I am having a 60 gallon bathtub installed and have only a 30 gallon
tank water heater. I am considering installing a 12 kw on-demand
tankless water heater to serve just that bath. My husband, and our
plumber (who seems to have no experience with these units) do not think
it will do the job, but I have done the math and, though close, I think
it will work.
We live in Florida and the tap water temp is around 70 degrees in the
winter, 80 plus in the summer. The flow rate for the faucet is 2.5
GPM. When I apply the formula, (Kw x 6.83 divided GPM) it appears I
would get a 32.8 degree rise in temp, which would be just barely enough
in the winter and ample in the summer.
Does anyone have experience with these units for this sort of
application? What happens if the flow rate exceeds the unit's ability
to heat to the desired temperature? Does the water cool off, or does
the flow decrease?
you might be better off to use the tankless as a pre heater for the
main hot water tank for everything
have you checked your main service entrance capacity to make certain
the load isnt too big along with everything else in your home?
I think we've determined that the electric can handle it.
The other issue is that this bath is the some distance from the hot
water tank, having the on-demand would eliminate the wait for the water
to heat up in that bath. It is a small unit that will fit in a hidden
spot in that bath. My concern is whether it can handle the flow rate.
You are probably looking at a cheap sink heater, a one shower Bosch
takes 120 amps. If flow is more you wont heat, if voltage is poor, you
wont heat. If electric supply is small you wont heat. You are just
calculating, so figure in temp drop from heater to tub and cool down,
maybe 20f, maybe more. 70f incomming, check that at different areas with
water running 10 minutes and still take off 5f for saftey. Are you sure
voltage is at rated needs, no you cant be. Improper-to small a wiring
will reduce heater output also. Do you have Ng or propane, these fuels
are still cheaper than electric. Unless you have a 200a service and 120
free forget tankless. People that hate tankless did not size, plan, test
or install them right. So far your numbers are incomplete and have no
margin for error. You also need a plumber that does this everyday and is
not "learning his trade" on your pay and possible mistake. I would have
a written temp output guarntee from the installer.
I can't say anything about your calculations, but if you think it is going
to be close you will be disappointed in the results. I have hooked up a few
of these over the years and based on my experiences the customers were not
satisfied with the units that were marginal. With instant electric water
heaters you should oversize it to get the results you want. Did your
calculations take into account any losses while the tub is filling up? If
this is a hydromassage bath tub I recommend the optional built-in heater.
Back to the electrical discussion, its not so much the volts as it is
the Amps. To get any decent kind of flow, you are probably looking
at a two 12 kW unit in series which is approximately 24 kW or 81888
Remember you are talking about filling a tub (not just shower flow).
At 240V, your heating array is going to draw 100A which is a
significant peak load to all but the most heavy duty residential
In my opinion, you are better off with a tank. It will heat up more
slowly, but you will have 80 - 100 gallons of hot water readily at
your disposal without all the problems of peaking current in an effort
to get instant hot water. Gas or propane would probably be a better
choice for you still.
read about natural gas units 15,000 BTU/H 180,000 BTU - (Fully
Modulating) first at:
your electric 12kwh is only 40,945.7 btuh. you will be filling the tub
from a tub spout at a much faster rate than 2.5 gpm i hope. if there
are no kids in the house just crank up the thermostat on the water
heater you've got. this way you will be diluting the hot with some cold
and filling the tub quicker.
for those others with kids visiting the house:
"TO TEST YOUR WATER HEATER'S TEMPERATURE
Check it in the early morning before anyone has used the hot water.
Go into the kitchen and turn on the hot water tap and leave it running
for two minutes.
Hold an outdoor thermometer or candy thermometer in the stream of
running water until the temperature stops rising.
If the temperature is between 120 and 125, good.
If higher, find the thermostat on the water heater and turn it down.
Gas water heaters have an external thermostat, near the bottom.
Electric water heaters have two panels screwed to the top and bottom of
the tank or one panel on the side. Set it to "low" or "Energy
Wait 24 hours and then test the water temperature again to see it is in
the safe range.
Consult a professional if the temperature did not go down." this and
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