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I'm debating installing a mixing valve on my shower to hopefully eliminate the routine scalding associated with taking a shower. I've read a bit online but I have questions.

  • I haven't seen a P&ID of an installation. With a two valve system, should the mixing valve be installed upstream of the hot water valve and cold water be plumbed in like normal?

  • I have seen advice suggesting check valves. Do I need them on both sides or just the cold side?

  • Any recommended brands or styles?

  • Any other tips?

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  • Where are you? What kind of water heater do you have: central tank, central tankless, or point of use? Sounds like you are thinking of combining components to get a system that performs well, b ut it really doesn't work that way nowadays. You install a single integrated system which meets the plumbing code. Do you want to redo the tile on the control wall? Is this a shower or a tub/shower with diverter?
    – Jim Stewart
    Commented Feb 22, 2017 at 1:28
  • Central tank. And it's a tub/shower with a total of four valves. Two for the shower and two for the bath. What would you suggest? I have access to the pluming from an access panel behind the faucet.
    – mreff555
    Commented Feb 22, 2017 at 1:31
  • So the tub and the shower have totally separate valves; no diverter, right? Interesting, I've never seen that or I don't remember. Is this in the US? You could leave the tub valves as is and only change the shower valves. What is the stem-to-stem spacing between the valve handles for the shower? I think the normal in the US would be 8", and 6" (150 mm) in Europe. Is this an old bathroom so the construction is plaster over metal lathe w or w/o ceramic tile over that? Be a shame to demo that; maybe there is a way to save the tile.
    – Jim Stewart
    Commented Feb 22, 2017 at 2:43
  • The thermostatic bar mixing valve is common in Europe but available on a limited basis in the US. The ones I have seen require approximately 6" spacing between the supply pipes to the shower mixing valve.
    – Jim Stewart
    Commented Feb 22, 2017 at 4:03
  • I don't have a tape measure handy but I believe it's at least 8. Also you are correct. White tile on rock lathe. The home is moderately old. 1944. I was looking at the watts brand valves.
    – mreff555
    Commented Feb 22, 2017 at 11:55

2 Answers 2

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If your existing shower is a two handle type (independently controlled hot and cold) the simplest solution would be to install a temperature actuated flow reducer. This devise can be added inline between the shower head and the shower arm. It will reduce the flow out of the shower head as the temperature gets too hot. This will give you time to adjust the temperature before getting scalded.

The best solution is to replace the existing shower valve with a modern pressure balanced one. This type of valve will adjust the flow of the hot if there is a drop in pressure on the cold side due to a toilet being flushed. This would require you to open up the wall in order to replace the valve (easy if the back side of the shower wall is accessible).

Update: Some manufactures sell a cover plate for converting two (or three) handle shower valves to pressure balanced ones. The cover plate conceals the leftover holes in the tile from the old valve. Here's a clip from Ask This Old House on installing one.

If you’re not able to replace the valve, but the water lines are accessible (in an attic or basement), you could install a point-of-use thermoplastic mixing valve on the hot line that feeds the shower valve.

You can also adjust the temperature of your hot water tank, however you don't want to set it too low as you would increase the risk of pathogens.

Lastly, adding check valves will not help if the issue is pressure balance.

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  • Thanks, I wanted to avoid replacing the tile. I would prefer to keep my water heater >=140F to prevent pathogens. I do have access to the piping. I was going in the direction of adding a thermoplastic mixing valve. The questions I asked above were in regard to that.
    – mreff555
    Commented Feb 22, 2017 at 2:05
  • So would this be a thermostatic valve in the hot water line to the shower valve. This would involve mixing in cold water with very hot water to go to the hot valve of the shower valve pair. This would reduce the temperature of the hot water to the shower mixing valve so that temperature swings would be greatly reduced. The temp of the hot water might be limited to say 110 F and this would be mixed with cold water at say 60 F to get to a comfortable temperature for a shower. Is the piping in the wall copper tubing?
    – Jim Stewart
    Commented Feb 22, 2017 at 3:28
  • Yes. All the pipes are soldered copper.
    – mreff555
    Commented Feb 22, 2017 at 11:56
  • So are you thinking of having the mixing valve accessible from the shower side or by opening an access panel on the back side? Do you plan to adjust the mixing valve only monthly or seasonally as the temperature of the cold water changes? Is this a thermostatic mixing valve? supplyhouse.com/…
    – Jim Stewart
    Commented Feb 22, 2017 at 15:11
  • Regarding the location of the thermostatic mixing valve: can be installed anywhere on the hot line that serves the shower valve. The cold would stay connected as is with the exception that a tee would need to be installed in order to connect cold to the thermostatic mixing valve. However, if you are experiencing scalding due to pressure balance issues, installing a thermostatic mixing valve will prevent the scalding but will not stop the nuisance of fluctuating temperatures.
    – pdd
    Commented Feb 22, 2017 at 19:44
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I followed the link to the Watts valve you are considering. The link states that some of these Watts valves are intended for installation only at the water heater or boiler and are not suitable for final tempering of water at a fixture. So one approach would be to locate some tempering at the water heater to take water at 140 F down to say 115 F in the hot water supply lines, then you would have a fixture or another tempering valve to insure that the shower could not deliver water over say 105 F.

You really need to check with plumbing code enforcement in your area and see what they think is best and what they will approve. Even if you would cobble something together that would work for you, it might not pass inspection when you get ready to sell this house. It's far better to do something which works for you and satisfies the code.

A Word about American Society of Sanitary Engineering (ASSE) Standards 1016, 1017 and 1070: ASSE 1016 covers the delivery of water at the individual fixtures that are adjusted and controlled by the user. Therefore, it addresses very precise and immediate temperature regulation requirements. This standard covers three types of valves: Pressure Balancing, Thermostatic and Combined Pressure Balancing/Thermostatic. Watts ASSE 1016 listed valves meet the thermostatic requirements of this standard.

Modern commercially available temperature and/or pressure balancing shower valves are designed to both work satisfactorily and to meet plumbing codes requirements.

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  • It's likely that your current plumbing is in compliance by being grandfathered in, but if you modify it and do not bring it up to current code, then it will not be in compliance.
    – Jim Stewart
    Commented Feb 23, 2017 at 12:41

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