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Shower gets cold and stays cold for more than 10 minutes

Eddie Gornish
2 months ago

We have a problematic shower and a problematic sink faucet.

When trying to use either fixture with hot water, both fixtures will stay hot for about 5 minutes.

After that, both fixtures will turn cold.

Once they turn cold, they stay cold for about 10 minutes each (then alternate back and forth between several minutes of hot water and several minutes of cold water).

More details below:

In 2018, we did a whole house remodel (including our entire plumbing system).

We have a tankless water heater and 5 fixtures (2 showers and 3 sink faucets) that use hot water.

Bathroom 1 has a shower (B1S) and a sink faucet (B1F).

Bathroom 2 has a shower (B2S) and a sink faucet (B2F).

The kitchen has a sink faucet (KF).

After 3 years, we started having an issue where B1S would start to get cold after about 5 minutes of hot water.

Specifically, if we turn the hot water all of the way up, it takes about 1 minute for BS1 to get hot. It will stay hot for about 5 minutes before getting cold.

The issue is not a cold water sandwich, as once the water gets cold it stays cold for about 10 minutes (then alternates back and forth between several minutes of hot and several minutes of cold).

We experimented with the other fixtures and noticed the same issue with B1F (we don't know when the issue with B1F started).

B2S, B2F and KF are fine.

We eventually realized that if we turn on the B1S and B1F at the same time, both will stay hot indefinitely.

We can also turn on B2F at the same time as B1S, and B1S will stay hot indefinitely.

(We haven't experimented with other combinations.)

Note - Despite the fact that B1S and B1F are in the same physical bathroom, B1S shares more plumbing with B2S and B2F. The latter 2 are on opposite sides of an inner house wall, shared with B1S.

Our tankless water heater has a hot water recirculation feature, but we don't think this problem is related to it.

When the hot water recirculation is off, B1S and B1F take longer to initially heat up (as expected); however, they still turn cold after about 5 minutes of hot water.

Before we realized that one could workaround this issue by turning on 2 fixtures at once, we had a plumber come out.

The plumber installed a new shower cartridge; however, that made no difference.

Can anyone help figure out what the problem is?


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