When taking a shower, I have the hot water faucet full open. Then, if I turn on the cold water faucet on more than half way the heater shuts off. It will not start back up until the cold water is completely off. So, summer and winter my showers are either scalding hot or freezing cold.

  • 4
    Was there a question in there?
    – Huesmann
    Feb 25 at 15:54
  • Welcome to Home Improvement, please take the tour. You've given us very little to go on and haven't asked a question, which is something of a requisite on this Question & Answer site. Feel free to edit into your question things like brand/model of shower handle, plumbing diagram (to the best of your knowledge), brand/model of the water heater. Where it's located (whole house WH or dedicated to just this shower/bathroom). The more info you give us, the more likely we are to be able to help you.
    – FreeMan
    Feb 25 at 16:20
  • Sounds like the cold water use starves the heater of pressure and/or flow enough to trigger a safety. Does the problem go away if you throttle the cold down below some threshold? Do you have low water pressure? Low flow?
    – popham
    Feb 25 at 16:58
  • 2
    "Ah, how sharper than a serpent's tooth it is to have a tankless child .."
    – keshlam
    Feb 25 at 17:20
  • 1
    Make/model # of water heater? Feb 25 at 17:27

1 Answer 1


We had this same problem with our Bosch gas fired tankless water heater (WH). There are different ways to fix this depending on your situation. What is happening is that you are using such a low flow of hot that the tankless WH is shutting off. All tankless WHs have a threshold for flow below which the WH shuts off. Tank WHs do not have that limitation.

First thing to do is to turn down the temperature on the output of the tankless water heater so that from the shower valve you have a higher flow rate of hot water and you don't need to feed in as much cold water. This helps in two ways: the higher flow rate of hot water raises the flow rate through the WH and then when youdo feed in cold you feed in less cold so this lowers the hot flow less.

The downside of lowering the output temp of the WH is that when you have multiple points of use simultaneously, the tankless WH maight not be able to supply all at the rate they would like, and the lower output temp may not be ideal for some uses, e.g., the dishwasher. (But IMO dishwashers should have their own water heaters, likewise to a lesser extent clotheswashers.

What temp is it set at right now? If it is currently set above 120 F, set it to 120 F. If it is already at 120 F, set it at 115 F and see if that solves the problem.

Your particular make and model of tankless WH might have a relatively high threshold for shutoff. If you have a low flow shower head, then, when only that shower head is being used, the flow may be so low that the flow is not above threshold for the WH. If you do not want to lower the temp on the output of the WH, then as a temporary measure, turn on the hot tap on the lavatory 1/3 or so while you are showering. This is a waste of water, but will keep the WH on while you are showering.

We had a problem in one shower after I installed lower flow shower heads than we had used previously. I had to modify the flow restrictor in that shower head to allow increased total flow. This worked somewhat but in the end to get saatisfactory performance I had to remove the flow restrictor in that shower head.

Modern shower heads may be designed to make it much more difficult to modify or remove the flow restrictor because of people doing this to evade water conservation guidelines. AFIK it is legally allowed for a home owner to do this, but not allowed for a professional plumber to do it, although some will at the direction of the home owner.

The reason that the shower is hot when only hot is on, but the WH turns off when cold water is turned on is that the flow of cold into the mixing chamber of the shower valve increases the (back) pressure in the mixing chamber and this reduces the flow of hot water from the WH. The shower head is a restriction which creates back pressure depending on the total flow through the shower head.

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