Or hot water is getting into cold pipe.
Symptoms:
Faucet turns on and water comes out, but hot water never shows up, or does not get very hot.
Or hot water turns cold and then hot again.
Or cold water seems warmer than usual.
Tankless: Crossover can cause tankless water heater not to activate. Read: No error code on tankless.
Many things cause crossover-like symptoms with tankless, so read manual, plus read information below.
| Step 1: Does water heater have hot water? 1) Put bucket under overflow tube and gently raise and then lower TP lever.  Keep feet, face, hands, kids, and pets away from hot water that will blast out of TP overflow tube.Apply cold water immediately to any burned area of skin. This action will reduce severity burns. |  Raise TP lever to see if tank has hot water |
3) If water heater temperature is set too low, then adjust temperature: How to adjust water heater temperature
4) If water heater is not hot, then you need to troubleshoot that problem.
How to troubleshoot electric water heater
How to troubleshoot gas water heater
Hot water is leaving tank, and water heater cannot keep up, so water seems cold at faucet.
Broken dip tube can cause less hot water. Dip tube
Broken pipe can run hot water out of tank.
Dripping and leaking faucets can use up all the hot water.
If water heater TP valve is streaming hot water into drain line, you might not notice the water.
Action: Look for leaks. Turn off all faucets and put ear against pipes and listen for sound of running water.
Larger image
Step 2: Review crossover causes:
1) Most common cause of crossover is bad cartridge on single-handled faucet.
When faucet cartridge goes bad, cold water can enter hot line (or hot water can enter cold line). When this happens, the cold water comes out of faucet, but hot is slow to arrive or never arrives.
2) Faulty check valve on recirculation system. Recirculation check valve allows water to flow one direction, and then stops water flow going other direction. Action: Dis-assemble cross piece on recirculation and clean with white vinegar.
Recirculation system
3) Faulty check valve elsewhere in plumbing system. Action: replace check valve.
Check valve can also cause closed pressure system and cause TP vlave to release water, and cause frequent TP valve failure. How to test and replace TP valve
4) Mixing valve can be installed in-line, or installed on top of tank.
Mixing valve is used on some water heater systems so washing machine receives very hot water, while lower temperatures are delivered to kid’s bathroom.
One method that mixing valve creates lower temperatures is letting cold water enter the hot line.
When mixing valve goes bad, it can let larger or uneven amounts of cold water into the hot line so that hot never arrives at faucet, or temperatures are unpreduictable.
Action: Replace mixing valve.
Note: Temperature setting over 140 degrees can distill minerals out of water that can clog mixing valve, recirculation system, pipes, inlets, etc.
Reduce temperature below 130 degrees to save money. How to adjust water heater temperature
1) Turn off hot water at tank –or- at tankless unit.
This is done by using the shut-off located above water heater.
2) Turn ON all faucets on HOT side.
3) Wait 5 minutes.
4) If any water is flowing, then you have crossover problem.
IF water is running at one or more faucets, then water is entering hot pipes from 1 or more locations.
Following test shows how to narrow down suspects.
1) Best suspect is the faucet(s) that ran duuring test. BUT this facuet may not be the problem.
2) Turn OFF all faucets except one faucet open on hot side.
One faucet is turned on Hot, and all other faucets are off, and hot water at tank is still turned OFF.
3) Go to each faucet in house. Put ear against each faucet and listen for flowing water.
4) When you hear water flowing, then that is the faucet with crosover problem.
5) Also put ear against check valves and recirculation pumps to help locate source of problem.
There may be multiple crossover issues, and the combination is causing the problem, so check everything.
6) Action: Replace faucet cartridge or check valve to solve problem.
Stacking is when several short draws of hot water cause cold water to enter tank.
Tank is already up to temperature.
Several short draws activate thermostat, and more heat is applied.
Temperature at top of tank gets hotter than usual.
 Larger image | Typical faucet has 2 gpm (gallon per minute) Typical tub spout has 2-1/2 gpm 2 gpm means faucet can fill one gallon container in 30 seconds. 2gpm water flow= in 30 seconds hot water will travel a distance of 83' along 1/2 pipe. Formulas If single handle faucet is lifted in center, then equal amounts of hot and cold water will flow, so distance is halved along each 1/2 pipe. Low hot water pressure |
| Flow rate of 2 gpm through the water heater is a flow rate of 17.43 inches/second through a 3/4 inch ID pipe Read | |
 30 seconds | Hot water slow to arrive at faucet 1) Use timer and 1 gallon container to see how many gallons of water come out of faucet in 30 seconds. 2) Measure distance from water heater to faucet. 3) Open hot water faucet. Use timer and see how long it takes hot water to arrive at faucet 4) Measure water pipe diameter. Most pipes from water heater are mixed 3/4" & 1/2" 5) Use chart above to see if hot water is slow to arrive at faucet. See chart |
