Have you ever experienced a toilet filled with hot water, a dish machine with extreme calcium build up or cold water coming out of what should be the hot water line or vice versa?  If you have seen this or other oddities then you have witnessed crossover. Hot or cold water crossover occurs when one overpowers the other and takes over the water plumbing system. 
How Can Water Pressure Change When It Comes From One Source Into The Building?
Water comes into the building at a set water pressure (psi), typically around 65psi. This 65psi goes directly to all the faucets and plumbing fixtures that require cold water. So naturally if you open up the cold water faucet the psi will be 65 or slightly lower due to friction loss of the water traveling through the pipes. Where the psi shifts is when a soft water systems is installed. 
Side note: Restaurants have multiple reasons for softening water:  1. Espresso machine need cold softened water to protect calcium build up on the internal boiler. 2. Dish machine typically need hot softened water to protect against calcium/hard water deposits that can damage elements, components and scale build up. 3. RO machines run higher efficacies with cold soft water.
When the 65psi cold water travels through the soft water system, the water pressure can lose between 5 to 10 psi (sometimes more is the resin in the tank is older).  When the water continues through piping it also will lose pressure due to friction loss as it heads to the boiler to get heated. If the boiler is in the basement or on the roof the friction loss causing the once 65psi to drop; say 50psi. 
With the cold water at the faucet coming in at 65psi and the hot water is 50psi it is understandable how the cold water will overtake the hot water plumbing system and why we call it crossover. This one example shows how crossover can impact changes in the plumbing system.  While most cross over problems are with cold water infiltrating the hot there are times the hot water can overtake the cold; this is mostly caused by booster or recalculating pumps.
How Does Crossover Happen?  
Crossover happens when check valves and or plumbing devises allow water to be tempered/ mixed fail. These devises typically are: 

  • Mixing valves (used for such areas as)
    • dipping wells
    • hand sinks
    • chemical dispensing equipment
  • Pre-rinse faucets
  • Special faucets that dispense water and chemical
  • Chemical rooms that require tempered water
  • Faucets that allow hoses and spray nozzle to be attached

Most mixing devices have built in internal check valves made of plastic. The same can be said with pre-rinse faucets where the check valves are built into the valve stems. When a mixing valve for a hand sink fails it can still temper water, but it can no longer control backflow.  Rather than replace costly mixing valves to solve the crossover problem simply install an independent inline check valve in the plumbing line prior the device/mixing valve; a very economically solution.  

This leads to the final two questions: 1.How to locate crossover 2. How to resolve Crossover  
 

How to Locate Crossover! 
The following are helpful steps to locate the exact point of crossover  
 

Pic #1

Testing at Pre-Rinse Faucet
•    Step 1: Making sure the cold water faucet handle is turned off, only turn on the hot water.
•    Step 2: Place your hand on the pipe/nut behind the valve stem of the hot water faucet while the hot water is running. The pipe should be almost too hot to touch. See Pic#1  
•    Step 3: While still holding the pipe turn off hot water the faucet (there should be no water running), if the temperature cools at all, then the internal check valve stem has failed. 
•    Step 4: While still holding the faucet being tested, have a helper turn on the hot water only at an adjacent faucet. This will cause the hot water to pull away from the faucet being tested drawing the cold water through if the hot water check valve has failed. 
•    Step 5: If the piping being tested cools in temperature within 30 seconds or less the crossover is found. Do this at each pre-rinse faucet making sure all other possible breaches are found
 

Pic #2

Testing at hand sinks with mixing valves 
•    Step 1: Make sure the angle stop for the cold water is turned off and turn on the hot water at the faucet. 
•    Step 2: Place your hand on the hot water angle stop while the hot water is running; the pipe should be almost too hot to touch. See Pic#2  
•    Step 3: While still holding the pipe, turn off the faucet so no water is running and turn on the cold water angle stop (no water should be running); if the temperature cools at all the internal check valve to the mixing valve has failed. 
•    Step 4: While still holding the hot water angle stop; have a helper turn on the hot water only to an adjacent faucet. This will cause the hot water to pull away from the faucet being tested drawing the cold water through if the hot water check valve has failed.
•    Step 5: If the piping being tested cools in temperature within 30 seconds or less the cross over is found. Do this at each faucet with a mixing valve guaranteeing all other possible breaches are found.

Other areas to test
For other larger check valves, have a plumber inspect for proper operations. The large check valves can be found at:
•    Check valve following the soft water tank
•    Check valve following a booster pump
•    Check valve following at the circulation pump return
•    Check valve leading to the boiler Water Supply line 
How to Resolve Crossover! 
Once the source of the problem is found, the solution is easy. While replacing costly mixing valves and faucets stem valves will work; a better longer term solution is to install spring loaded check valves prior the devises/mixing valves . These checks may already exist and simply need to be replaced due to age but last longer than plastic built in checks. Be sure not to install Swing Flapper Check Valves as they get stuck open and are not as robust. If cost is an issue, install a spring loaded check valve on the weaker water line, which is typically the hot water line, this will solve the problem.  
This guide should keep your toilet filled with cold water and dish machine calcium free. 

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