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Community and Q&A

Limescale in Heat-Pump Water Heater

Jeff Cooper | Posted in Mechanicals on

I had assumed that a heat-pump water heater would produce much less limescale than a conventional electric water heater, because one of the partial remedies for elements that get coated with limescale and deposit piles of it in the tank is a low-watt-density element that spreads the heating over a larger surface area.  A hybrid water heater like the Rheem wraps the heat-pump heating coils around a huge surface area of the tank, so I figured it must produce less limescale when running strictly in heat-pump mode, but no one at Rheem is willing to say it does, so I’m beginning to question my assumption, and with my extremely hard water (740 mg/L), I don’t want to spend $1400 on a water heater only to have it ruined in a few years.  In a conventional water heater, the limescale falls off the elements as flakes that can be scooped out of the bottom of the tank quite tediously.  (They don’t flush out.)  If limescale forms on the inside of a heat-pump water heater tank, I wonder whether it will just be a solid coating that can’t even be scooped out.

A water softener is out of the question, because I can’t stand the slippery feeling, and using all of that salt is problematic.  There’s a TAC water conditioner that leaves the minerals in place but converts them to a non-clinging form, and a couple of reputable studies (one at ASU and one in Germany) have found it better than 90% effective at preventing scale (unlike the magnetic and electronic devices), but it’s around $600 plus very expensive media that has to be replaced every 3-5 years.

I only use hot water at the kitchen sink and for showers, no laundry or dishwasher, and it’s just me.  My house is all-electric; natural gas is not available, and propane is prohibitively expensive.

Any thoughts/knowledge about limescale in a heat-pump water heater or suggestions as to the best solution?

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Replies

  1. Wannabegreenbuilder | | #1

    I feel your pain. I have hard water to and didn't want to soften with salt for a variety of reasons. I bought a TAC system and only use it for the portion of water to be heated as that is where the scale deposits are a problem. It's too soon to give you a report on how it worked. Perhaps in the future.

    1. Jeff Cooper | | #3

      Thank you. Good reminder to use the TAC only on the input for the water heater, especially given the expense of the media.

  2. Trevor Chadwick | | #2

    Been there, in the past I've removed 5 gal buckets of scale from my water heater.
    You're gonna have to bite the bullet and run a water softener, even if its only for hot water, or plan on replacing the tank as part of regular maintence.

  3. Jeff Cooper | | #4

    A TAC conditioner is highly preferable to a softener for me, at least, as I find softened water truly awful. For anyone reading this who still has a conventional tank, I found that the best way to remove the flakes of limescale is to attach a piece of tubing that will fit through the lower element's hole to a shop vac. You still need a stick or something similar to move the flakes to where the tubing can reach them, and it's still quite tedious, but it's much better than scooping them out without the suction.

  4. Expert Member
    Akos | | #5

    I don't think that there is anything out there that has more pseudoscience equipment than water scale treatment systems.

    That one that seems at least based on science is the zinc electrolytic systems. There doesn't seem to be much out there for residential though with proper electronic controls, I'm not sure I would trust that passive ones to work in the long term.

    One option is Ranni's scalecutter. Not cheap, but I would assume it work (or at least helps) since these folks know water heaters. Looks like there is also a 3M Aqua-Pure part that is much cheaper.

    1. Jeff Cooper | | #7

      Thank you for recommending the 3M filter. Strangely, neither the 3M website nor any of the retailers provide any data on how much it reduces scale.

      Here's the ASU study showing TAC conditioners reduce scale by 99%:
      https://www.uswatersystems.com/media/pdf/ArizonaStateUniversityWaterSoftnerStudy.pdf

      I checked another link to the study to make sure the one above is unaltered.

      This report from the City of Scottsdale has similar data and more info:
      https://www.scottsdaleaz.gov/Asset64369.aspx

  5. Christopher Welles | | #6

    Dosing polyphosphate into the water is a technique that a lot of home water treatment systems use. I haven't spent much time looking into the cons of that approach since I just stick with a water softener, but it's supposed to be effective at preventing scale.

    1. Jeff Cooper | | #10

      Thank you. I haven't seen much about that; I'll check it out.

  6. Roland Daoust | | #8

    Roland,

    Seems to me that some brands of heat pump water heaters have their heating elements on the outside of the water tank....

    1. Jeff Cooper | | #9

      The Rheem hybrid water heaters wrap the heat-pump heating coils around the outside of the tank. Whether this prevents scale is the essence of what I've been trying to find out. Rheem won't say it does, and given what a plus it would be if it did, I expect they would say so if they could. They also have conventional elements inside the tank as a supplement. I expect other brands have essentially the same configuration.

      1. Charlie Sullivan | | #11

        You could imagine the scaling vs. temperature working different ways.

        1. Most of the lime drops out between 100 F and 140 F. Going up in temperature doesn't matter much because it's all dropped out of solution already.

        OR

        2. What you get with the water at 140 F is just the tip of the iceburg. As you got up hotter and hotter, the amount dropping out increases exponentially.

        If it's 1., the heat pump water heater will have just as much problem as anything else. If it's 2, the heat pump water heater will almost completely solve the problem.

        Here's a plot from a 1940 study that shows that it is number 2. Thus, you should be fine. Note that the X-axis is the exiting water temperature of a gas-fired heater. The element temperature is much higher.

        https://inspectapedia.com/plumbing/Purdue_74_Scale_Rate.jpg

  7. Jeff Cooper | | #12

    Thank you, Charlie, but if you could elaborate, I might better understand your reasoning. My current, conventional water heater heats to 120 and, with standard elements, produced enormous mounds of limescale flakes till they piled up so high, they broke the lower element, which prompted me to replace both elements with low-watt-density ones whose effect I don't yet know. The graph you cited and similar info I've seen elsewhere indicate that I would have accumulated scale much faster at, say, 140, but even at 120, I accumulated too much, and the water where I'm building my new house is much harder than where I am now.

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