Takagi T-H3J-DV-N tankless hot water heater goes cold in the middle of a shower

[Ninar]

Hi,

We recently completed a gut renovation of an 8 family building in Brooklyn. We have the Takagi tankless hot water heaters and a couple units have complained that the water will go freezing about 5 minutes into a shower. We cleaned the internal filters (full of city grit), and it is still happening 1-3x a week in one of the units.
When this happens there is no error code, but the standby light is flashing.

this apartment is on the second floor of a 4 floor building.

The plumber who installed has been over several times, but has basically thrown his hands up.
My tenants are flipping out, this is my first time dealing with these units-- any advice is VERY appreciated.

 

[Dana]

Call Takagi.

Best guess (truly a WAG): If the gas plumbing is undersized and/or has multiple branches off to the units from a main line, the pressure blips from other big burners coming on/off can interfere with these things. Most installations require a 1.25" dedicated gas line between the regulator at the meter and the tankless, with no other gas appliances sharing that line. If these water heaters were dropped in to replace tank type water heaters without upgrading the gas plumbing, this sort of flaky behavior would be pretty likely.

The total length of the gas line (including the "equivalent lengths" of all the ells and tees along the path) can also be an issue when the path is complex, as would be common in an apartment building. See page 30 of the manual.