high-flow faucets they just trickle and the tankless simply will not increase flow rate beyond 1.4 gpm, no matter how high the demand on the system. I haven’t found any answers online so I’m hopeful you fellas (or ladies) have ideas.
First year Navien warranty includes labor and any parts for the following 5 years.
Confirm flow capacity out of your filter and if still an issue within the unit call an NSS for no labor charge service. I've not ever run into Navien contributing to a flow issue. There is not really any part inside to constrict flow except maybe the mixing valve, but that would maybe affect temperature, not flow.
You should be able to pop the pressure relief valve and have full flow under any conditions.
 
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