So you just come home after an intense day at work, and now want to take a nice bath, but discover that you can’t get the right mix of hot and cold water from your shower. It’s most probably because of a little part that you might have never heard before, which is a mixing valve. Today, you’ll find out what it is that, why it fails to work, and how to test and fix it quickly.
What Is a Mixing Valve?
A mixing valve, as the name suggests, mixes the cold water with the hot water so that you don’t scald yourself. It does so by keeping your faucet temperature steady, even if anyone in your house uses another one. In it, there is a small check valve and strainer to stop debris and backflow from affecting the valve.
This adjustment screw lets you set your desired maximum water temperature. When you turn it, it moves the adjustment piston, which pushes down on the flow regulator to set the temperature. This gives you some control over how hot the water comes out of the faucet.
Then, there is a flow regulator that moves up and down to increase or decrease the hot water flow depending on the outlet temperature. It doesn’t move much, just a couple of millimeters, about the thickness of a nickel. But even that small distance is enough to give you the precise temperature changes.
The sensor in the mixing valve is the core of the operation. Inside is a wax element sealed in a copper housing. As hot water flows through the valve, the wax absorbs heat and reacts to the temperature it touches. At a certain point, the wax melts and expands more than it would in solid form. This expansion creates pressure to push the actuator rod outward. Then, when the water cools down, the wax hardens again and contracts, which allows the spring to push the rod back in.
Note: All of this science is not in the manual mixing valves, where you can just adjust the hot and cold water levers to set the temperature yourself.
What Happens When a Mixing Valve Fails?
Almost all the modern showers and sink faucets have some sort of mixing valves that mix hot and cold water. In older fixtures, you can do this manually with the turn of the handles, but the newer ones have a thermostatic mixing valves that use pressure balancing to get you consistent water temperature with better safety.
You can also install a mixing valve at the water heater outlet to control the temperature of the water that flows to all of your fixtures.
When these valves go bad, here is what you’ll witness:
1. Sudden Bursts of Hot or Cold Water
When you turn on the fixture and the temperature jumps back and forth, the mixing valve no longer balances hot and cold. That tiny part inside controls flow in a steady way, so once it slips, you get random bursts that are uncomfortable and unsafe.
2. Water Feels Too Hot
If the water from your shower or any other faucet that has a mixing valve comes out hotter than it should, the valve may not shut off enough hot water. Since water heaters reach up to 140°F, this is risky because your skin can burn quickly if the balance is not in place.
3. Water Never Gets Warm Enough
On the flip side, you might notice the water remains lukewarm even with the hot side fully open. That usually means the valve is stuck and allows in more cold than it should, which turns usage into a hassle and wastes energy.
4. Pressure Drops When Others Use Water
If someone flushes a toilet or runs another tap and the pressure at your fixture drops along with the temperature, that points to the mixing valve that has lost control. This is because a functional one always holds the flow steady, even when water is used elsewhere in the house.
5. Drips or Leaks Around the Valve
Another sign is that you see visible drips coming out from the mixing valve housing, which points to the fact that the internal parts are worn down or clogged. This puts the rest of your plumbing system at risk if it is left alone.
Can I Replace a Mixing Valve Myself?
Yes, you can replace a mixing valve if you know your way around a toolbox. Here’s what you’ll need: a screwdriver, pliers, a utility knife, a hex key, plumber’s grease, and a few new O-rings.
Replacing Manual Mixing Valve
If you have a manual mixing valve, the type you twist or turn with a handle, you can swap it yourself:
- The first thing is always to shut off the water. Most valves have shut-off screws you can turn with a flat screwdriver.
- Once the water is off, remove the handle by unscrewing it, take off the cover plate, and then you’ll see the valve body.
- Unscrew the brass screws on the sides and gently pull out the old unit with pliers. Be careful here so you don’t scratch the inside of the pipes.
- Before dropping in the new valve, hold something like a paper plate under the opening, crack the shut-offs open for a second, and flush out any grit that’s sitting in the line.
- Now it’s time to slide in the new one. Coat the O-rings with plumber’s grease so they seat properly, then line up the notches of the valve with the slots inside the housing. Push it in, tighten the brass screws back down, and secure the mounting plate.
- Open the water again slowly and check for leaks. Once it looks good, put the trim and handle back on: screw the base, snap the cover, fit the handle into the “off” position, and tighten it with the hex key.
Fixing & Replacing Thermostatic Mixing Valve
The thermostatic mixing valve at the water heater or near a faucet is another story. It rarely goes bad. However, debris or calcium buildup and clogging inside it do not let enough hot water in to mix with cold water to make it hot enough.
There is a screw on top of these things; lift it up. Then take the wrench, grab onto the lever, turn it counterclockwise to cold to let the water run and dislodge the debris, and then turn it all the way back to hot. Next, turn on the faucet nearby, come back, and wiggle the mixing valve halfway from cold to hot and hot to cold a few times.
This lifts up the mixing valve to kind of move the trapped debris from where it’s lodged in. Then, finally, turn it all the way back out, and that’s about it. If you get low water pressure on your faucets after this, it might be that the crud that dislodged has made it up through the water, and it’s stuck in the screen. So just open it, and you may see the debris inside. Clean it up, put the screen back in, and you are good to go.
If, in any case, the problem is still there, unscrew the hot, cold, and the outlet pipe of the mixing valve, swap in a new one, and test it.
Conclusion
So now you know why your mixing valve is not working, and how to clean or replace it. If you’re not too tech-savvy to go through with this process, the safest move is to call California Coast Plumbers, because working with water lines can get messy fast.