I have had Bosch 250 sx ng heater for the last three years with continuing problems maintaining consistent hot water. Either the unit will not heat up or during a shower when you add cold water to cool the temperature the heater shuts off usuall with a A2 error code. I have changed the overheat cutoff fuse temperature limited sensor loc 3070. This helped for a few weeks but the problem is back either machine wont heat up or in middle of shower unit shuts off and must be restarted. I have had little or no luck with Bosch support and had a rep out once. I am sick of this machine and don't know if I should junk this large investment. I have heard that this model has had problems similar to this. Thanks for any help.

http://www.bosch-climate.us/files/201109162028550.250SX_English.pdf

Bosch 1600H NG??? I suppose. If so you need a digital manometer.
Take a static (off) reading then leave it hooked up and start flow. Watch pressure draw down and you should see the heater draw Water Column down below 5" as the pocket of NG in the pipe just before the unit draws down to minimal to keep heater lite. In that case problem in gas line size/gas line run length, Meter pressure.
If you have and maintain in operation 5" WC then is probably the water valve needs rebuilt along with a new diaphgram and also the water filter should be replaced too. This occurs esp after 2-3 year as Bosch specs rebuild at 3 yrs on the Maint Schedule. Also clean all the faucet aerators for good flow above 1.25 GPM Min at all faucets.
Under a single faucet flow (laundry sink) of 2.25 GPM the flame bed of the Unit should be 1"+ high and uniformly Blue.
I assume at this time you either have a Maint problem or you could be overflowing the heater's Winter capacity. This unit @ 70F rise only makes about 2.2 GPM. It's strictly a single use faucet heater at 117,000 BTU.
When you do the Maint look to see if there is corrosion between the GV pushrod and the water valve pushrod. Notice there is a huge mis- match in the 2 rods sizes.These units occasionally (more the 125B) corrode there and the GV will seize on and boil the heater dry and open a seam so you really want to maintain it to Bosch Specs and Schedule.
Note: This is a gas appliance & requires skill and knowledge to service. While Bosch may say you can DIY, I'd readily advise a Bosch approved Service Tech or Plumber do any rebuild. The Digital Manometer alone is a $100-150.00 item and it's the ONLY way to get accurate gas pressure readings.

Water leaking into elements. Bad PC board. Undersized breakers. Undersized main panel.
Bosch Powerstar is a computer that happens to heat water.
If your 5-year-old laptop computer was malfunctioning, you would not be surprised.
But for some reason (false marketing by manufacturers?), folks believe a tankless computer is going to last 20 years.
Moreover folks also believe tankless will provide endless hot water while saving money.
The whole idea of tankless saving money is a load of sales-BS that defies physics and common sense.
There are no service manuals... except bosch website has troubleshoot section in product manual.
Call a bosch-qualified service tech.
Go to Lowes and buy ordinary tank heater, and fix your heater today for only $250.
Tank-type electric water heater can be easily repaired and maintained by homeowner to last forever... that is a fact
http://waterheatertimer.org/How-to-maintain-water-heater.html

http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html

Gene

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

The following link has general troubleshoot steps for Bosch A2 error
http://waterheatertimer.org/pdf/Bosch_A2_error_code_troubleshooting.pdf

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

Q. EC - Ionization failure during operation
A.

1. Verify gas type (See Fig. 2 in installation manual) corresponds with the type of gas supplied to the heater.
2. Verify the three yellow wire connections to the set of electrodes (underside of the burner assembly) are secure.
   
3. Verify that venting meets specifications per chapter 3.6 of the installation manual. Improper venting may cause premature failure of the flame sensor rod.
   
4. Check gas pressure. See Bulletin TWH-G2-03.
   
5. Check and adjust CO2 readings. See Bulletin TWH-G2-12.
   
6. Gas valve or flame sensor may be damaged. Call Bosch Water Heating with observations and results of previous steps.

BOSCH error codes:
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/2400ESTechSupport/Troubleshooting/tabid/1295/Default.aspx

Best place to start is troubleshoot section in manual. Page 24
http://www.boschhotwater.com/Portals/7/TechManuals/250SX_English.pdf

Problems range from air supply, gas supply, venting, part failure or combination of listed things.
Don't forget, tankless require yearly maintenance, outlined in manual, and cannot be exposed to more than 11 grains hardness in water, and should be de-limed each year.

All costs are energy.
I suggest reading entire manual several times, so you are pro about your tankless water heater.
Even with this knowledge, trained service technician will have to be consulted some day, and tankless should never be installed if factory trained technician not available in area.

Bosch has online tech support form:
http://forms.boschhotwater.com/Home/BoschTechSupportForm/tabid/614/Default.aspx

Install tank type water heater that can be put online when Bosch is not working
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Add a comment for more free help.
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.

Hi,
The unit is plugged up with mineral buildup from the water going through it... it will need to be cleaned internally....this is done by running an acid solution through it to clean out the mineral buildup...

Then it will work again...

heatman101

Sounds like hard water. Service the water valve. Parts are available from your local Bosch Service Provider (Contact Bosch if you don't know who the service provider is).

This sometimes happens in undersized gas delivery pipes. Does the unit have at least a 3/4" supply pipe?

Aquastar 1600H NG Natural Gas Tankless Water Heater Bosch AquaStar Model 1600H NG Natural Gas Whole house tankless gas water heater, 117,000 Btu input, natural draft with hydro-generated ignition. Provides endless hot water for one major application at a time . Can replace 40-gallon storage tank water heater. Optional Accessory: Model AQ1- Power vent with built-in... More freeze protection when venting horizontally. Capacity: 1 to 2 Bathrooms Dimensions: 8.75D x 18.25 W x 29.75H Weight: 40 Max GPM @ 45F Rise: 4.2 Energy Factor: NG .80 LP .84 Minimum Input: 28,000 Maximum Input: 117,000 Vent Size: 5 Gas Connection: 3/4 Water Connections: 1/2 Warranty: 12-year warranty on heat exchanger, 2-year on parts Features: Endless hot water Can save up to 50% off utility costs annually Perfect for one major application at a time No standing pilot, hydro generated