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Built my house in 2005. We have a tankless hot water heater. It appears to be working fine. Plenty of hot water from other faucets.

Guest bathroom shower is all from original contractor install. Single-lever ball handle, American Standard - lift to start water flow, turn handle to adjust temperature.

About three weeks ago, I noticed that the water in the shower is either totally cold or too hot. I can't get a "warm" temperature. If it's too hot, for example, and I "nudge" the lever to the right, the water becomes completely cold. And vice versa.

The water pressure, regardless of temperature, seems fine. I do not notice a drop in pressure whether the temp is all cold or all hot.

So, before I call a plumber, I am wondering if this is possibly the sign of a bad cartridge, which I think I can replace on my own and skip the repair bill. However, I really don't want to waste my time disassembling the faucet if a bad cartridge could not be causing this problem.

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Would appreciate your thoughts. Thank you!

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  • Not all shower valves use a "cartridge", what brand is yours? Can you post a picture of the entire handle and escutcheon (trim plate) assembly? Because you are new to the site, you may need to post the picture on Imgur, then stick a link to it in a comment here, we will then add the picture to your question.
    – Jimmy Fix-it
    Jul 19, 2017 at 1:26
  • It's American Standard. I don't have Imgur; will OneDrive do? 1drv.ms/i/s!AstRPdyson_Vm3vTHaqhiyASvpOH 1drv.ms/i/s!AstRPdyson_Vm327ys02y7Jy6gN5 1drv.ms/i/s!AstRPdyson_Vm3xx1OERXPqPT5rC
    – Squirk
    Jul 19, 2017 at 2:23
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    This could be due to low flow from the shower head in this one shower causing the tankless heater to shut off. Does this problem only occur when this one shower is the only outlet delivering hot water? There is a flow restrictor in the shower head; removing it may solve the problem. The restrictor is easy to remove from most shower heads and removing it is reversible if that isn't the source of the problem. Look on youtube, e.g., youtube.com/watch?v=vE87Vq-Xefc, youtube.com/watch?v=_5HnQF5KihU
    – Jim Stewart
    Jul 19, 2017 at 19:39
  • Thanks. No, not a low-flow head. Have been using this head for a few years now with no issues.
    – Squirk
    Jul 19, 2017 at 20:34
  • Shower heads can get deposits in them which restrict flow. it occurs so gradually that this process might go unnoticed. At least some of these shower heads have a screen in them to catch grit or mineral grains to prevent clogging of the orifices in the shower head. Sometimes these screens clog. This occurs frequently with kitchen and lavatory faucets. A neighbor asked me why she had "low pressure" at her bathroom lavatory faucet. My first thought was the aerator was clogged. I unscrewed the aerator, disassembled, and flushed it--high volume flow restored!
    – Jim Stewart
    Jul 19, 2017 at 21:59

3 Answers 3

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Yes a bad cartridge can be the issue.

I speak from experience, you may want to forgo buying said cartridge until you take it apart and inspect the valve and parts.

Often times pieces of plastic, rock, minerals, bad sweat 'balls' and other contaminants can get in there and bind the mixing valve in the cartridge.

Also, if you find it is cracked, warped, or otherwise damaged, you might want to get the replacement from a reputable parts supplier. I was stunned to see that Home Depot was selling all plastic and Kohler OEM part was 90% metal (OEM, pronounced as separate letters, is short for "original equipment manufacturer").

I tried the Home Depot part to no avail. The OEM part worked and continued working at least for 8 years until I sold the place.

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I think this is very likely a problem in the thermostatic mixing valve of the shower, but to definitively rule out your tankless water heater as the problem do the following.

When the water stream in the shower goes completely cold after your making a tiny adjustment to less hot, leave the handle alone and immediately check or have some else check to see if the tankless heater has turned off. If it has, then you know it's the tankless heater. Tankless water heaters require a minimum flow to stay on.

Our tankless heater became erratic when I installed low flow shower heads. When I removed the flow restrictors in the shower heads (increasing the flow rate) the heater performed reliably.

One way to test if the shower valve or the tankless heater is the source of the erratic behavior is to set the lavatory faucet to run a medium low level of hot water and see if this affects the delivery of hot water from the shower. When I realized that our problem was our tankless heater shutting off, I had the user of the bathroom where the problem was worst run hot water from the lavatory faucet during showering. This kept the heater on, but of course this is rather wasteful. Once I removed the flow restrictors in the shower heads we of course discontinued running the lavatory faucet.

Our Bosch tankless heater has been in service 13 years and is a basic discontinued model Bosch which has entirely mechanical controls. It has no digital temperature control, and does not require a source of electric power. It is possible that a proper adjustment of the gas flow 'slider' and the temperature knob would allow the heater to stay on with the low flow restrictors in the shower heads.

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  • When I run the hot water tap in the sink adjacent to the shower at medium-low pressure, the hot water comes on and stays on. Seems like if there is a flow issue, it is somewhere between the heater and the shower faucet.
    – Squirk
    Jul 19, 2017 at 20:39
  • Do you mean the hot water in the shower comes on and stays on (when the lavatory faucet delivering hot water)? If so, this does not necessarily mean there is a flow issue between between the heater and the shower valve. The most likely explanation is that the shower alone is not pulling enough hot water to keep the tankless heater on. Try running hot water in any faucet (say the kitchen faucet) and see if that keeps the shower water hot.
    – Jim Stewart
    Jul 19, 2017 at 23:15
  • Your method seemed to work. I left the hot tap running in the bathroom sink, and the shower temperature adjustment seemed to be back to normal. Next question (and this is probably stupid): what is the difference between the mixing valve and the valve cartridge? Do they do the same thing? Would a cartridge in my faucet handle indicate that there is no mixing valve behind the wall (thus avoiding the need to cut a hole in said wall to make a repair)?
    – Squirk
    Jul 20, 2017 at 12:22
  • There are virtually no shower installations that have a hidden mixing valve without an access port. The term "mixing valve" refers to a valve that allows hot and cold water to mix in a chamber and come out of a single spigot. In the old days kitchen faucets were entirely separate for hot and cold water--two separate spigots. These were NOT mixing valves. All ordinary faucets nowadays incorporate a mixing chamber and emerge from a single spigot.
    – Jim Stewart
    Jul 20, 2017 at 16:52
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The cartridge is part of the mixing valve. The cartridge is the replaceable part of the mixing valve which is accessible without tearing into the wall. But your findings indicate to me that the tankless heater is shutting off when you reduce the amount of hot water.

What is the model of the heater? What do you have the output temperature set at? If you lower the output temperature of the water heater, this will cause an increase in flow rate by thermostatic mixing valves and the water heater may stay on.

Two possibilities come to mind:

(1) for some reason the tankless heater is requiring more flow to stay on than it did before

(2) the shower heads have plugged up some and they are drawing less water than before, so much less that it is not enough to keep the water heater on.

Does your shower mixing valve have a variable flow rate or is it always on a certain flow and only the temperature is variable?

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