How to Tell If Your Thermostat Valve Is Plumbed Backward

Quick Fix:
If your thermostat valve only gives you very hot or very cold water, the supply lines are likely reversed. The hot line must connect to the left side of the valve and the cold line to the right. Re-plumbing restores proper temperature control.

Quick Answer

In normal operation, turning the thermostat handle clockwise should make the water cooler, while turning it counterclockwise should make it warmer. If your valve is plumbed backward, the opposite will happen: turning to the right will give you only hot water, and turning to the left will suddenly give you only cold water.

This occurs when the hot water line is connected to the right side of the valve and the cold line is connected to the left, rather than the other way around. The fix is simply to re-plumb the valve so the hot supply is on the left-hand side and the cold supply is on the right-hand side when looking at the valve from the front. Once corrected, your thermostat will regulate temperature as designed.

A Deeper Dive Into Why This Happens

A thermostatic valve depends on a precision cartridge that expands and contracts in response to changes in temperature. When plumbed correctly, hot water enters the left port and cold water enters the right port. The cartridge continuously adjusts an internal shuttle to balance the mix, ensuring the temperature at the outlet matches the setting you select on the handle. When you turn the handle clockwise, the mechanism reduces the flow of hot water and allows more cold water, lowering the overall temperature. When you turn it counterclockwise, the opposite occurs: cold is reduced, hot is increased, and the water warms.

If the supplies are reversed during installation, the cartridge still tries to regulate based on its calibration, but the inputs are swapped. When the cartridge moves to limit hot water, it is actually restricting the cold line, which makes the outlet temperature rise instead of fall. When it moves to increase hot water, it is actually opening the cold side, which makes the outlet temperature suddenly drop. Instead of achieving a steady blend, the water output flips between extremes, giving the impression that the valve is malfunctioning even though it is functioning exactly as designed.

This situation underscores that the valve itself is not at fault. The cartridge is doing its job, but it can only perform correctly when the hot supply is on the left and the cold supply is on the right. With the lines reversed, the cartridge cannot physically create a stable mixed output because its internal logic is working against the swapped connections.

The Solution

The only way to resolve the problem is to re-plumb the valve correctly. The hot water supply must be connected to the left-hand side and the cold water supply to the right-hand side, when viewed from the front of the valve. Once the supplies are reconnected in the correct orientation, the thermostatic element will immediately begin working as intended, providing smooth and reliable temperature control.

How to Confirm

We have prepared a short video that demonstrates exactly how to test your valve. By slowly turning the thermostat handle and observing the water temperature, you can quickly determine whether your supply lines are reversed. If you notice that turning to the right produces only hot water and turning to the left produces only cold water, the valve has been plumbed backward. Once identified, a plumbing correction will fully restore proper operation.

👉 How to Tell if Your Thermostat Valve is Plumbed Backwards.mp4