I have a question about how filling a bathtub would work with a tankless system. Due to my panels load/capacity I'm looking at the RHEEM RTEX-11 with a Flow Rate: up to 2.68 GPM.

I'm worried about filling my tub as it's quite large, but don't understand what happens when GPM is low.

  • I only have 1/2 Pipe in my house for one thing, not sure the PSI but it's low.
  • If the GPM is low, and I crank the tub to 100% Hot, will it just be a slow hot water pour or will the GPM basically match my homes pressure and the water won't be warm/hot as it's not spending enough time in the tank?
  • I don't mind if it takes longer to fill my tub, just want it to be hot. It just occurred to me that slower fill means more heat will dissipate before filling, but the entire stream will be hot. As it stands, my tank is only about 3/4's my tub and then it fills the rest with cold water. I expect the two would be about the same, if that's how it worked, but I'm not sure if the stream would be hot with low gpm.

3 Answers 3


Results vary. A tankless heater I've had for many years regulates the output flow to assure that the output temperature is as hot as expected, however, not all tankless heaters do this.

The "installation instructions & homeowners manual" document from Rheem for the RTEX-11 states the following (page 7):

To ensure the optimal temperature output and overall performance of your tankless water heater you may require a flow regulator. These flow regulators are installed on the outlet connection of your tankless water heater and limit the maximum volume coming out of your unit to a specified flow rate to prevent the exit temperature from becoming too cool. To learn more about flow regulators or find out how to purchase one for your tankless water heater contact Manufacturer’s national service department at 1-(800)-374-8806.

Based on that statement I expect that you could manually reduce the flow by partially closing the hot valve on your tub filler until the output rises to a temperature you like. Or, if you don't want to worry about that, install a flow regulator device to force the issue.

  • I don't have separate knobs for hot cold, just a spin dial, not sure if that would work for your idea, but basically that tank in question does not have a regulator built in but I can add one... Hmm accepted! Ill have to look into this, probably something I would add afterwards if I found I had issues but I'd like to research first Commented Oct 21, 2019 at 23:11
  • Right, if your dial is the kind where you turn it a little and the water comes on cold and you turn it more to make the water get warm and then hot, then you'd be out of luck. That kind of valve doesn't let you control flow. The single-knob kind that push/pull in and out do allow controlling the flow to some extent.
    – Greg Hill
    Commented Oct 22, 2019 at 1:15
  • 1
    Do note that installing a flow regulator of the right size would only affect the tub because other points of use already have a low flow. For example faucets are limited to less than 2 gal/min already so you wouldn't even notice a difference.
    – JPhi1618
    Commented Oct 22, 2019 at 16:12
  • @JPhi1618 your comment made a lot of sense and helped me take the dive, just ordered a bunch of AWG 6 to get started, can't wait! Commented Oct 25, 2019 at 2:21

Here's how it works.

  • With a tanked heater, you get hot water to exhaustion, then you get cold water.

  • With a tankless heater, you get unlimited hot water. Really. However, how hot depends on flow. The heater will put the exact same number of BTUs of heat in the water, no matter how fast you make the water flow. However, the faster you blow teaspoons of water through the heater, the fewer BTUs each teaspoon gets, i.e. The cooler it is. Does that make sense?

Imagine if instead of a tankless, you are holding a propane torch up to the stream of water. If it takes 2 minutes to fill the tub, you get 2 minutes of torch heat. If it takes 4 minutes to fill the tub, you get 4 minutes of torch heat, which is twice as much. Make sense?

So, to get the hottest water out of a tankless, reduce flow to where the tankless is able to fully heat the water. This is counter-intuitive, but it really works.

On showering water into a tub. That doesn't work. The shower drops will cool off. That is, after all, exactly how a sparger (cooling tower) works. You need to use a spigot, or at least, a showerhead on a hose which you set in a bucket.


Tankless GPM ratings refer to the flow rate for which the unit can maintain its set output temperature. If you click through to the PDF Specification Sheet on the site you linked, you will see that there are several variables that contribute to the GPM you will actually achieve. The GPM rating is a lot like a car manufacturer's MPG rating... it uses certain assumptions to create comparable figures across different products, but individuals certainly see varying results.

The flow rate your house can provide, given your supply PSI and plumbing, will result in the highest GPM you could see at the tub. Your tub fixture will most likely be a more limiting value. After that, you need to consider the input temperature (cold side) and desired output temperature, to see if the unit can keep up. Look at the Temperature Rise chart on the spec sheet to be sure you will get the amount of water you want, heated to the temperature you want.

I have a dedicated tankless unit serving our bathroom. I set the temperature to about 110 because I want the water to come out nearly shower-temperature. (There's no point in heating water up just to mix it with cold to cool it down). But when I'm filling our tub, I turn the temperature up at the control panel, because I want more water and I want it hotter.

I think if you study the Temperature Rise chart and understand what it means, you will be able to see what is going to happen in your particular situation.

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