Hot water to the sink, but not to the shower, same bathroom!

by Walid
(Oakland, CA USA)

We have a bathroom about 70 feet away from my Takagi tankless heater.

When we turn on the shower, we run it for over 5 minutes, the heater does not turn on, and thus no hot water. This is a new situation, for many years hot water has been delivered, although it takes a few minutes to heat up.

so I ran this experiment.

If I turn on the hot water in the sink, in the same bathroom, the heater turns on immediately, and in 2 minutes I have hot water in the sink.

Then I turn off the sink, and the heater turns off.

I then turn the shower on again. Hot water is there, but the heater is not turning on! The hot/warm water lasts about 5 minutes.

Keeping the shower running, I then turn on the sink. The heater starts immediately, warm to hot water arrives in the sink in 20 seconds. After 2 minutes I turn off the sink, but keep the shower on. The heater keeps going. Hot water in shower continues, for over 8 minutes.

what is the problem? Why does the sink start up the heater, but the shower doesn't???

thank you!

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Sep 05, 2019
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Question - similar issue
by: Garica

I have a similar problem. So you took it apart, cleaned it and assembled back on and it fixed the issue. Thats was it?- so what fixed the issue- was there a clog?

Apr 28, 2019
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Thank you for your help!
by: Walid

I just wanted to thank Marti for his help and advice on my problem. I took apart the shower head and piping to the wall, cleaned it all out and reassembled it, and now the hot water heater kicks in and works like a charm! I know to any actual plumbers or even handymen that the problem and solution were probably pretty obvious, but to a no-nothing like me, it was just great to have this laid out for me. Thank you again.

Feb 23, 2019
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low water flow in the shower
by: Marti

It looks like your Takagi tankless water heater doesn't kick on due to low water flow in the shower. In the experiment you ran, when both the fixture and shower were running, the total water flow was more than the minimum required by Takagi. And the water was hot enough inside the plumbing to provide some shower.

What I would do, remove the shower head and run it in the full blast. If Takagi is running and does not turn off, your shower head is clogged with the sediments and has to be either cleaned or replaced with a new one (but not a low flow).

Since you didn't mention the model, Takagi mentions for one of the models that the water flow of 0.4 GPM is required for continuous fire after initial ignition.

Hope it helps.

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