The title is a little misleading. I know why it’s illegal, but I think what’s really important is the end result (users, especially children, don’t get burned). I’m curious why it isn’t equally as legal to turn the temperature up as high as supported by your water heater, but use a thermostatic mixing valve to regulate the temperature to 120°F. With this approach you would effectively increase your hot water capacity without having to install a larger tank.

  • How would raising the temperature increase your hot water capacity?
    – Gunner
    Jun 4, 2020 at 1:04
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    Because you’re mixing it with cold water to cool it down. If the water is warmer then you need less of it and more cold water. This is assuming you’re not actually using the water at the highest possible temperature.
    – Jordan
    Jun 4, 2020 at 1:08
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    Furthermore, there may be usage points (such as dishwashers and clothes washers) that want, or maybe even need (due to listing instructions, especially if a single HWH is serving a mixed occupancy) water heated beyond 120degF...using thermostatic mixing gives you the flexibility to provide multiple temperatures instead of forcing the use of booster heating Jun 4, 2020 at 1:19
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    Since when is it illegal to adjust your water heater thermostat? Who told you that? I have always cranked mine up to increase capacity... and I do have "scald-guard" type shower fixtures btw... Jun 4, 2020 at 1:37
  • 3
    Where is it illegal? Jun 4, 2020 at 2:07

1 Answer 1


You can do this, but not as completely as one'd want to

The approach you describe (running a tank full-hot and then downmixing it to a scald-safe temperature using an ASSE 1017 central thermostatic valve) is indeed permitted by the Codes, although it does not negate the need to provide scald-safe fixture valves at showers and baths.

However, one drawback of this approach is that it limits the protection from Legionella growth provided to the hot-water piping system. In most applications we consider here, that's not a huge drawback; however, it does mean that care should be taken to avoid "dead legs" of hot water plumbing, as those are common hangouts for pesky Legionellae.

The ultimate system uses an anti-scald mixing valve feeding directly into an anti-scald fixture at baths and showers, and an anti-scald mixing valve at bathroom (and perhaps kitchen) sinks as well, allowing untempered water to be fed to clothes washers and dishwashers to minimize the need for booster heating. This is more commonly done in the more complex commercial/institutional plumbing environment, but is theoretically doable for residential work as well, although the standards for mixing valves are in a bit of flux at the moment.

  • Surely, having the water at a higher temperature would reduce the risk of Legionella growth? Or have I missed your point entirely? (very possible, I've only been awake for an hour)
    – SiHa
    Jun 4, 2020 at 5:55
  • @SiHa -- you missed my point -- it's that using a mixing valve at the tank limits the zone of protection against Legionella to the tank Jun 4, 2020 at 11:49
  • "at the tank" Thank you - that was what I was missing. I told you it was early.
    – SiHa
    Jun 4, 2020 at 13:42

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