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I just remodeled my bathroom and had all new plumbing installed about two months ago. All faucets and fixtures are pretty much still brand new. I have a gas tankless water heater also that is about 7 feet from my shower (it's a couple of years old). Just during the last couple of weeks, when I'm showering, if I turn the Delta handle more towards the cold side, the hot water goes away completely. I can turn it on all the way hot and it never gets even warm again. But, if I turn the water completely off and turn it back on, it goes back to normal like nothing ever happened. Anyone have any ideas?

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This might be a problem with the water heater or it might be the shower valve. Or it might be due to an interaction between the water heater and the Delta shower valve which surely must have an antiscald feature.

The first thing I would try to resolve this is to turn the output temperature of the water heater to a lower setting. If you have the water heater output temperature set at 120 F, then lower it to 115 F. If that doesn't resolve the problem, lower the output temp by 10 deg F from the original setting. The purpose of this is to cause a higher flow rate through the water heater so that it does not go below the minimum and so shut off when you lower the temperature at the Delta valve. When the season changes to colder input water you could raise the temp of the output of the water heater.

What is happening is that when you turn the Delta valve to a lower temp the valve lowers the water flow rate from the water heater to such a low value that the heater shuts off the gas flame entirely. That is standard behavior for any tankless water heater. In deep summer the cold water going into the water heater is pretty warm so the water heater reacts by restricting the flow rate of water compared to the winter when the input water is much colder. So when you ask for lukewarm water the heater reduces the flow rate so low that the heater stops heating entirely. Water keeps flowing through the hot water pipes but it is not heated.

This does not explain why the heater does not resume heating when you turn the Delta valve back to the original higher temp. This could be a fault or a wrong setting on the heater or on the Delta thermostatic antiscald valve.

Also if your new Delta valve had a new shower head with it, then the shower head might be a lower flow rate "water saving" head. Code is requiring this in a lot of places, but what can happen is the the flow is so low that the water heater shuts off when you lower the temperature.

We have an old, outmoded Bosch tankless WH and find that we have to set the output temp much lower in the summer or the heater will shut off in mid shower when one reduces the temperature. But our heater will come back on if we turn the valve back to hot. However, our shower valve is an old two valve Price-Pfister that does not have an antiscald function. This type of valve is no longer allowed by code and would get flagged if we would put the house up for sale. We are not "in violation" beause the valve was installed before the code required anti-scald shower valve. Our valve is said to be "grandfathered in", but if we would replace it we would have to use the a valve with antiscald.

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  • Thanks so much. I'm inclined to think it might be an issue with the Delta shower valve itself. I never had this particular issue before with the old stuff and same water heater. I'm not sure the older one had an antiscald feature either though. I will try the temp on the water heater first as you suggested. I appreciate it. I spent enough on the plumber this year, so I prefer not to have to call him back. :)
    – AmberD
    Aug 16, 2023 at 2:41
  • What is the current setting of the output water temp of the tankless WH? Aug 16, 2023 at 2:51
  • Where is this, country, state? Aug 16, 2023 at 3:00
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    Don't rush to attribute the fault to the Delta valve. If you want to try to separate the function of the Delta valve and the tankless WH, do the following. With the WH output temp set at the value which causes the bad behavior, before you start the shower, open the hot valve on the bathroom lavatory to give a low moderate flow of water and leave it running during the shower. Does the bad behavior not happen when you shower (does the WH keep heating when the temp of the Delta is reduced to lukewarm)? If you do this, please report the result. Aug 16, 2023 at 3:11
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Most tankless heaters have a minimum flow rate of 0.5-2 gallons per minute in order to activate the burners and avoid damaging the system. This explains why you get no hot water when your delta handle is requesting a very low amount of water.

However, it does not explain why turning it back to hot does not activate the heater. You should recreate the problem, walk over to your tank, and check if you can hear hot water flowing on the outlet side of your heater.

It's possible that your shower cartridge has a clog which only activates under the scenario you describe but unclogs when you spin it to the off position.

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