Tankless electric hot water heaters

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    • #276821
      Rachael Clark

      Hi, first time on the forums so bear with me…

      I’m looking to replace an old water heater in a small “granny flat” area. The current heater only lasts for about 10 mins of water in the shower, and whereits mounted, a kitchen is going to be built, so the heater must be put in a cupboard.

      Although the current heater is only small, its too big to fit in the cupboards that will be installed.

      So, this got me thinking to tankless systems. I really dont know much about them, so i was wondering if anyone could give me some information? Firstly, the only main disadvantage with the electric systems that ive heard about is a low flow rate, but are there any others compared to a standard storage hot water heater? Keep in mind that the current heater is not too crash hot, so we’re not fussy here

      Secondly, what about size? Physically, it would be great to get the smallest system possible, to save on space, but I’d like for the system to be able to support the number of faucets. These are: 1 shower, 1 bathroom sink, 1 small kitchen sink, and possibly 1 washing machine. I can safely say that 90% of the time these will not be run at the same time (maybe just the kitchen sink and the bathroom sink, occasionally). So what im getting at (eventually) is, what size would I need to support these taps (dont worry about the washing machine) and get a decent flowrate, etc?

      Last question is: can anyone give me some names of companies that make (good) tankless water systems, that would be good for the scenario above?

      btw, it has to be electric.

      Thanks very much

    • #294837
      Retired plbg1
    • #294838

      I will try to give you a practical answer top your question about flow rate. The hot water demand (in gallons per minute) of various fixtures is: bathtub=3.6; Dishwasher= 1.5 (and less if you have a dishwasher with an electric heater option); kitchen sink= 1.6; lav=0.3; shower=2.5; and clothes washer=3.3.

      Your setup requires 2.5 + 1.6 + 0.3 +/- 3.3. Depending on whether or not you plan to run your washer with the other fixtures, you will need a unit capable of delivering at least 4.4 gal/min, and perhaps as much as 7.7 gal/min.

      As I ststed in an earlier post on this subject, you need to calculate the temperature rise that your unit is capable of (temperature rise is defined as the number of degrees (usually Fahrenheit) difference between the inlet water and outlet water temperatures (at the maximum related flow rate of the water heater). Thus, a heater may be rated for a 70 degree temperature rise (that’s a fairly common specification), which means, for example, that the heater can deliver 120 degree water from a 50 degree input at the unit’s specified maximum flow rate. All of this should be posted on the manufacturer’s specs for the heater.

      One thing I must mention is that the manufacturer’s specs assume that the unit will be installed in a well-designed system that allows for good water flow. If you have an old house with a low pressure supply, clogged supply pipes, or blocked fixture outlests, you will still have problems if you merely replace your conventional water heater. Indeed, a well designed plumbing system with a good tank-style water heater should provide adequate hot water, especially for the minimal demand you describe for your system. If it doesn’t, you have further problems that may need more than replacement of your conventional water heater with a tankless one.


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