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Community and Q&A

How do I reconcile an electric, tankless water heater and low flow faucets?

dacremodel9 | Posted in Mechanicals on

I have installed my first tankless electric water heater. It is an RP27PT single phase Powerstream Pro Redring, made by BBT North America Bosch Group. It works fine at the washing machine, or, if you turn on multiple valves but any one, by itself, will not activate the heater. After a couple of hours on the phone with tech support, I believe our low flow shower valve, vanity faucet or kitchen sink faucet, do not allow enough volume of water to pass through the heater to activate it. We have 45psi water pressure at the heater. California requires a min 20psi in muni systems so I would think we should be within an acceptable range. Any advice?

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  1. Riversong | | #1

    The most water-conserving residential fixtures (EPA WaterSense) should have a flow rate of no less than 0.8 gmp at 20 psi, which is the minimum flow rate to activate your Bosch heater.

    If you're using ASME A112.18.1 standard commercial fixtures (max. 0.5 gpm), then you might have a problem. If not, then it seems you have a problem with the heating unit or the installation.

  2. homedesign | | #2

    David, Are you saying that even with no cold water turned on and you turn hot on all the still get no hot water?

    I have noticed that with my low flow shower head if I open the cold too much that it sort of overpowers the hot side and restricts how much hot gets thru and therefore the tankless shuts off.
    If I am careful not to crank up the cold too much then no problem.

  3. dacremodel9 | | #3

    John, Yes, I am only using the hot water valve. And it is not in the shower only but at any valve in the house except the washing machine.

  4. dacremodel9 | | #4

    Robert,you sound more knowlegable than I so I hope I don't test your patience too severely. We have residential fixtures sold from a retail show room in California. Moen Chateau tub/shower valve, Price Pfister t533-5ss kit sink faucet, Moen TL462 lav faucet. We have removed the aeraters and restricters and get a gallon of water in less than a minute through the fixture. We have approx 45psi at the house. The supply goes through a water softener before it gets to the water heater but Culligan assures us that the softener will not significantly reduce the flow. I interpret your answer then to leave me with a problem with the unit or the installation. Did I hear you correctly?
    Thanks for your time and trouble.

  5. Riversong | | #5

    Though I install plumbing and teach intro plumbing, I am not a licensed plumber (I am a master builder). But I would say that, if you're getting little more than 1 gpm with aerators and restrictors removed, then you're not going to get 0.8 gpm (minimum flow for heater activation) through one faucet.

    45 psi at the water supply should be adequate (normal well pressure range is 30-50 psi). But you most certainly will have a dynamic pressure drop across the water softener (can be as much as 15 psi). I would suggest testing the water pressure at the heater input with water flowing at various rates. A doubling of the flow rate will quadruple the pressure drop.

    Are there long plumbing runs to the fixtures? Were large-diameter trunk lines used or was 1/2" pipe run all the way? Is it a conventional branched supply system or a parallel (home run) system? Copper or PEX?

    If pressure drop is not the culprit, then another possibility is cold/hot crossover somewhere in the system (is there a tempering valve anywhere?).

    When I said that the problem might be in the installation, I meant the installation of both the DHW system and the entire plumbing system. It all has to work in harmony.



    We often will put an expansion tank between the water softener and the demand water heater to mitigate the pressure drop across the water softener. Keep it close to the water heater to optimize the effect, I think this will solve your problem. The cold water to the house can be on either side of the tank and a 20 gallon unit should get the job done fine, since it's on the cold side it doesn't need to be one of the little heat resistant ones we use on radiant and solar systems but can be the regular big blue one like what you would use on a well pump.

  7. sheeschen | | #7

    Just wanted to write in and say I had a similar problem (just faucets, not shower valves, though). Rinnai said .6 gpm should be enough to initiate the burner, but a .7gpm aerator didn't work. Ended up putting on a 1gpm aerator and that worked well enough. I guess there's some variability in the flow-rate sensors.

  8. dacremodel9 | | #8

    This is a response to some of your most appreciated suggestions. To start with, the residence is supplied with a 1” copper line. The water softener is on that 1” line and approx. 2’ beyond the softener it reduces to ¾”copper. 8’ beyond the reducer is the water heater. The shower is directly above the heater, approx. 7’ from heater to shower valve. The lav is approx. 9’ from heater to faucet. The laundry tray is approx 17’ and the wash machine is the same. The kitchen sink is about 30’ from the heater. All runs are ¾” copper under the floor and reduced to ½” at the vertical run to the fixture. All the copper is newly installed as a part of the project. All flow restrictors, (that we can find), have been removed from the fixtures. This morning’s test seemed to indicate that everything was working except the kitchen sink. The kitchen sink would run hot for a minute, sometimes two, then the heater would shut off and it would run cold. If the valve was allowed to continue running, the heater would eventually start up again and heat the water. This intermittent on and off of the heater was only at the kitchen sink.

    Michael suggested an expansion tank between the softener and the heater. I am ready to try this but do not understand the dynamic. What is the function of the expansion tank? Why would it help? What is happening now that is causing the on/off cycling of the heater. How would the expansion tank remedy that problem?

    Thanks all for your consideration.


  9. Steve Konstantino | | #9

    Tankless electric water heaters make little sense. Their objective is to save the standby losses common with tank style heaters. The unit you have installed draws 26,000 watts (120amps) to keep up with demand. It makes more sense to use a super-insulated Marathon tank with 3800 watt elements. The standby losses are minimal and the peak electric draw is 85% lower than the tankless.

  10. David Anderson | | #10

    Good point Steve, I agree with you in most situations. We have a fairly unusual situation here. It is an office/residence. The office is set up for two people to be used year round. The residence is intended for an intern, grad student learning the trade, 3 months of the year. This dairy operation has been using this intern system successfully for several years and is committed to continuing. So, for 3 months of the summer we need the full service of shower and washing machine, while for the remaining 9 months all we will need is the occasional hand washing and coffee maker cleaning. It seems like a worthwhile experiment in this situation. We will track its effectiveness over time. Thanks for your response.

  11. Jerry Harman, GAR | | #11

    Tankless water heaters are notorious for this low-flow / single faucet problem. .8 gpm out of a faucet will not activate most tankless units because that's NOT .8 gpm of just hot water - you're tempering it with cold water. My understanding is that a circulator pump will correct this issue. Navien in fact includes a pump in several of their tankless models. There are also after-market circulator pumps that don't require a separate dedicated return line. is one brand and they have a good description of the problem on their website

  12. Jerry | | #12

    I have the same problem with my electric tankless heater. To get the flow rate when you take a shower, you can turn on some hot water in the sink also. Thus, you get increased flow. Yeah, ok that is stupid, but it works. Another solution is to turn down the heat, so that you can increase the percentage of hot water through the mix. My heater has three elements. I turn off the breaker switch for the first element and let water flow only through #2 and #3. In the summer this works just fine.

  13. wjrobinson | | #13

    More evidence that tankless is simply wrong for most residential living.

  14. user-659915 | | #14

    "So, for 3 months of the summer we need the full service of shower and washing machine, while for the remaining 9 months all we will need is the occasional hand washing and coffee maker cleaning."

    Why oh why are the simple, obvious, dare I say natural solutions so often ignored? Are we really trying to minimize our resource use in supplying our home energy needs or are we trying for the greeniest gadget user award? Unless this is a medical or food processing facility hot water is NOT required for occasional handwashing, nor for coffee maker cleaning. A tank water heater used for the three-month internship could just be SWITCHED OFF for the remaining nine months of the year.

  15. GBA Editor
    Martin Holladay | | #15

    You're almost right -- except you must live in a mild climate. Here in Vermont, my water enters the house at about 35 or 38 degrees F all the way through May. It's possible to wash your hands in water that cold, but it really hurts if you are trying to be thorough or if you just came in from the outdoors.

    One solution is to install an uninsulated water tank upstairs, where the water can be tempered to room temperature before being used for hand washing. Of course, it still takes energy to raise the temperature of the water from 35 degrees F to 70 degrees F. That energy would need to be supplied by your space-heating system.

    I imagine that in California and Florida, water doesn't enter the house at low temperatures during the winter. When I visit warm climates, I'm always amazed to discover that there is no cold water tap. Instead, there is a hot water tap and a warm water tap.

  16. user-659915 | | #16

    Martin, good point. We're so spoiled here south of the Mason-Dixon. The simple fix: a small electric instant heater like this: Place it right under the sink and you'll save energy and water too (no standby OR run-out losses).

  17. Carlos | | #17

    The probeme is the activation flow for the Model/Brand tankless you got. Most tankless water heaters activate at .50 or .75 gpm, all low flow shower heads and aerators are rated Showerheads 1.5 GPM @ 80 PSI and low flow aerators at 1.0 gpm @ 80 psi, the lower the psi the lower the flow, so at 45 psi you have you are most likely below the activation flow for your tankless brand.

  18. trish | | #18

    We just installed a Rinnai V53e, the smallest unit available from Rinnai, because we live in South Florida where the incoming water temperature feels like it is >80F, at least in the summer. We use low flow shower heads because we believe in water conservation. The problem is that if we only run one shower, the hot water does not turn on. The unit is set to heat the water to 120F. Has anyone tried lowering the temperature setting on the water heater so that a greater proportion of the hot water delivered goes through the heater? I haven't measured our water flow from that shower head, but it is supposed to be 1.5 gpm and no hot water comes through when the knob is turned to all hot.

  19. BobHr | | #19

    Have you checked any shut off valves before the facuets. The valves can get hard water buildup which will reduce the water flow.

    I am not a plumber but I helped a friend install a new kitchen facuet in a 20-25 year old home with copper pipies. He did have some water restriction issues at the facuet. I pulled the shutof under the sink and the hot water was 75% restricted and the cold about 50%. A few months later I installed new counters,sink and faucet. My home is about 18 years old. Though not as bad I did notice a considerable amount of restriction.

    Another thinkgto try is to bypass the water softner. My water softners I have seen have the plumbing setup with a bypass allowing for water flow around the softner with just the switching of a couple of valves.

    I have heard of some complicated setups to make tankless systems work particularly if you have newer antiscald valves. I have heard of a small tank heater after the tankless. That way you get the instant hot water of a tank system and the benefit of the tank system. Another benefit of a tank after the tankless is that the antscald valve doesnot activate if there is hot water in the pipe followed by cold that hadnt yet been heated by the tankless.

  20. Chris | | #20

    I recently installed a Richmond tankless unit and when I completed all the plumbing the thing worked great from all my faucets. One thing to note is that I had my water softener looped or bypassed when I first tried it. When I put my softener back in service things went to hell in a hand basket.

    Talking to the company and reading this thread it appears as though in my case my softener is reducing the amount of flow or pressure required for the system. Since I have hard copper plumbing it isn't that easy to install something to take a reading. Now it appears as though I will have to add a heat pump or pressure tank.

    Hope this helps anyone else scratching their head for hours, try bypassing your softener if you have a loop and see if this is the root cause of your problem. Good Luck!

  21. David Meiland | | #21

    No one has mentioned measuring flow at the fixtures by simply putting a bucket under the faucet, turning on the hot water, and seeing how much comes out in 60 seconds. The paint store here sells a 1 gallon plastic tub with graduations marked on it. My tankless needs .62GPM to fire, and that's not a lot of water--I checked flow from my fixtures before buying the unit. If yours won't fire with the washing machine filling then it's hard to see how a faucet will do it.

    If you're on a well with a pressure tank, the variations in pressure can confuse some of the tankless heaters. Our pressure tank runs from 40 to 60 PSI, and when the pump kicks on and increases pressure in the lines, it can cause temperature variations from the heater. I installed a 40PSI regulator in front of the heater so it never sees higher pressure and now it's consistent.

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