bosch 250sx NG works at high flow; gives EC error code at moderate flow

[cmccready]

I just installed a Bosch 250sx ng ( natural gas ) tankless hot water heater.   I also installed the Webstone Isolator bypass valves, which have been a huge benefit in troubleshooting.  If I use the bypass portion of the Webstone valve at the egress and fully open the ball valve, the unit senses the flow and turns on, heating the water nicely, though it does make a little noise (sounds like a mild groan or roar) until it stabilizes.  If I turn on a valve like a sink or shower, the unit senses the water flow and tries to start combustion that promptly goes out (about 1 second).  This process repeats 3 times and then the display shows a code EC.  I must have enough gas (pressure and volume) if it runs "full-out", but why is there a problem at standard volumes?  I am suspecting the CO2 setting but I have no way of measuring it.  I am tempted to try some "quarter turn" adjustments on this portion of things to see if it improves.  Any advice?  

[dkpd1581]

use this link for Aquastar's error code pdf....your code advises that the system is not sensing a good flame.  You can check the installer's guide most likely for more info on how to correct that issue....

aquastar.pdf

[cmccready]

thanks for the reply.  I read the online info from the Bosch site.  Here's what I can confirm.

1. gas type is correct (not trying to use ng on an lp system)

2. flame ionization rod connection appears good

3. new device so rod should not be dirty

4. gas pressure is 8wc static and 5.5wc dynamic ... within specs (made a manometer from some plastic tubing, water and food coloring).  I converted piping to 1" (what a pain that was) so volume should be good as well.

5. anybody out there know how to measure co2?

6. ionization rod is probably not damaged as it is new, and visible from the display window, and the unit does operate at large capacity.

overall, the puzzling item is that there is no error code if i am passing a large volume of water through the bypass valve ... only if i pass a smaller volume.

[hvacdrd]

Measuring CO2 without a combusion analyzer isn't possible. Here is the Measuring & Adjusting CO2 information -

[cmccready]

Can I try some "quarter turn" adjustments without the co2 analyzer? If I record the adjustments I should at least be able to return it to its current "non-working" state.

The least expensive combustion gas analyzer I can find is $500.  My neighbor is a plumber and it is not a part of his arsenal.  Is there some place that would rent such a beast?  I live in L.A. ... with this many people there ought to be some place that would have one available.  I read the manual for the Bachrach analyzer.  There's not much to it ... other than having one in hand.