Shower gets cold and stays cold for more than 10 minutes

last year

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?

Thanks

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Comments (5)

A Mat
last year
What is the flow at bs1 & 2

What is the flow (draw) on your tankless
Eddie Gornish thanked A Mat
dadoes
last year
Seems to be a flow-rate issue related to keeping your tankless activated. Multiple sources on simultaneously (B1S + B1F or B1S + B2F) apparently generates sufficient flow-rate to keep your tankless running. Does B1S have a temp-limiting valve that perhaps is throttling-down the hot water flow rate once it reaches a high-enough temp? B2F have a low flow-rate or perhaps the aerator is obstructed with debris or mineral deposits accumulated since the 2018 plumbing renovation?
Eddie Gornish thanked dadoes
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Eddie Gornish
Original Author
last year
A Mat - I measured the flow rate of 4 of the 5 fixtures. They are (approximately):

B1S - 1.1 gpm (problematic shower)

B1F - 0.7 gpm (problematic faucet)

B2S - 1.5 gpm (good shower)

B2F - 1.0 gpm (good faucet)

(I used the "time how long it takes to fill a gallon bucket" method for the numbers above.)

Note - I live in California (lower mandated flow rates), so 1.5 and 1.0 for good shower/faucet seems reasonable.

But "bad" shower and faucet seem low.

Thanks for the suggestion to measure this!

dadoes - The shower fixture "dial" for B1S has a safety feature that does not let you accidentally turn on the water too hot. (You need to push a button on the handle to turn it higher). I've bypassed this "safety" button when the water has turned cold, and it makes no difference (water is still cold). I'm not sure if this is what you meant with "temp-limiting valve". I don't know if either B1S or B1F have accumulated debris. If it comes to that, I would probably call a plumber (not sure how to take these fixtures apart).

Thanks
dadoes
last year
A sink faucet, simply unscrew the aerator on the spout to check for debris.

Check the user manual or specifications documents for your tankless water heater, or look it up online by the model number to find the required GPM flow-rate to activate the unit and if there's a reference for what is the required sustained flow to keep it activated.

Perhaps the safety-limiter on B1S is malfunctioning or needs to be adjusted to a higher trigger temp.

Set B1S running hot, until it goes cold, leave it running and quickly go check the tankless to see if it has shut off.

Have you tried pressing the bypass button *before* the flow runs cold? Pressing it *after* the shower has gone cold probably doesn't change the hot flow-rate. Your tankless may need the flow to stop for several seconds and restart to trigger it to re-ignite (assuming it's a gas unit).

What is the setpoint of your water heater? As a test, try setting it to approx 105°F to check what is the effect on B1S and B1F. It's possible that your setpoint is high enough that showerers are turning the hot water flow down so low that it's dropping below what's needed to keep the tankless activated. Setting the tankless lower such that more hot flow is mixed-in for a comfortable shower temp may eliminate the problem.

I set my tankless *at* the desired shower temp ... between 101°F and 103°F, sometimes low as 92°F to 95°F after a session of summer yard work ... and run the faucet at full-hot flow with no cold mixed in. Haven't had a problem with a shower going cold in 17+ years.
Eddie Gornish
Original Author
last year
I had a plumber come out. There was some "gunk" in the bad faucet head, but the shower head seemed relatively clean. The plumber then removed the O-rings from the faucet and shower head, which allow for a greater flow. As predicted, this solved the hot water issue (it was related to water pressure).

I believe these O-rings are used to regulate water flow. That being said, we can control the water flow manually to be just enough to keep the water hot, and we no longer have to waste water waiting for it to warm up. So overall, I think our water usage will also go down.

Thanks to all who replied with advice.

Shower gets cold and stays cold for more than 10 minutes

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A Mat

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Shower gets cold and stays cold for more than 10 minutes

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A Mat

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