Why is hot water coming out of the cold side of the tap?

November 12, 2014

When hot water comes out of the cold side of the tap, it can be a real problem. Here's what is likely causing the problem and what you can do about it.

Why is hot water coming out of the cold side of the tap?

The pipes are too close together

The hot water pipes may be in close proximity with the cold-water pipes. What's more, there may be a water heater nearby radiating heat. The result?

  • Over time, this could lead to warming up of the cold water.

Recirculating plumbing could be the cause

There are some homes that have hot water recirculating plumbing that utilizes cold water as a return medium.

  • In such plumbing systems, it is not unusual to get warm water in the cold-water tap until the hot water reaches the temperature valve that controls the recirculation.

Ventilation ducts may be the answer

If hot water only appears in the cold-water tap during winter, it could be that the cold water pipes may be touching the ventilation ducts of your forced-air heating system.

  • In the cooler months, the ducts are often warm to the touch from the hot air being pushed through them. The heat could gradually warm up the cold water given enough time.

Improper heat trap installation

During the installation of the piping system, the heat trap may have not been placed in the water heater inlet.

  • In such cases, hot water may flow out of the water heater and mix with the cold water.
  • Try to feel which section of the cold water pipe is hot first thing in the morning, and that could be your explanation.

The "heating effect"

Hot water can cause copper lines to heat up. As such, there are times when the hot water can heat the entire faucet tubing, including the cold water lines which are located adjacent to the faucet.

  • The warm water in the cold-water tap is only transient. As soon as the cold water runs though the line, it eliminates the heating effect.

There's too much water pressure

Another thing to consider is the status of the water expansion tank, usually located near the water heater.

  • If you have installed such a system, it requires a pressure lowering valve or a one-way valve in your water meter. Otherwise hot water will flow into the cold water pipes.
  • Many experts believe that such a problem may be due to the thermostat in the immersion heater. The thermostat can fail and this leads to hot water overheating and boiling over into the cold-water tap, making the stored water warm. Within a short time, this comes out as warm water via the cold-water tap.
  • If the thermostat is fine, it could be due to incorrect layout of the pipes.
  • Homes with tankless water heaters, whereby water is heated 'on demand' vs. stored in a large insulated tank, may possibly not have the water expansion tank located next to them. Some searching may be required to find it.

What to do

Whatever the reason, the best advice is to consider calling a plumber and let him or her decide how to fix a problem with your hot and cold water taps – especially if you have small children and/or seniors living in the household for whom the risk of a serious scald or burn could be a threat.

The material on this website is provided for entertainment, informational and educational purposes only and should never act as a substitute to the advice of an applicable professional. Use of this website is subject to our terms of use and privacy policy.
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