[Shop-talk] Whole house tankless heater experience. Was: tankless/instant water heater

[David Scheidt]

On Fri, Jan 28, 2022 at 6:31 PM Jim Stone <1789alpine at gmail.com> wrote:
>
> Thanks, Dave.  That is an interesting way to put it.  I think I will time the heat and try contacting Rheem.  You never know.
>
> On Jan 28, 2022, at 8:13 AM, DAVID MASSEY <dave1massey at cs.com> wrote:
>
> Jim, your plumber may have made that recommendation based on reliability and not necessarily on performance.
>
> A coworker of mine installed a tankless heater because he has a wife and two daughters who would regularly empty the 30 gallon tank when showering.  He noted that the heater would delay heating water to the point that the front-loading washer would never draw water long enough to get it to kick in.  My speculation is that the manufacturer programmed in a delay to confirm a good, solid hot water demand in pursuit of their energy rating.

Tankless water heaters have a minimum flow, not out of efficiency
concerns, but out of not cooking the heat exchanger.  They modulate
their output, but even at lowest fire, if there is insufficient flow,
they can boil the water in the exchanger, which tends to result in
sudden, dramatic failure, and boiling water being sprayed about.
People seem to think this is undesirable, so if the flow switch
doesn't detect enough flow, it won't allow the burner to fire.  And,
yes, lots of people complain about that when they try to run a device
with intermittent demand, like a dishwasher.

I agree that your unit shouldn't be working the way it appears to be,
and there's either something wrong with your unit (and your plumber,
for not wanting to fix the problem), the design of that model is crap,
or it's not suitable for the application (in which case, shame on the
plumber for selling it to you.)
-- 
David Scheidt
dmscheidt at gmail.com