I'm considering the old tankless vs tank water heater question and
reviewed some of the archived discussions. A couple questions
occurred to me that I didn't see addressed:
I understand that people (sinks, showers) like 110F water, while
machines (clothes and dish washers) like 140F water. So does anyone
make a tankless water heater with two hot water outlets for two
different temperatures? Tankless water heaters have those remote
control units, but that seems like trouble if someone is showering
while someone else raises the water temperature to use the dishwasher.
Since tankless water heaters are flow rate bound, it seems to me it
would be useful to limit the flow rate to clothes and dish washers. I
don't really mind if my clothes or dishes take a few extra minutes to
wash, if it would impact other uses less. Are there inline flow
limiters available that could be installed after the supply stop for a
dishwasher or clothes washer? About 1.0 gpm would be about right,
since the tankless heaters require 0.5 - 0.75 gpm to start.
Good gas tankless will keep output temp the same whether it puts out .75
-5 gpm, so reducing flow is not going to do anything for your laundry. I
set mine so the shower head output is 100 - 102, I do not add cold water
for a shower. My tankless is next to the laundry so output is higher,
no loss in pipes, clothes come out clean, A remote thermostat is good
if your winter ground temperature drops cooling input temps since
tankless work on heat rise temp of water, not tank temperature as tanks
It is probably easiest to set the temperature either to a high one that
machines like (140 F) or a lower one that humans like (120 F) at the tank
and take care of the water temperature differential at the use point. What I
mean here is set the high temp and control the shower temperature by
installing a thermostatically controlled shower blender similar to those
used in Europe or install a cold water shunt at the existing shower valve to
reduce the output temperature at the shower. As for kitchen and bathroom
faucets, you might have to take more care. The alternative is to set the
lower temperature and use the machine's internals to heat the water to a
higher temperature for use. For example, always use the "Pots and Pans"
setting on the dishwasher to wash normal loads. The only difference between
Regular and Pot/Pans settings on most washers is that the machine uses a
heat coil to warm the water on the Pots/Pans setting.
I'm presently doing the latter (i.e. setting the temp to 120 F) with my
Rinnai Combo unit until I upgrade the installation next year. I planning on
installing an air handler unit to replace the gas fired burner in the
furnace. When that is done, my house will be forced air heated by hot water
from the Rinnai. The overall efficiency of the Rinnai (90%) means I'll save
lots of money with reduced gas usage than with the two gas appliances I have
today. My present furnace is only 75% efficient. During the air handler
installation, the mixing valves the installer put in will split the hot
water between the air handler and domestic points of use. The air handler
lines will use the water directly from the Rinnai. They will install a
thermostatic mixing shunt in the domestic line that will blend cold water in
to reduce the output flow temperature. Thus I set the remote to about 160 F
for the air handler and the mixing valve will reduce the output to about 120
F. Cool stuff.
Your scenario can't occur because when you set an over-ride output
temperature on the tankless unit, it maintains that temp until the water
flow ceases (or goes below the minimum flow rate of 0.5 gpm). Thus someone
can't come along and reset the temperature until the priority source stops
using the water).
I don't think this would buy you anything. Most good tankless heaters limit
flow rate to favour output temperature so this mechanism is automatic
depending on your daily hot water use pattern.
The Rinnai is a natural gas heater. I guess it depends on which model you
saw on the site. Mine's a Continum 2520 Combo for domestic hot water and
heating. There are other models just for domestic hot water use.
I see your point that this can be done, but it strikes me that if the
supply system can do be designed to take care of this for a reasonable
price, that is easier. Maybe a small electric "booster" water heater
for the machines supply? Of course, as you mention, the dishwasher
already has a heater. But I don't think the clothes washer does.
Ignorant question: the water the Rinnai heats for the air handler,
what happens to it after it gives up its heat?
If you have a thermostatic mixing valve to make 120 F water for
people, why not have a second one to make 140 F water for the
machines, and run separate supply lines to the machines? I guess the
separate supply lines is a significant cost. There is also the
problem of the cooled water sitting in these separate supply lines and
the need to purge it before use.
Is it a concern that if the Rinnai is set to 160F, that one small load
won't trigger enough flow to activate it? E.g. cold water is 60F,
small faucet wants 1 gpm of 110F water, so Rinnai is to provide 0.5gpm
of 160F, perhaps below its threshhold. The thermostatic mixing valves
don't figure into this calculation, it doesn't matter whether the
mixing is done at the point of use or earlier.
I don't see how this prevents the following scenario:
Standard water temp is 120F
Person A is having a shower (pressure balancing valve)
Person B wants to run the dishes and overrides to 140F
Person A's shower suddenly gets hotter
It gets fed back to the cold water inlet for domestic use or reheating in
the Rinnai. On my Rinnai, I have two valves connected to a "Y" on the cold
inlet labeled (DCW and AH return) and two connected to a "Y" on the hot
outlet labeled (DHW and AH).
The thermostaic mixing valve is automatic. I understand from the installer
that it is a purely mechanical device (not electronic nor
electro-mechanical) designed to introduce the correct amount of cold water
to reduce the temperature of the outgoing hot water from the Rinnai. It
senses water temperature based on the expansion properties of a piece of wax
integrated into the valve (like a car radiator thermostat). So, a 1gpm
outflow rate request to the Rinnai from the point of use will automatically
mix in the correct amount of cold to give a 1gpm delivery at the
approximately correct temperature of domestic hot water. This is all guess
work on my part at the moment because the installation is roughed in for the
air handler but will not be completed until next year (my choice). So I'm
not exactly sure of all the equipment that eventually will be installed to
control the temperature.
So long as Person A continues to shower, the tankless heater is sensing a
hot water flow. It will be impossible for Person B to reset the remote to a
higher temperature for the dishes until the flow rate from Person A's use
ceases (i.e. Person A finishes the shower). At that point in time, Person B
can reset the temperature and use the dish washer. However, if Person A
thinks that they are not clean enough yet and re-enters the shower, they may
have to re-adjust the hot and cold taps for a comfortable water temperature
since now the outgoing water is at 140F instead of 120F. This problem could
be solved automatically if the shower was equiped with a thermostaticly
controlled faucet. These units are common in Europe. You have two
controls -- water quantity and water temperature. Once you set the water
temperature to a comfortable value, it is maintained independant of the
water quantity setting. So, in this new scenario, Person A re-enters the
shower and the faucet automatically delivers the comfortable shower
temperature without Person A needing to readjust the water temperature
My point is that a 1 gpm 110F flow request at the faucet translates
(regardless of what mixing mechanism is used) into a 0.5 gpm 160F flow
request at the Rinnai (under the stated conditions), which may be
below the Rinnai's threshold. Just something to be aware of.
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