Should hot water pressure be reduced after hot water is used?

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I'm noticing reduced hot water pressure (on all faucets/shower) after significant hot water use (2 long showers, 2 loads of HE (High Efficiency) laundry, and using a sink for about 5 minutes). Cold water pressure is normal.

Is this hot water reduced pressure normal, or indicative of a problem?

Note that this reduced hot water pressure is a new thing. It was not an issue in the past.

The water heater is a Rheem natural gas tank-type storage water heater, with a capacity of 40 gallons.

UPDATE #1: I visually inspected the exterior of the water heater, and there are no visual defects. There is no water around it, either.

UPDATE #2: When I looked at the water heater, it has not two, but three pipes attached to the top. I found this QA, but I'm not sure if the answer applies. One of the three pipes is not insulated, so I'll assume that's the cold water intake. Both of the other pipes have insulation on them, so I'm guessing hot water is coming out of...

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I'm noticing reduced hot water pressure (on all faucets/shower) after significant hot water use (2 long showers, 2 loads of HE (High Efficiency) laundry, and using a sink for about 5 minutes). Cold water pressure is normal.

New Answer Derived From Exhausting Initial Answer & more:

It's your Water Heater, here are some things to try in determining whether or not there's actually a problem.

1 - Drain & clean the tank properly. ONLY do it when the pressure's good. Turn off the Hot & Cold Water & WH, open the T&P valve & drain the tank (see manual) to somewhere that contents can be seen. Blast on the cold water when you can start to "slightly" rock the tank to stir & break things up & periodically continue this until it's empty. Operate the hose bib handle to crush any big particles or Poke into your fully open hose bib if the hose flow drops. Then, when empty blast on the cold water & let drain & repeat until nothing comes out. Then, do just a 2-minute fill &...

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A new hot water heater has nothing to do with cold water - correct? That is the way it is as cold water is fed into the hot water tank, is heated and stored until needed. The cold water pushing to get into the tank is the pressure.

Simple test is to turn on the cold water. If the pressure coming out of the cold is Greater than the hot, then there is a problem in the install.

Usually the water to the house is shut off by a tap under the stairs or in the corner of the concrete floor of the house. Heater can then be swapped out. Then, as you have copper (meaning soldering the connections) the water is slowly let into the house, so you can check to be sure all the soldered joints there is no leak(which would become evident by a drip of water). Then if there is a leak, you can still fix it before the water is turned on full.>a few drips don't mattergushing out of a joint does.< So kind of the "chicken shed" method but it is used by many experienced plumbers. "Those first...

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Before calling service technician, open following link for Rheem troubleshooting resources.


http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

List of causes MIGHT include one of following:
Bad household circuit breaker that powers electronics, or loose wire inside outlet where tankless plugs in.
Tankless fuses and control board.
Water flow rate too low. Minimum .8 gallon required (depending on model)
Tankless inlet filter clogged.
Household aerators clogged at faucets.
Crossover at single-handled faucet
If installed, there can be crossover at mixing valve.
Faulty household check valve, including check valve on re-circulation system if that system is installed.
Restriction in household water line, such as clog at elbow.
Unequal pressure in household hot and cold lines, caused by restriction in hot line. When single-handled faucet turns on, the cold water pressure is higher so hot does not flow...

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If you've just installed a new water heater and find that the water pressure is lower than you expect, a series of tests can help you isolate the problem, which may not be the result of replacing the heater. A heater input or output pipe could be blocked, or the main pressure to the house could be low. If the problem happens only at a particular fixture, that fixture may be blocked or defective.

Water Tank Hum

New water heaters often are equipped with heat traps to improve energy efficiency, and you may hear a humming or whistling sound from one of the traps in a newly installed heater. The sound is produced by water flow being restricted, and that restriction also can cause low water pressure. The solution is to turn off the tank, close the water valves, unscrew the heat traps from the pipes and replace them with dielectric nipples that don't restrict water flow. Dielectric nipples have an internal plastic coating to prevent metal-to-metal contact between copper and...

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Ok I think I may know what is happening.

You say that this only happens after you use the shower or the washer.

You then get a high pressure spurt and hot water a few seconds later with normal cool water after that. If you turn it off and wait a few minuets it does this one more time, then operates normally untill you again shower or use the washer.

To cause the pressure build up you need the line to be sealed at both ends of the system. There is the water meter at one end and the faucets at the other end. Now to have higher pressure in the cold side and not the hot side the hot side needs to be isolated from the cold. This is done with an anti back flow check valve somewhere after the point where the hot water system Tees off of the incoming main line.

Next you need a heat source to heat the water in the cold water pipes. As the hot high pressure water occures after the hot water heater is used this must be the heat source. How it is heating the pipes...

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If problem is located at one tap:
Clean areator and check if problem solved.
Clog will probably be located at low point in water line.
Remove and replace shut off valve.
While shut-off valve is removed. Use stout wire, electrical-wire-puller or drain auger to run into pipe down to next elbow, and then flush line.
Hook air compressor to water line and push air into water line to help release clog. Flush line again. Read more and see image
If problem continues, then pipe has to be traced back to next elbow, and that elbow removed and cleaned or replaced.

If all hot taps have low pressure,
then problem is usually at or near water heater.
Clogs can be found inside the heat-trap nipples at water heater, or regular galvanived nipple, anywhere along the pipe, at elbow, tee, or valve.
Cogs can also happen at check valve, or mixing valve, or pressure valve if these things are installed.
How to save hot water using mixing...

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Water pressure within the home can vary based upon the location, type and height of the property, as well as the type of appliances used and how much water is being used by other customers.

Water pressure can vary at different times of the day. Pressure is normally higher late at night when very little water is being taken from the mains network and most people's taps are turned off. In the morning when people are taking a bath or shower, or watering their garden on a hot evening, there is a bigger demand for water which can cause low pressure.

Water suppliers’ statutory service standard level of mains water pressure is 10 metres/head (or one bar). This means there is enough force/pressure to push the water to a height of 10m.

This is measured at the point where the water leaves the water suppliers’ pipework and enters yours (usually the outside stop valve or property boundary).

As a guide, if you have a suitable single service pipe, the first tap in...

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Water leaving the hot water heater should flow at the same pressure as the cold water in your home. If it does not, a problem exists at the water-heater tank itself or in the lines that distribute hot water throughout the house. Annual maintenance helps to fix the problem.

Poor Pressure

Hard-water buildup in the pipes can block water flow into and out of the hot water heater, resulting in low pressure. Sediment in the hot water tank has the same effect. A kinked flexible water line -- especially the accordion-style copper pipes commonly used with hot water heaters -- can also reduce flow. If the shut-off valve leading to the water heater is partially closed, water pressure from the unit drops. Too-small water distribution lines and too many sharp bends between the hot water heater and faucets exacerbate the problem. When the water pressure coming into the home is low -- beneath 40 gallons per minute -- this also affects the pressure of the hot water, but you should...

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High water pressure is major cause of leaks, pipe damage, and wasted water. Learn how to detect it, prevent it, and fix it.

While some might consider high water pressure a good thing, water pressure that is too high can cause annoying and expensive damage.

Symptoms of water pressure induced problems include leaks in multiple fixtures in the home, leaks that only appear intermittently - like at night, and toilets running occasionally without being used.

The most common source of excessive water pressure is the municipal water supplier. The water company sets the pressure to meet their own needs, such as delivering water to fire hydrants, high elevation buildings (or tall ones), and other reasons. This water pressure is often well over 100 psi, and too often over 150 psi. However, the maximum recommended pressure for a residence is 80 psi.

You can easily test your water pressure with a gauge, such as the one pictured here, that screws on to a hose bibb....

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I do have a washing machine that would be the closest hot water valve

Leave the main water supply on and disconnect the hose on the hot side going to the washing machine. See if the pressure drops at this point or if it is as high as the the pressure at the main coming into the house. If the pressure drops then the problem is from that point back to the hot water heater.

Okay so I drained my water heater again, and I got more green stuff and the water was a bit discolored. But still I have little pressure.

Leave the main water supply on when you drain the hot water tank. Does the pressure drop when you are draining the tank? If so then the problem is from the tank back to the main. It could be sediment in the bottom of the tank. You have hard water and the tank is 12 years old so it may be that the tank is near the end of its life.

If the pressure doesn't drop then the problem is as the water leaves the hot water tank or further down the line. The...

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Water Pressure Reducing Valves

Ensuring Practical, Safe Working Water Pressures Municipal and private water supply companies use pumps and pumping stations to boost water supply pressures in supply mains to be able to supply water for fire fighting, high rise buildings to overcome loss of pressure as the elevation increases, and to maintain water supply in water towers and supply tanks. Pressure in water supply mains can exceed 200psi. Most plumbing codes require water pressure reducing valves on domestic systems where the municipal water main's pressure exceeds 80psi. Higher pressures could rupture pipes, damage fixtures,and injure the people using them.

Promoting Water Conservation

High water pressures waste water. Many municipalities today not only charge homeowners and businesses high rates for water consumption, but also charge consumers equally high rates for the disposal of wastewater. Furthermore, reducing water consumption,...

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