Tankless water heater - broken or is it the gas?

[rubor]

Hi everyone,

Trying to help my parents avoid a big repair bill. They have a tankless water heater, model Noritz NC250-SV-ASME at the business they run. They've had it for about 4 years, but for the last few months, it turns off and flashes error code "90" after it has been running for a few minutes.

I googled this error code, and it comes up as a combustion abnormality. I checked the flue, but there is no blockage. I checked for debris in the fan motor and couldn't find any - the fan seems to be working fine. The other thing that is listed as a cause is an insufficient gas supply. Like if the heater is not getting enough gas, and it combusts poorly, it turns itself off.

I called Noritz, and they said this is likely a gas issue, and to call the power company. I called the power company, and they said I have to deal with it with Noritz. My dad called a plumber, who said the unit needed to be descaled - he charged my dad $250 and it did not fix the problem.

At this point, I feel like I'm going in circles. Should I persist with the gas company, or try something else? Does it even make sense for the gas to be the issue when it was working fine until recently. I would appreciate any advice or guidance. Thanks!

[Spork]

(I'm guessing.)

Is it at all possible that some repair work was done on the gas line (either before or afterr the d-marc) that introduced a bunch of air?

Are there other gas appliances in the building and are they working normally?

I might be tempted to try to purge the line a little... If nothing else, multiple successive lightings.  I know when I've worked on the lines in my house, it's taken me a while to get appliances to light.

[lordrtype1]

My comment is more for verification: Your said the error code could indicate a gas issue.  One thing I would ask is if it was piped with a ¾" gas supply to it. I would assume so, since it's been working this long, but if not, if something else is pulling gas when it does, there won't be enough from a ½" line (what a regular water heater uses). Tankless need more gas to heat, because of being 'on demand,' so gas supply is critical.

[Greg]

A "combustion abnormality" can be one of three things; gas supply, air intake flow problem or exhaust flow problem. 

If it uses a coaxial flue that includes the intake, it can be difficult to check for blockages.  Birds, rodents and insects are the most likely problem causes, but even ice can be a problem at the outlet depending on the weather.

I suggest that you strongly suggest that the gas company come check their regulator.  If it goes bad it can limit gas flow.  There's likely a way to test for sufficient gas pressure/flow at the heater, but it requires special equipment.  You may want to let the gas company know that if you have it tested and it's insufficient you'll be sending them the bill.

Another place that can cause problems is in the burner itself which can build up soot and other stuff that needs to be blown/gently brushed out/vacuumed.  The burner can usually be accessed after the cover and inside cover are removed.  Not necessarily DIY unless you're super handy.

The descaling was probably to remove minerals from inside the water pipes in the heat exchanger, not exactly your issue.