On 2009-08-06, Phisherman wrote:

I plan to purchase a tankless water heater to replace a failing water
tank.

I am definitely a fan of tankless water heaters, but one alternative
to consider is a condensing tank water heater. It will be even more
efficient than a non-condensing tankless water heater. One issue is
that a condensing tank will cost a lot more than a non-condensing
tankless, and while that is often offset by less installation labor
for the condensing tank, since you said you'd be installing it
yourself, that wouldn't apply.

I need a 7.4 GPM unit (4 bathroom house with 3 showers) and
considering a Rheem GT199PVN.

A couple comments. That GT199PVN won't really provide you with 7.4
GPM of hot water--that flow rate is for only a 45 degree Fahrenheit
temperature rise. It's mainly a marketing exaggeration, it would only
apply if your incoming cold water is 60 degrees already and you are
happy with 105 degree hot water. Where are you located? If your
incoming water is actually 50 degrees, and you want 110 degree water
in your shower, then that's a temperature rise of 60 degree, so you'd
get about (45/60) * 7.4 = 5.55 gpm.

Another comment is that 7.4 GPM (if you get a bigger unit that can
handle it) is enough for three simultaneous uses, e.g. 3 showers, or 2
showers and a dishwasher, 2 showers and 2-3 vanities, etc. Depending
on your lifestyle and how many people are in your house, then 2
simultaneous uses might be enough, in which case the GT199PVN is
likely to be adequate (depending on where you live and the incoming
cold water temperature).

What are the pros and cons of two separate pipes than just one pipe
with an interior pipe? Are two separate pipes functionally better
than one pipe inside the other?

Good question, I don't know if separate pipes is a functionally better
arrangement, but it certainly seems like a combined pipe is more
convenient.

Cheers, Wayne