Temperature control problems in shower in the cool/lukewarm range

Hey, everybody. I’m new here, just hoping to get a few perspectives on the issue I’m having.

I have a stall shower with a Moen L2300 series “Chateau” single-handle temperature control. Also, I have a tankless water heater (brand/model can be found if needed). My goal is to be able to take cool to lukewarm showers when I want to.

Symptoms:
I can get very cold water, and I can get very hot water. When my single-handle faucet is in the 6:00 position, it is off. When it is in the 12:00 position (the midpoint based on the labeling of the unit), it gives me very warm/almost hot water as expected. For the hotter range (9:00 to 12:00), everything seems to be working fine. Between 12:00 and 3:00 is where my problem lies. Between 12:00 and roughly 3:00, there seems to be almost no change to water temperature. It maintains a very warm/almost hot temperature, no graduation to a cooler temperature as I would expect. At 3:00, there is an abrupt change in temperature… it jumps to the cool/lukewarm temperature I’m seeking and works fine for maybe a minute or so. Here’s where it gets really weird… the temperature then COMPLETELY bottoms out, and I cannot recover it to a warmer temperature at any point on the control, even in the hot range. It stays cold until I turn it off completely and start again, at which time it seems to act like I’m turning it on for the first time – which usually takes 60 seconds or so for hot water to arrive.

What I’ve done so far (at Moen’s advice):
Changed out the original cartridge
Removed new cartridge and flushed pipes
Installed a third cartridge

At this point, I’m questioning whether it may actually be an issue with my water heater. I’ve been blaming Moen, but I also have other Moen faucets in the house (in sinks though) that seem to manage temperatures properly with a full range of temperatures.

Any suggestions or diagnoses?

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Is the tankless water heater gas or electric?

I am not too familiar with the electric tankless, but the gas models have both a minimum flow rate and a minimum BTU requirement. Just a guess, but you might not be demanding enough hot water to keep the heater operational when you turn the temperature down.

To test this, try running a sink faucet on hot while you adjust the temperature on the shower valve and see what happens.

phishfood said:

Is the tankless water heater gas or electric?

I am not too familiar with the electric tankless, but the gas models have both a minimum flow rate and a minimum BTU requirement. Just a guess, but you might not be demanding enough hot water to keep the heater operational when you turn the temperature down.

To test this, try running a sink faucet on hot while you adjust the temperature on the shower valve and see what happens.

Click to expand...

Makes sense. I'll give it a try. It's a gas heater. Tried Moen one more time today, and they suggested I turn the temp down on my heater. Therefore I wouldn't have to push my temperature so low. I don't think that conflicts with your theory. Correct me if I'm wrong. I'll try your suggestion first.

phishfood said:

Is the tankless water heater gas or electric?

I am not too familiar with the electric tankless, but the gas models have both a minimum flow rate and a minimum BTU requirement. Just a guess, but you might not be demanding enough hot water to keep the heater operational when you turn the temperature down.

To test this, try running a sink faucet on hot while you adjust the temperature on the shower valve and see what happens.

Click to expand...

I tried your suggestion by turning my bathroom sink faucet on hot while testing my shower. In the 3:00 position, the water maintained a cool temperature. Good news, I thought. However, when I turned the sink off, it continued to maintain the temperature. I would have expected it to bottom out once the sink stopped calling for hot water. That was your theory, right? So I don't know what to make of that. This is an example of why this has driven me insane. It's been very hard to reproduce the issues at times.