To an existing storage-tank (conventional) electric water heater, I wish to add a thermal expansion relief valve (in lieu of an expansion tank) and a mixing (aka. tempering) valve, so that the temperature may be set at 160-degrees or so for add'l hot water capacity.

The water heater is in the basement, and currently both the cold and hot connections are soldered 3/4" rigid copper pipe going straight up from the tank connections into the two living floors of the house (with a shutoff valve on the cold inlet pipe).

I propose modifying the plumbing as shown here.
enter image description here

The thermal expansion relief valve will probably be: https://www.supplyhouse.com/SharkBite-25704LF-3-4-x-3-4-x-1-2-TER-1-Thermal-Expansion-Relief-Valve-Lead-Free which incorporates a shutoff valve (so that the existing one may be deleted). The mixing valve will probably be: https://www.supplyhouse.com/Resideo-Braukmann-AM101-SB-1LF-3-4-Union-Push-Connect-Mixing-Valve-3-9-Cv-Lead-Free

First question, is this a suitable configuration ?

Second question, what is the best way to make the connections involved ?

I am fairly comfortable with soldering (sweating) copper pipe, and there is plenty of clearance; however, I think it'll be difficult, because I'll have to deal with water trickling down from all the house plumbing above.

The proposed parts have push-to-connect (aka. Sharkbite) connections. That should be a simple way to make the connections. But is it the best way ?

On the cold side, since the pipes coming from above are rigid, it may be impossible to fit the two push-to-connect parts (thermal valve and tee) in the vertical stack above the tank's cold water inlet. So I wonder if I'll need to use a flexible connector between the cold-water inlet and the bottom of the tee.

On the hot side, I'm confused about whether it's possible to remove the push-to-connect fitting from the mixing valve (the product text suggests that it is, and see the image below) and use a flexible connector with MNPT at the the top end to go directly into the mixing valve (instead of two push-to-connect joints with a short piece of pipe between).

enter image description here

I also wonder if I should consider using compression joints instead. I am comfortable with PEX (barb & crimp) and have the tools, but I prefer to avoid the flow restriction involved there (the house has 3 bathrooms). So I'm pretty confused about what's the simplest and most reliable way to connect all these parts, and seek advice.

  • Most hot water tanks should already have thermo relief valve on them. Mixing valves are usually placed at the sinks/tubs where needed, so if you need hotter water somewhere(dishwasher) you have it. If at the tank just reduced the temperature of the tank.
    – crip659
    Commented Oct 10, 2022 at 23:27
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    The overpressure should be on the outlet line
    – Traveler
    Commented Oct 11, 2022 at 1:18
  • @Ruskes All of what I'm seeing says it should go on the cold/inlet line (don't think I can add a pic in comment). Commented Oct 11, 2022 at 3:10
  • This house definitely does not have thermal relief. It's one of a number similar houses in these parts and the lack of thermal expansion (be it tank or valve) is well known. Commented Oct 11, 2022 at 3:12
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    Oh, of course there's a T&P valve. Here I'm talking about the "thermal expansion relief valve" which is an alternative to the expansion tank recommended in any closed system. Commented Oct 11, 2022 at 18:08

2 Answers 2


Question 1: Is this a suitable configuration?

The component arrangement of your drawing looks okay. It is always best to review the manufacturer's instructions for whatever thermal mixing valve you use.

The instructions for the valve you linked to are here.

The drawing they provide is slightly different as they show a system with a recirculation line which I assume you do not have.

Question 2: What is the best way to make the connections involved?

There are pros and cons to all the different connection types, and the best one will depend on the use case and personal preference. Old-school plumbers will mostly say soldering is the best (but that is somewhat biased as it requires the most skill).

Sharkbite fittings carry a 25-year warranty and being that this work would be exposed, there isn't the risk of a future leak of a concealed joint causing unseen and unknown damage.

I am a plumber by trade and have access to all the tools of the trade and yet I still have a few Sharkbite fittings in my house.

As for physically installing the thermal expansion valve on the cold side, you can use a slip coupling that is extra long and allows for it to be 'slid up the pipe' to make room to install the valve, and then be slid down to its final location.

  • I just got a chance to look at this (vacation place) and there is an 'L' in the copper pipe above the water heater - the pipes run vertically to the ceiling joists (in the basement) and then an elbow to a short horizontal run of pipe. I imagine that'll give all the flexibility I need. Commented Oct 14, 2022 at 3:51
  • Tempted to remove the cold pipe from the water heater and solder in the tee for the pipe over to the tempering valve and then use the Sharkbite thermal relief valve to make the coupling. How risky to try to unscrew the pipe from an old water heater ? I imagine rupturing the tank by twisting out the fitting, by I'm paranoid by trade ... Commented Oct 14, 2022 at 3:54
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    It should be fine as long as you back-wrench to eliminate any torque into that tank. If the tank is older, you should take this opportunity to check the tank's anode rod and replace it if needed.
    – pdd
    Commented Oct 14, 2022 at 17:34

There's a neat plumbing fitting called a union that is designed to solve this type of impossible to assemble arrangement by allowing pipes to be joined without rotating them or moving them axially. enter image description here

In this photo the brass nut is connected to neither end of the fitting, instead it works to clamp the two parts together, when loosened it is free to move away from the join allowing the pipes to be separated laterally.

You can either fit them inline, or choose a tempering valve that has unions instead of sweat or compression connections.

enter image description here

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    it does require some precision cutting off the pipes to the right length and soldering.
    – Traveler
    Commented Oct 10, 2022 at 23:51
  • @Jasen I'm sorry but I'm still confused about how the unions work. Do you solder pipe to the pieces of the union that get clamped ? If so, why add that step instead of just using compression fittings ? Or do the ends of your pipe go directly into them, and if so, how are they different than compression fittings ? Commented Oct 11, 2022 at 3:19
  • they solder in, before you tighten the nut the advantage is that the join withoin needing to move the pipe or the fitting much in the "stretch the pipe" direction. so you don't need the black high temperature resistant hose in you drawing
    – Jasen
    Commented Oct 11, 2022 at 3:25
  • Oh the black was just the best way I could show one of those braided-steel flexible water heater connectors. Commented Oct 11, 2022 at 3:43
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    prop it up with something, (eg a hook attached to a clamp on the pipe), or deform the pipe or fitting slightly
    – Jasen
    Commented Oct 19, 2022 at 0:22

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