How are mixing valves installed? Do they affect just the hot water line?

So my basic understanding is that the valve connects inlet to hot and cold, but outputs only hot and you set it to your desired temperature and GPM.

So, for example, if you set it to 120F, then the water in the hot line will always be 120F, in theory.

However, when I take showers I find that as the shower goes on I have to adjust the hot-cold balance constantly. So, does this mean that as the temperature from the tank goes down, the GPM from the hot goes down so the overall water stream cools?

  • Are you talking about the thermostatic mixing valve on a water tank or a shower diverter? Or is the question about the interaction between both?
    – Joe Fala
    Apr 1, 2019 at 13:59

2 Answers 2


The purpose of these thermostatic (static: constant, thermo: temperature) mixing values is to give you a controlled-temperature output even when the hot and cold input temperatures can vary.

When you take a shower, the temperature can fluctuate because of the cold water (water from pipes in the walls vs water that just came in from underground) and because the water heater is "running out" of water and getting cooler. The value works by mixing cold water in with the hot to give a constant output.

So, let's say you set the valve for 120, and have hot water at 140 and cold water at 60 degrees. I'm going to make up some math on the fly and say that the valve will mix about 80% hot water with 20% cold to cool it off down to 120. As the incoming temperature drops, it uses less cold. So when the water heater gets down to 125, it's going to be more like 95% hot and 5% cold. Finally when the heater is down to 110 degrees, you're going to get full hot water and then you're left on your own to adjust the shower handle.

The speed at which the adjustments happen will be device dependent an "adding hot water to the cold" or "adding cold to the hot" is really just two ways of saying the same thing. Also I believe that some models only have one "valve" to change the amount of hot or cold and others have two separate valves (maybe for greater temperature swings or faster action?).

  • Okay, so the constant adjustments I have to do on my current shower is that because I have a sucky pressure balancing mixing valve, and if I get a thermostatic valve, I will stop having to micromanage the shower temperature, or will I still have to adjust the knobs during the shower even with a thermostatic valve (as opposed to a pressure balance valve)? Mar 1, 2019 at 19:45
  • The pressure balance valve is to make sure you don't get scalded if the cold water pressure drops drastically and isn't really designed to make sure the temperature stays the same. The mixing value is mostly sold so you can set your water heater temperature to something over 120 degrees (to kill legionnaires disease) but also prevent scalding by making sure you only get 120 degrees or less at the point of use (showers, sinks). So, the mixing valve will make your hot water a constant temp, but really only if you set it lower than the the water heater.
    – JPhi1618
    Mar 1, 2019 at 20:36
  • Just note that it's not safe to have your heater over 120 unless these mixing valves are installed at all points of use except for maybe the washing machine and dishwasher (not sure? These are more common in other areas, but not regularly used where I'm at, Texas).
    – JPhi1618
    Mar 1, 2019 at 20:37
  • @JPhi1618 well it's not safe to leave a storage heater below 130, either. So pick your poison: legionnaire's, scalding, or having to spend $75 on a new faucet. Mar 2, 2019 at 14:50
  • In my case, there is a thermostatic valve at the water heater specifically to let it run hot but produce safe output, and mixing valves at point of use to let the user select temperatures below that maximum.
    – keshlam
    Feb 24 at 14:44

They mix the hot water and cold water to give a mixed water. Some cheep ones react slow and fluctuate output at steady flow rates. Cheap one cost 75 and good ones cost 150. For extra 75 I don't know why people use cheap ones.

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