I have all the tools and aptitude to do the job.

I was seeking information about the system. The way the plumbers are
talking, there are technical things they are not comfortable with.

I will check the filters and then flush the unit. If we decide to keep
the unit, I will add service valves.

The problem is that when taking a shower, the water suddenly turns
cold. I imagine this could be a heat exchanger issue, a sensor issue,
or a flue gas issue.

The unit is a Bosch 125 natural gas . It has not been serviced for
five years. The daughter called local plumbers and they declined to
work on it.

A new unit will sell retail for 900 to 1000 dollars. If verical space
exists in the attic were the unit is located, we probably will buy a
Rinnai. I don't know what the additional cost will be.

I'm guessing at this stage if we get a plumber to replace the unit
costs will be $1500 to $2200. I am at the age where I really don't
care to work that hard, but I will.

The plan now is to flush and service the unit ourselves and then if
change out is required, a closely monitored contractor will be used.

Two days ago, I couldn't even spell tankless water heater. :-)