"John Gilmer" wrote in message
net...
On 8/3/2011 8:05 PM, Mikepier wrote:
I currently have a standard 40 gallon electric water heater on a 30
amp breaker. The panel is 100A service which also services an electric
dryer. I was thinking about getting a tankless water heater to save
space, but after reading some electrical specs on these units, it
seems I would have some issues with the electric.

I have 1 bathroom and 1 kitchen and a washer. From the calculations I
would need at least a 5 gpm unit.
If you look at the specs for the Rheem units, the RT18 needs 2
seperate 40A circuits, with maximum power being 75 amps.

http://globalimageserver.com/fetchDo...2-5934e3043c7d

Obviously my panel cannot support that. I could maybe drop to the RT9
unit that requires 1- 40A breaker, but I'm concerned it might be
undersized at 3 GPM, and also concerned that it uses 38 amps. If my
dryer is on, thats another 26 amps, and my 220V water pump using 5
amps, totaling about 70 amps.
That leaves me with about 30 amps to spare.
So I could either upgrade my panel, or just stick with what I have.
This is for a seasonal home in the summer. Just wondering what
everyone else thinks I should do.


You can easily "do the math" and determine EXACTLY what the temperature
rise is when you run 40 amps at 240 volts through a water heater.

All that really matters is the temperature of the cold water coming in and
the flow rate.

If you are "marginal" you can help by reducing the flow rate and if your
cold water coming from outside is "too cold" you might want to help it
along by running it around the basement. It's important to "temper" the
cold water to the shower mixing valve since the warmer that water is, the
less "hot" water you will need for a comfortable shower.

I can give you a "data" point. We have a conventional electric water
heater. Even when I "run out" of hot water, if I run 100% hot water into
the shower, it's tolerable. Bottom line is that it depends on just how
hot you want your shower.

You first task is to: 1) measure the temperature of the cold water; and
2) "do the math" and determine the temperature rise at various flow
rates; & 3) decide whether it's "gud enuf."

The only "justification" for a demand type hot water heater is to "save
energy" by not keeping 30/40 gallons of hot water sitting around & heating
up the basement.

There's another justification and that is availibility of an unlimited
amount of hot water, ie there is no tank to run
out.


You can "save" much of that energy by adding
insulation to the heater, installing that gadget that prevent circulation
of the hot water above the heater (I think it's just a "leaky" check
valve), and if your circumstances permit, just shutting down the water
heater when you don't need it.

Frankly, the cost of a new panel or the equipment and wiring costs of a
"demand" WH will take years to recover.