Adjust the temperature on your water heater down. "Mixing" with cold is reducing the flow of water through the burner and it is turning off. You can't request a trickle of hot, you probably need nearly full flow of hot to keep that burner going. Regarding the time it takes to receive water at your faucet; the hot water is produced right away but the hot water pushes the stored cold water in the pipes out. You are waiting for your hot to arrive. In my old house, the kitchen sink was furthest from the on-demand water heater. If I didn't want to wait for that stored cold water to be pushed out, I washed my hands in the bathroom! We had to change our habits to accommodate saving all that cash not storing hot water. (We switched from electric to on-demand gas, huge savings.)

Try turning down the setpoint of the water heater so the hot water isn't as hot. Setting the heater temperature high results in turning down the hot flow at the
shower and adding more cold ... which is what happens when your husband
showers at a lower temperature than your preference. Tankless units require a specific (gallons per minute) flow-rate to activate, and stay activated. Too slow of a flow, they shut off. Lowering the setpoint results in less cold mixed-in at the shower and more water flow through the heater.

Your experiment of running a faucet with the shower solved the problem by essentially the same effect ... increasing the flow rate through the water heater so it didn't get too low and shut off.

I take that an extreme and set my tankless at 102°F to 105°F as the normal temperature and run only the hot water when I shower (no cold mixed in). I've never had a shower go cold in 13 years. I raise the setpoint temporarily when needed for tasks such as washing dishes or clothes in hotter water.

Interesting...thanks to all who responded. We will try this and report back.

We took the flow restrictors out of the faucets and shower heads and that helped quite a bit although we're still having a problem that we think is more related to the unit itself.

Set the temp on the heater to no more than 120 F.

What is the model of the heater?

Removing the flow restrictors from the shower heads is not at all ideal.

You need to turn down the water temp to 115 - 120.

It requires 0.4 gpm flow to stay running. When the temp is set too high, it doesn't take 0.4 gpm to keep the shower hot... so it shuts down.

For other readers: The tankless heater with the lowest flow rates to stay on that I have found is the Rheem - they only need 0.4gpm to start and 0.26 gpm to stay running. I have had a couple of them and never run into this problem.

What kind of heater do you have? I have a small Bosch (rated for one use at a time). Similar issue with delay in getting hot water, but only one minute. There is a small flow-operated generator which creates the ignition spark to start the heater. Once the hot water is flowing, it almost never quits. I have a thermostatic temperature valve, but as long as one fixture is running (sink, etc.), no problems.

Update to anyone following this thread (and a thank you for all of the responses): After changing out all of the energy saving low flow shower heads, and changing the temperature on the unit to 130 degrees, I am happy to report everyone is taking warm/warmer/hot showers again with no cold blasts! It took some tinkering, but it is working great now.

Question - why do you have it at 130? Why not 120?

Yes, you do know that with tankless the idea is not to mix, but to use the full hot water?

It's reasonable to mix the temp down from 120, for example. Reasons to do that might be to have hotter water for the dishwasher and/or washing machine.
Also, filling a tub may require water hotter than 105 initially because it cools off by the time it has filled.

That said, there's no reason that I can think of to heat water above 115-120 F in a tankless heater.

In a tank water heater it can increase the effective capacity of the water heater. In a tankless it does no such thing... but it does cause the problem noted by the OP.

Sara Sitardites, I'm curious what *is* the temperature reading at your shower. Please check it with an instant-read kitchen thermometer (or whatever you may have) and report back.

"We tried 120, it wasn’t hot enough for me, even turned all the way to the end it was still just “warm”. Weird I know, but 130 was the magic number for everyone to be satisfied."

120 coming out of the showerhead would be uncomfortably hot. Not dangerous but too hot.

This isn't so odd. I'd bet it's caused by one of two or two of two of the following things:

1) Bath/shower mixing valves have anti-scald devices as a safety feature. They prevent big temperature spikes and limit how hot the output flow will get by mixing in cold water. Some monitor temperature, others monitor relative hot and cold line pressure flow. They are all adjustable, usually by taking off the front plate of the valve. Often to take off the front panel, you need to first take off the knobs/handles.

2) Sometimes hot water lines have tempering valves plumbed in that add cold water to the hot water line, to limit how hot the the water in the hot water pipe is. If there is one, you'll see it on the pipe from which the output of the heater flows. I believe by code they need to be visible. These are also adjustable.

Do you find either or both of these in your home?

agreed. 105 is the design temp for showers. Much beyond that and it's just too hot for people to tolerate.

I'm talking the actual temp of the water here... Like others, I suspect something else is going on that is reducing the water temps at the shower head.

Yes, I'd like to know what the shower head temp is too.

Third degree burns happen after 30 seconds @ 130F. They can happen after 5 minutes at 120 F. My point isn't about scalding though, it's that something else is going on that you need to set the water heater temp to 130F to get a hot shower. A cheap digital thermometer will give you an accurate readout of the actual water temp. There is a mixing valve someplace.

What you don't want to do is remove flow restrictors from shower-heads. That just wastes water and gas.

I agree, I know it is hot - I don't think I'm a crazy hot coal walker who can tolerate scalding water or anything like that. ;) All I know is when the plumber left it was set to 120 (and he said that was the normal temp he always set these particular heaters to, in fact, he said it was illegal for him to change it to a hotter temp, but that we could), but that was not cutting it for me in my shower. My husband was fine with it, I was not. I don't have a kitchen right now so I am not sure if that would have been super hot either (we're going through a remodel).

Elmer Fudd (cute name), I have no idea about either of the two things you have mentioned. This is a relatively new house (14 years old) so maybe it is common to build in those safety features you described, not sure.

I will attempt to figure out actual water temp as soon as I have my kitchen back to normal after my remodel and can find the kitchen thermometer!

I will also let you know if my water usage has changed at all once I get my new bill.

We did not take the flow restrictors off, but we did buy the highest flow shower heads we could find. It said in the description both the flow rate and water consumption was 2.5gpm.

Hope this helps anyone going through similar issues.

Hi does anybody know the solution to this problem. I turn on the kitchen sink and it gets hot. When I turn on the water in the bath it gets slightly warm then gets cold and doesn’t warm up again? Could it be mineral deposit in the line?

How do you get it going again? Does it have an error code? Do you have to turn it off and reset it to get it to work? What is the temp you have it set at?

When was the last time it was flushed? It should be done every year or 2, depending on the hardness of your water. I just had mine flushed after a year, and it could have gone another year or 2, I was told. But I have moderate hard water, and I don't really get any calcification around my fixtures.

We just put in new faucets in our master bathroom. They are low flow compared to the 15 YO previous models. They produce no hot water unless we turn on the shower at same time. After much consultation with plumbers, tankless water heater mfr., Delta help desk and local plumbing supply store - we have new 2.2 gpm aerators on the way from Delta. They will replace the 1.2 models that came with faucets. There is just not enough pressure to make water heater turn on. Hope the new aerators do the job!!! Would be interested in any other remedies just in case!!

Tankless heaters require a minimum flow to activate, and a minimum flow to stay activated.

For example, the Rheem Prestige unit requires 0.4 gpm to activate and 0.26 gpm to stay running.

These are both low numbers and not every unit has the same flow rates - you should be able to look yours up.

A typical bathroom faucet is 1 gpm - which is plenty to turn on the water heater.

You should set the temperature of the water heater to 110-115 at the very top it should be 120. There is no benefit at all to setting it higher than 115 or so.

The problem is when you set the temp on the water heater high, then try to mix it with cold water to make it 105 you end up with not enough hot water to keep the heater running.

Instead, set the water heater to 110 - 115 and then use mostly hot water.

When you set the water heater high and then mix it down, you also have a problem with it taking a long time to get hot water.

Jake - our water heater is set at 114 and has been in the four years we have owned our home. No problems prior to new faucets. Tech support for water heater admitted that there are problems with low flow faucets, especially on the other side of house as ours is. It was obvious from my conversation with them and also Delta that this has happened to other homeowners. Thanks for your comments!

I was having the same issue, what I did was I lowered the temperature down from 120F to 100F and the flow went from .3 gal to .6 and that fixed the problem.

Thanks to all who contributed to this discussion, solved my issues immediately after struggling with cold water issues. jim

How does one adjust the temp to actually use only hot water?

@Karen Williams, simple as setting the tankless water heater to the temperature of water desired at the faucet, then turning on only the hot water ... whether it's a single-lever faucet (set it on hot so no cold is mixed in) or a hot and cold double-handle faucet (turn on only the hot). The tankless unit will moderate itself to heat the water to the set temperature. Example, 102°F for a shower, 125°F for washing dishes. Of course, the tankless unit must be easily accessible to adjust the temperature. Many tankless units support one or more remote control panels that can be placed at convenient locations such kitchens and bathrooms for ease of adjustment when the tankless unit is installed in a basement, garage, attic, or on exterior of the house.

@dadoes I have no idea what you are referring to about setting the temperature "at the faucet" or about any remote control panel.

Ok i turned the temp up to 140 still waiting 15-20 minutes for hot water i understand the standing water has to be pushed through but how do tou explain the water running cold again after the hot has finally arrived?

Check the size of the valve/ orfice where the natural gas comes into your house. The plumber used the wrong size and we were not able to get enough hot water to finish a shower. Once the correct size was installed we had no problems.

Yes, start with getting rid of the flow restrictors they putthem in EVERYTHING it is ridicoulouse we put in a touchless water faucet in kitchen i ALWAYS had MORE THENENOUGH WATER PRESSURE, in hot AND COLD

after installing the new fawcet, our hot water had 1/2 the prssure it normally had for the last 13 years turns out it was the screen in the hose to the hot water side removing that screen helped ALOT i think it maybe part of the waterless tank problemss too

@sharon bradshaw

I'm sorry, but no. Flow restrictors are just fine unless they are getting clogged up with gunk. It's not unusual for "crud" to break loose when doing plumbing work or replacing faucets. You will typically have to flush out aerators and such after doing work like that.

Faucets with solenoid valves (like your touchless faucets, washing machines, dishwashers, ice makers, etc.) need to have a screen in them to prevent crud from getting to the valve.

@Karen Williams

This is the control panel on my tankless water heater. Water Set Point ... the output temperature the unit feeds into the hot faucets ... can be set in 1°F increments between 50°F and 140°F.

Temp Up and Temp Down buttons adjust the Water Set Point temperature. Then press Temp Set to make it the normal at-all-times temperature if desired ... otherwise the adjustment is temporary and will revert back after a number of minutes. I have 103°F as the "normal" temperature and raise it when needed for washing dishes or clothes, without pressing Temp Set, and it reverts back to 103°F after 30 minutes so I don't have to remember to do that. That's just how my unit works, they're not all the same.

Setting the temperature "at the faucet" means I set the water heater to the temperature of heated water I want/need "at the faucet" ... such as 103°F for a shower, instead of 120°F or 135°F which is much hotter than needed for a shower ... and I turn on only the hot faucet (or set a single-lever faucet to full-hot with no cold mixed-in). Thus, the tankless unit has a high-enough flow rate passing through it for the duration of the shower to insure it doesn't shut off due to too-low flow. 120°F, 135°F, 140°F output temperature requires mixing in cold water and turning down the hot flow at the faucet to get a comfortable shower, which reduces the flow through tankless possibly below the required rate which can cause it to shut off completely and the shower goes cold.

Remote control panel means exactly that ... some tankless water heaters can have an optional extra temperature control panel installed at a convenient location such as bathroom or kitchen so the homeowner doesn't have to go down to a basement or out to a garage or wherever the heater is located to adjust the temperature. My heater doesn't have a remote control but it's located in a broom closet in my laundry room behind the kitchen near the center of the house so is easy to access any time I want to change the setting.

I have not removed the flow restrictors from any showers or faucets. I've never had a running shower or faucet go from heated to cold in the 18+ years in this house with the tankless water heater.

Turning down the temperature or GPM output always solves a insufficient flow to trigger the burners issue. It sounds counter intuitive, but it is just basic science.