We have a plumbing technician to repaire the water leakage out side of our house, which need to turn off the water supply to our house. After that, we found that hotwater will not turn on (burner light will not on and no hot water). How can we solve this problem.

There are several potential causes of low water pressure with a tankless water heater like the Tempra 24 Plus:

1. Flow rate limitations: Tankless water heaters require a minimum flow rate to operate properly, typically around 0.5-0.6 gallons per minute (GPM). If the flow rate drops below this threshold, the heater may shut off or fail to produce hot water. In your case, the initial flow rate of 3.4 GPM is well above this minimum, but the drop to 1.6 GPM suggests that there may be a flow restriction or other issue causing the lower pressure.
2. Water supply issues: Low water pressure can also be caused by problems with the water supply itself, such as a clogged or damaged pipe, a water main break, or problems with the city water system. Check with your local water utility to see if there are any known issues or maintenance activities that could be affecting water pressure in your area.
3. Water filter or softener issues: The whole-house water filter and softener that you installed could also be causing low water pressure if they are clogged or not functioning properly. Check to make sure that the filters are clean and that the softener is operating correctly.
4. Plumbing issues: There could be issues with the plumbing in your home, such as a clogged or damaged pipe, a faulty pressure regulator, or incorrect pipe sizing. Have a licensed plumber inspect your plumbing system to identify any potential issues.

If you have already checked for these potential causes and are still experiencing low water pressure with your Tempra 24 Plus, it may be helpful to contact the manufacturer or a licensed technician for further assistance.

Buy igniter at local plumbing supply house or call manufacturer as shown on label side of unit; might want to test the device:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#igniter
http://waterheatertimer.org/Water-heater-manufacturers.html
http://www.thetankatwaterheaterrescue.com/forums/forum3/
Gene
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If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

The rinnai is a computer that heats water. It is not ordinary water heater most people are familiar with, and repairs are not do-it-yourself, requiring a trained technician qualified in that brand who can diagnose both computer and water heater problems and plumbing problems.
Call rinnai for recommended local technician. Click on support tab. Note their webpage is very short on repair details, unlike companies that sell tank-type heaters... because tankless units are not do-it-yourself.
http://www.rinnai.us/tankless-water-heater
Problem might be simple blown glass fuse on circuit board or complete failure of computer and sensors.
Here is page written for Rheem tankless, that explains general complexity of tankless computers, and showing required maintenance, such as deliming each year:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
To save problems with tankless computers plus anticipated expense of maintenance and repairs, I recommend homeowner install tank-type gas water heater:
http://waterheatertimer.org/How-to-install-gas-water-heater.html
Also install one-time-use whole house surge protector, and install grounding rods to prevent further damage... lightning frequently likes the same places over and again:
http://waterheatertimer.org/How-to-wire-Whole-house-surge-protector.html
Gene
h

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Before calling service technician, open following links for resources including Rheem chat and possible manual:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
http://waterheatertimer.org/pdf/No-Error-Code.pdf
http://waterheatertimer.org/Crossover.html

Typical tankless manual says to have yearly service which includes checking household plumbing, gas supply, vent and condensation, check sensors for loose wires, clean and inspect burner parts and ignition factors etc.

There are two solutions:

1) Replace TP valve with same temperature/pressure as existing TP valve.
Buy the TP valve at hardware store or home center or plumbing supply.
Turn off water heater so water cools.
Hot water inside a tank is explosive ... so open the TP valve and test water temperature.
If TP is on top of tank, you don't need to drain much water before unscrewing
If TP is on side of tank, use the TP valve to drain water -or- use the drain valve
Unscrew TP to remove.
Put teflon tape on new valve threads so it will seal against leak.

2) If the new TP replacement starts doing the same thing, there may be a thermostat problem -or- thermostat is set too high.
If you keep the water temperature set high, you can add expansion tank to incoming cold water line.
Home Depot sells expansion tanks. Plumbing supply carries them,
The tank for residential water heaters is about same diameter as a football and it goes on cold water line.

Check out the upper element and on the side of it should be the same number for the bottom element.
If not write information down and model number along with name and go to a plumbing supply house or Lowes,Home Depoet for a new element.

check with your local plumbing store they may have in stock or can order for you look in phone book

you might also want to check into the piping in your house is it gavinized, cpvc or copper, this could also make a diffrence, you could also be pulling mud from the bottom of your well, the anoid rod in the water heater will keep you from seeing this in the hot side lines,suggest to have a test done on the water take a jar with a lid and take it to your local plumbing supply house and see if they can't have it tested then you will have all your answers, the test i'm in md. it cost us aprox 40.00 to have the complete water test,

Water source. City...New line..."T" the line so cold water services the house and the other line runs direct to the inlet side of the heater. Make sure you are connecting to the line after it passes through a pressure regulator. If you have a pump and water tank...."T" the line after the tank so cold water services the house and run a line directly to the inlet side of the heater. The outlet side gets connected to the main feed for hot water to the house. These do not require a storage tank. It does take a short period of time to heat the water, but once they get to temp they have an endless supply. Make sure you vent the heater if gas. After you get hot water, you can adjust the temp to the required temp according to the code in your area.

they make a long stem wire brush for this,, you will need to go to a plumbing supply house to get one turn it on and gently rub it over the burner make sure it is lit so the pressure from the gas will blow out what is there good luck