I have a jacuzzi tankless gas water heater Model # J-SN180F. In the evening I can't get hot water in my kitchen. It works in my bathroom where the unit is placed but the priority button is lit in the evening as well. It's a rental and our landlord is several states away so he's no help. I generally do my dishes at night and it's seriously screwing with my routine here. How the heck do I fix this?

Check the pipes that go to your bathroom from the heater. Possible blockage can be found by how hot the pipes are.

Access your product manual.
I cannot find a product manual.
Some major company, like Rheem, makes this computer that heats water.

Call service technician to repair the unit.
Copy following link for overview of type of problems found with tankless computers.
For example tankless computers require continual maintenance on yearly and monthly basis to prolong expected life of product.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

If tankless computer is older than 5 years, then assume it is like your 5-year old laptop computer.

Save problems, and repair the unit yourself, same day, by installing tank-type water heater:
http://waterheatertimer.org/How-to-install-gas-water-heater.html

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

For my Jacuzzi tankless water heater model #J-SN180F: It turned out to be an exhaust issue. If I take off the exhaust vent completely then the talks heater works with 3 baths at full hot water. I get the error code 12 again as soon as I put event an elbow of the vent. So my conclusion is I need some sort of straight vent.

Before calling service technician, copy following links for troubleshoot resources:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

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

1) Guesses: Sounds like a bad single-handled faucet cartridge, or low water flow at faucet, or clogged water filter.
Replace faucet cartridge, clean the aerator, and/or clean sediment out of pipes below sink, and clean water filter at tankless unit each month.

2) For exact measure of problem:
Typical faucet can have 2 gallon per minute flow rate.
2 gpm flow rate means faucet fills 1 gallon container in 30 seconds. Test faucet using 1 gallon container and a cooking timer to get flow rate.
Clean aerator to improve flow rate. Compare kitchen flow rate with other faucets in home.
http://waterheatertimer.org/Low-hot-water-pressure.html

Most homes have 1/2" diameter pipe running to kitchen faucet.
100 feet of 1/2" diameter pipe holds 1.2 gallons of water. This assumes no sediment build-up inside pipes.
http://waterheatertimer.org/Water-heater-recirculation-system.html

Assuming only hot is turned on: If faucet is 100 feet away from water heater, and pipe diameter is unobstructed 1/2" diameter, and faucet has 2 gallon per minute water flow, then hot water should arrive in 36 seconds.
50 feet away, and hot water should arrive in 18 seconds.

If kitchen has single handled faucet, and the handle is raised in the middle, then equal amounts of hot and cold water are drawn at same time. This doubles amount of time for hot water to arrive.

If single handle faucet has a bad cartridge, then faucet can have crossover.
Crossover means that cold water can enter the faucet even if faucet is turned to hot.
Bad single-handled faucet cartridge can delay or stop the arrival of hot water.
http://waterheatertimer.org/Crossover.html

Tankless require maintenance including cleaning combustion parts yearly, de-lime heat exchanger frequently, and clean water filter each month.
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf
http://waterheatertimer.org/pdf/Bosch_Overcoming_fluctuating_temps.pdf
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

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Fixya is always less expensive than a service call.

Since you have hot water at all the other fixtures, the problem lies in the sink, not the water heater. It could be that the sink is farther away from the water heater than the other fixtures. It would take longer to get hot water.

I had the same problem with my Paloma a year and a half after installation...contacted Rheem\Paloma\Richman customer service (800-432-8373) and they recommened a local installer in my area. The technican diagnosed the problem as the "flow control valve"; he managed to get Rheem/Paloma to cover the part under warranty...I only had to shell out the money for the install.

1/overheat stat 2/fan 3/micro switch, best of luck.

Usually when you are having a problem at only one fixture, and not any others it is a sign that you have a faulty fixture. Hope this helps.

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