I have a newly installed Bosch hot water heater. After installing, it seemed to be working fine. It is in a small weekly rental unit. If anything, the unit is over-sized for the demand. Now that it is getting regular use, the people are complaining that when they are in the shower, the water is nice and hot, then after a couple of minutes, the water turns cold. After a few minutes, it returns back to its normal hot temperature. I checked the shower heads for restrictions and found none. The Model is Bosch 2400 EO (LP Gas)

1) Cheapest and most reliable solution: Install tank-type electric water heater from local box store for $250. Get store coupons, offer 10% off store price. Repairs are made same day with generic off-the-shelf parts.
http://waterheatertimer.org/pdf/How-to-install-electric-water-heater.pdf

2) Install grundfo water pressure boosting pump.
http://www.ronhazelton.com/projects/how_to_increase_water_pressure_in_your_home
http://waterheatertimer.org/How-to-solder-pipes.html

3) Remove areators and low flow shower heads, Maintain unit as shown in product manual: Delime tankless unit yearly, Clean water filter monthly.

4) Call city and report low pressure and report issue as fire hazard, move to different apartment. Call apartment complex and ask why they didn't read flow requirements for tankless before installing tankless.

5) Call Bosch 866-330-2729 and complain that nobody read the flow requirement or tested local water flow before installing tankless.

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

http://www.digelair.net/pdfs/TECHLIT/BOSCH/MANUALS/Bosch-ProTankless_AM.pdf
http://www.bosch-climate.us/files/201009031238250.GWH715ES_English_12.2008.pdf

Above are the manuals required to diagnose and service your error code.

EA: No flame ionization detected with water flow.
Verify that all manual gas shut off valves are open. Check gas type. Reset error code and open a water tap to cycle the heater in an effort to purge air on new installations. Cycling hot water tap on and off multiple times may be necessary. If heater still faults with EA error code, have a licensed gas technician properly purge air out of the gas line leading to the water heater. Check three wire connections to ignition group on the lower front of the heat exchanger are secure. Check gas pressure. Check venting specifications are met. Improper venting may cause premature failure of the flame sensor rod. Check that the minimum power fan speed has been adjusted to the proper value. Observe inside the viewing window of the heat exchanger when a hot water tap is opened. Sparking should be followed by a steady blue flame. If flame is unstable or yellow with proper gas pressure.

Tankless should never be installed with plastic pipe, PVC or CPVC etc.

Before calling service technician, open following link for Bosch tankless troubleshooting resources, including Bosch contact information, typical manuals, error codes, tech support, parts, fuel-saver calculator, service bulletins, water heater forum, fixya links, disclaimers, and links for finding service technician.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Read manual cover to cover, make sure tankless receives yearly maintenance as outline in manual.

Add a comment with your experience so others with same-similar problem can benefit.

Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

It seems unprofessional that Bosch, or any tankless manufacturer, would suggest self-installment when each tankless manufacturer pointedly says their product should be install and maintained by person specifically trained by manufacturer. Especially when it is widely known that self-installed units can void the warranty.

That being said.
Cover on your unit should come off. Unit is attached to wall with screws/toggles at each corner.
Gas line (assuming gas heater) should have a union located on gas line near tankless unit.
Hot and cold pipes are located on bottom of unit.
Vent stack is attached to top.

Specific plumbing techniques are used for each item shown above.
If you are not fully qualified to work with gas appliances, or have no plumbing experience, I suggest taking advantage of fixya paid service where expert walks you through each step while you work on unit.
But you need the tools and vocabulary, and remember the expert cannot see what you are doing or know exactly what you are talking about.

Frankly, if you are not qualified, call a service technician.

Install DIY tank-type gas water heater and avoid the whole headache
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Before calling service technician, open following link for Bosch tankless troubleshooting resources.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Hi,
Heating of water output depends upon the wattage of the unit and the condition of the water source (how cold). The lower the wattage the slower heating capacity at the same time the colder of the water source it is harder to keep warm. Also to consider the quality of the newly installed heater. Contact the manufacturer and demand a site visit as possible to check personally the performance of there unit.
Hope it may help you.
Have a nice day!
JDVillanueva

http://www.bosch-climate.us/files/201109162006340.GWH2400ES_English_10_2007.pdf

sounds like the inlet water screen needs to be cleaned or the unit is scaled up with minerals. see attached link to manual

In order to clean the unit you need to connect a small sump pump to the cold inlet, and run a hose from the hot side into the same bucket. Pour about a gallon a white distilled vinegar in the bucket, and turn the pump on. Let the solution run through the unit for about 15 minutes, then reverse the hose connections and run the pump for another 15 minutes. Reconnect the water supply and outlet. Turn the water on in the bathtub first on the hot side and let ir run for a few minutes before turning the power back on to the heater. While the water is still running turn the power back on and reset the thermostat. If you get a lot of debris in the bucket, you may need to consider installing a water-softener to prevent scaling.

I'm experiencing the samething it's been installed 3 years and christmas day nothing. Please help.

Are you using a single lever faucet? The install guide does indicate that you may need to reduce the input from the cold water side to ensure that water stays at a consistent temperature (see page 7 of the installation and service manual). Also, remember, this is a SMALL hot water heater. So if you're running for any length of time at full output, it may not keep up with you (see page 8 of the service an installation manual). I found the manual online at http://www.airwater.biz/images_templ/bosch-water-heaters-ae3.4-manual.pdf.