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Test and repair Crossover/ Cross connection
Tankless flow rates Crossover: when cold water gets into hot pipe
Or hot water gets into cold pipe

Hot water never shows up, or is slow to arrive at tap.
Hot water not very hot, but tank has hot water.
Hot turns cold and then hot.
Cold water seems warm or hot.

Tankless: Crossover can cause tankless water heater not to activate. Read: No error code on tankless.
First shower in morning does not get very hot water.
This might be crossover problem, or it might be 'deadband' issue caused by a variation in thermostat calibration. Deadband is temperature difference between burner shut-off and when water has cooled enough to come back on.
Typically the problem of thermostat calibration is considered 'normal' by water heater manufacturer's. Spec sheets say water temperature inside lower part of tank can drop 20-30F before thermostat turns on burner, and during periods of usage, the top of tank generally stays hot and the calibration is not noticed.
However, if no hot water has been drawn from tank for several hours, the entire tank can cool down. Result: first usage in morning seems cool by comparison.
Replace thermostat on electric water heater
Replace thermostat on gas water heater
Test temperature Step 1) Check tank for hot water
Raise TP levera) Put bucket or can under overflow tube and gently raise and then lower TP lever.  
-If water is hot at TP valve, but no hot water arrives at tap, it indicates crossover issue.
Keep feet, face, hands, kids, and pets away from hot water that can blast out of TP overflow tube.
Apply cold water immediately to any burned area to reduce severity.

b) If water is putting off steam, then water is hot. Use cooking thermometer to measure water temperature and compare with thermostat setting to get base line. If water heater temperature is set too low, then adjust temperature.

c) If thermostat set to 130, and water is 90, then problem is at water heater. Thermostat calibration can vary by 3-5%, but water coming from top of tank should be consistantly hot.
Pocket thermometer

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temperature gauge Temperature gauge
Typical bath shower is 104F. Typical thermostat setting at tank is 120F. Use pocket thermometer to measure water temperature. Tip: compare with thermostat setting to get base line for setting temperatures.
Or install permanent temperature guage on hot pipe leaving water heater.

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Dip tubeStep 2) If Hot water runs out quickly
The problem might be broken or cracked dip tube.
Let hot run for 10 minutes. Then recheck Hot water at TP valve. Water coming from top of tank should still be hot.

-If the water at TP is still hot, but water coming out the tap cools, then it's either a cross connection or direct-return recircirculation line without a working check valve.

-If the water at tank is cool after just 10-15 minutes of running hot, suspects include a) dip tube, b) bad thermostat, c) water leak causing all hot to run out of tank.
Dip tubes are generally good for at least 15 years. After that, they get brittle from constant immersion in hot water.
Troubleshoot dip tube
recirculation system
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Delivery test: is the tank producing enough hot water to match size of tank?
"Residential water heater can deliver 70% of tank capacity at the thermostat setting minus 20 degrees.  What does this mean?  If the thermostat is set on 120 degrees, the unit should deliver 70% of tank volume at a temperature between 100 degrees and 120 degrees."

Do a draw-down test:  Turn off heater. Connect hose to drain. Run hose into 5 gallon bucket.
With a 50 gallon tank. Expect minimum 70% or 35 gallons of hot water before it starts to cool.  Run water fast before it cools.
If result shows correct amount of hot, then tank is functioning.
Step 3) Check for Leak
Water heater cannot keep up, so water seems cold at faucet.
Turn off all taps, and put ear against pipe to listen for leak.

-Broken pipe. Turn off all taps and put ear against pipe.
-Dripping and leaking faucets can consume hot. Fix leaks.
-Leaking TP valve.
How to troubleshoot and Replace TP
watts recirculation systems Crossover valveStep 4) Recirculating system with crossover valve under sink? No ... then skip this step.
If home has recirculation system, and cold water or hot water never arrives at faucet, then crossover valve should be replaced.
The crossover valve can go bad and allow hot-cold water to mix, resulting in crossover. Replace crossover valve.
Buy from my affiliate links:
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Watts sensor valve kit

Water heater recirculation installation, troubleshoot, manuals

Step 5) Recirculating system with direct return line?
No ... then skip this step.

Recirculating system with direct return line?... then problem might be bad or missing check valve.
Turn off the recirc pump overnight and check if problem still exists next day.
Check valve is usually installed horizontally on the line. A swing check valve will not work installed in vertical position.
If the check valve is not functioning or missing from installation, then water pressure can backflow through the return line and create crossover effect at tap.

Recirculation system
check valve Step 6) Faulty check valve in plumbing system.
One way valve: Lets water pass one direction, but not other direction. Inspect plumbing pipe for check valve.
Action: replace check valve. Make sure check valve is installed horizontally and not vertically
Check valve can also cause closed pressure system and cause TP valve to release water, and cause frequent TP valve failure.
sharkbite check valve
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mixing valve Step 7) Other plumbing suspects
a) Reverse Osmosis system, where hot and cold lines cross connect with each other.
Action: disconnect system temporarily.

b) Faulty Mixing valve located on top of water heater, or can be installed in-line.

Water heater mixing valves

How to save hot water using mixing valve
Step 8) Check single-handled faucets
a) Turn off hot water at tank –or- at tankless unit.
This is done by using the shut-off located above water heater.

b) Turn ON all faucets on HOT side.
c) Wait 5 minutes.
d) If any water is flowing, then you have crossover problem.
Step 9)  Locate faucet with 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.

a) Best suspect is the faucet(s) that ran during test. BUT this faucet may not be the problem.

b) 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.

c) Go to each faucet in house. Put ear against each faucet and listen for flowing water.
d) When you hear water flowing, then that is the faucet with crossover problem.

e) 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.
f) Action: Replace faucet cartridge or check valve to solve problem.
Install dual handled faucets
Do not turn on hot tap when using cold water.
Single handled faucet that is lifted in center will draw hot water from tank.
Always lift single handled faucet over onto cold side when using just cold water

Each time hot is turned on, even for a moment, cold water immediately enters water heater. Water heater runs more.
This problem is more likely to affect tank-type water heater than tankless, since tankless require minimum water flow before heating is activated.

Dual handle faucets at Amazon
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Other tips for saving money:
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Related problems

Stacking can be caused by crossover
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.
water flow rate
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: How far does hot water travel in pipe in 30 seconds
How long does it take to fill 1 gallon jug
10 seconds = 6 gallon per minute
15 seconds = 4 gallon per minute
20 seconds = 30 gallon per minute
30 seconds = 2 gallon per minute
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
Hot water slow to arrive at faucet
Larger image
Typical faucet delivers 2 gpm (gallon per minute)
Typical tub spout  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.
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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.
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