I had a contractor install a Rheem electric tankless water heater in my casita. I initially wanted a much larger one but he talked me into the Rheem 13 KW 2.54 GPM. When the unit was installed the water was luke warm at best. The only way to get the water hot was to turn the ball valve down but then the pressure was too low. He then installed the 18kw Rheem3.51 GPM tankless heater and had the same problem.

I called Rheem today and they mentioned that an inline water regulator could solve the problem. The contractor did not seem to think so though. Honestly, I'm starting to question if this guy knows what he's talking about and would like some advice from someone who has experience with these types of water heaters. Do you think the water pressure is the problem and will an inline water regulator likely fix the problem?

I live in Santa Fe New Mexico so the ground water is probably in the 40's?

Any thoughts would be greatly appreciated.

Thanks, Mike

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    Stick a bucket under the shower and time to get an "Actual" gallons-per minute figure on the showerhead you have installed. Changing the showerhead may be all you need to do.
    – Ecnerwal
    Commented Feb 2, 2021 at 1:04
  • How did you produce hot water before you embarked on this tankless trip? Are the hot water lines routed through soil under a slab? Commented Feb 2, 2021 at 2:52
  • An 18 kW electric tankless water heater probably requires two 40 A , 240 V breakers. This heater would draw 18 kW / 240 V = 75 A. I think with your cold input water for a central water heater you would need a heater of twice that power (36 kW) to get close to the temp and flow rate we expect in the US. This would draw 150 A at 240 V. Commented Feb 2, 2021 at 4:54
  • What kind of electric service do you have? 100 A? 150 A? 200 A? Electric tankless water heaters require too much power for use as a whole house central water heater. They are only useful as point of use water heaters. Commented Feb 2, 2021 at 5:07

5 Answers 5


You need a water heater unit with a higher flow rate to bring the water temperature from 40 degrees to 120 degrees. Here in the Pacific Northwest, with year round water temps of around 45 degrees, it is advised to buy a MINIMUM of 7.4 gallons per minute flow rate in a tankless, whole house unit. At 2 or 3 GPM, your purchases are incapable of doing the job. (A 75° F shower is too cold for most of us at 98.6° body temps).

Flow rates out of showers and faucets have little to do with this. They are regulated by the government to save water from, for example, around 1.0 GPM at faucets to 2.2 GPM for a shower. I am referring to the flow rate of water through the tankless heater.

Many people think they can buy and install what is essentially a point-of-use tankless water heater (serving 1 shower or sink, for example) for whole house use to save money and get into trouble.

Is your fixture intended for whole house usage? (Serves all water heating needs in house, showers, sinks, dishwasher etc.) The answer to this question will guide my additional advice.

  • There is no point in getting an electric tankless water heater rated at 7.4 gal/min or higher because the house electrical system will not supply enough power to make it work. The limiting factor for an electric tankless is the electrical system. 200 A electric service will not support a whole house electric tankless water heater in a location where the incoming water is cold. Commented Feb 2, 2021 at 4:43
  • Good answer. It's best to ask the detail seeking questions (like "is this intended for whole house hot water") on the question, though.
    – FreeMan
    Commented Feb 2, 2021 at 13:31
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    I have seen 36kw electrics that the owner was unhappy with (more than once) +
    – Ed Beal
    Commented Feb 2, 2021 at 16:40
  • Thanks for your response. The water heater needs to serve shower and a sink at the same time. It sounds like a tank heater or gas tankless might be the best option.
    – Mike W
    Commented Feb 4, 2021 at 23:37
  • If you want to try to make this electric tankless heater work, get a very low flow, hand held shower head rated at 1.0 gal/min. This will give the best shower from low flow. Commented Mar 1, 2021 at 19:59

Everyone dismisses the regulator recommended by the manufacturer. You may need to regulate the main pressure to be in spec. if your tankless heater has a diaphragm used to adjust heat level relative to flow, but probably not matter if you have a hall effect flow meter.

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    – Community Bot
    Commented Apr 20, 2022 at 7:39

I cannot see how a separate regulator would do anything. A low flow shower head is a flow regulator; you don't need another regulator. Get a 1.0 gpm to 1.4 gpm hand held shower head and that will be the best you can do with an electric tankless water heater.


I had the same problem installed the 18kw system and was getting warmish to the odd hot spurts I had a 80 amp service put in as well... It was after the fact thT I was about to toss the box that it came in and found a fitting of some sort.. It turned out to be a regulator I didn’t notice it in the installation guide It goes on the output of the tank I now get 141°f

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    So this fitting is a flow restrictor which increases the temperature of the output water by further limiting the flow rate. If the WH comes with this, then by all means use it, but it is not magic. If the OP`s unit did not come with this extra flow restrictor, then he could simply throttle down the flow on the shower head to get a lower flow of hotter water. I am of course assuming that his shower valve allows lowering the flow rate. Are there modern single lever shower valves which have fixed flow rate and only allow variation of temperature? IIRC some hotel shower valves were that type. Commented Mar 1, 2021 at 19:51
  • Limiting the flow is not a solution, it just hides the problem. It would be much less expensive to get an electric countertop kettle. Then you get a nice 100 celsius, at a flow rate of 4 cups in 5 minutes.
    – Jeffrey
    Commented Apr 20, 2022 at 14:29
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    I have always had the gas tank heaters in my home. I even use them for heating and I do not run out of hot water even with two showers and the laundry operating.
    – Gil
    Commented Apr 20, 2022 at 23:09

Rheem's manual suggests an "optional" inline flow regulator in order to "Limit maximum volume to the specified flow rate to ensure the exiting temperature is within an acceptable range." The 13kV unit it's sized to the RTE10001A In-Line Flow Restrictor with the 1 GPM insert for 40-degree water installed on the outlet side. My theory is that the restrictor is optional so they don't have to include a possibly unnecessary ~$50 part for some installations.

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