Tankless Water Heater Shutting Off During Shower? Here is Why.

Why You Get Cold Water

Few years ago, when the awesome tankless water heaters of today were not available, not a day
used to pass without someone complaining about a tankless water heater going
cold in the middle of a shower. We still come across similar problems nowadays
and generally, the explanation is simple.

The most common cause of this problem is the hot water flow falling below the minimum
water flow required by the unit to keep running. This happens when you lower
the hot water flow to adjust the final water temperature coming out of your
shower head. Cheap tankless water heaters can have high minimum flow rates like
1 gallons per minute or 1 gpm. Falling below this flow rate is not a big issue.
Better models have like 0.5 gpm, which is still high. Rheem and Rianni tankless
water heaters are the best in this aspect. They still heat the water even if
the flow goes down to 0.26 gpm. These two brands are the least likely to
surprise with a cold shower.

There is a second scenario, although much less common, that can cause your tankless water
heater to stop. In the South, where the incoming water can be like 75 °F, you
need no more than 25 °F temperature increase for taking a shower in the summer.
If you are demanding 1 gpm hot water flow, the minimum flow rate of the water heater
is met so you would think it should not shut off. Yet, 25 °F temperature
increase at 1 gpm flow rate needs an heat input of only 12,500 BTU. Wait a
minute, Rinni Ultra RUC98i, which is one of the best tankless water heaters in
the market, has a minimum 15,200 BTU! This unit would stop rather than send you
water hotter than you ask for and risk scalding your skin! In other words,
during a Southern summer, the temperature rise you desire could be too miniscule
for your tankless water to produce. To combat this problem, turn up the water
flow rate or just get a cold shower for a change.


We Didn’t Change Anything!

Sometimes people insist that they are not changing anything in the shower and the water
is going cold for no reason. If this is happening only right after you start
using the hot water, it could be the infamious cold water sandwich. Hot water
is left in the pipes from your last hot water draw. When you open the tap you
get the hot water in the pipes but back inside the water heater some cold water
passes through before the unit ignites and starts heating up. So, you get hot
water and then cold and thne hot. The problem is many people will mess with the
shower handle the moment cold water comes out of the shower head. As a result,
the water flow changes and fluctuation after fluctuation in the water temperature
is what you get. To fix this, do not touch anything and let the intruding
volume of cold water go down the drain.

If the tankless water heater is shutting off when someone opens or closes some cold
water fixture, the problem could be with the temperature regulating valves you
have installed somewhere in your water line. These valves automatically change
the hot water flow to prevent you from getting scalding water if someone steals
the cold water from you by opening a cold water tap. As a result, the hot water
flow is reduced and the water heater thinks your shower is over. You might even
have inherited these valves from the former owner of your home and not know
they even exist.

At the end of the day, the best way to avoid all thse problems is to get a good tankless
water heater. Reading reviews and doing research on products are essential for
making the right decision.

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  1. I have a Noritz tankless water heater. And for years it had worked correctly. Over the past few years only while you’re taking a shower it’ll suddenly get cold when you get out of the shower to check the remote it is blinking basically that unit is shut off you can turn it off back on and you can take a shower again what the problem is only gotten worse over these years sometimes it’s down to the point when you turn it off and on again it’ll turn around and shut off again the unit does not shut off when washing clothes washing dishes or running hot water tabs once in a blue moon in May when you’re running a hot water tap but mostly it’s only while taking a shower I ordered a new remote but the problem still exists what can I do to correct the problem

  2. I have a Rheem tankless water heater and I have had nothing but problems from day one . Rheem sent me a new mother board and
    burner. That worked for a while and now it is back to shuting off half way thru a shower. It seems to go off just about the time you put
    soap on your face. I am a plumber and I bought a Rheem because their tank W/H are GREAT. I have installed many Bosch and I have
    not had 1 call back . I am about done with the Rheem!!

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