what is the best
supplemental
heating system?
Jerry D. re NINger
In my opinion, the best
technical match for solar preheating is a water heater that
will heat the water only when
necessary (on-demand) and
only as much as needed to
reach the set-point temperature. Until 2010, I would have
said that the only technology
that can do the job properly
was a fully modulating electric tankless water heater. This
year, a condensing gas tankless
water heater (rated EF = 0.95)
with 2 gallons of built-in storage (covered under the legal
definition of instantaneous
water heaters) became available and appears to be capable
of doing the job properly and
efficiently. The photos at left
show a solar water-heating
system that uses a modulating
electric tankless heater as the
supplemental water heater.
This solar water-heating system, installed on a Florida house, serves
six to seven people on average. The storage tank, the controls and
the electric tankless heater are shown in the lower photo. Note the
subpanel to the right of the heater.
I know that there are issues,
real and perceived, with the use
of tankless water heaters.
the solar heat transfer and tends to result in very
high storage-tank temperatures.
It is, of course, possible to turn off the element or burner so that it does not fire automatically — assuming you remember to reset the
controls on cloudy days. But since most customers want seamless solar, if they forget to reset the
controls on overcast days, they are unlikely to
want to wait for the water to get hot when they
come home. I suppose we need an app for that!
It is also possible to purchase storage tanks in
which the supplemental heater heats only the
upper portion of the tank, leaving room for the
sun to do its job in the lower portion. That is a
better option, but unless the burner or element is
large enough or the volume at the top of the tank
is large enough to keep up with the hot water
needed for at least one shower, our customers
may be unhappy on some occasions.
Gas tankless units are
expensive to retrofit, because
we’re likely to need to increase
the size of the gas line and
change the venting. Incoming hard water also
may necessitate increased maintenance, something we are unaccustomed to doing with our
water heaters. Electric tankless units have their
challenges, as well. They require space in the
distribution panel, the capacity of the unit may
be limited by the amperage rating of the panel
(according to the National Electric Code, a
200-amp service can handle a 30-kilowatt unit),
and the wiring to the unit needs to be larger
in diameter than that used for standard electric water heaters. As with gas tankless heaters,
the hardness of the incoming water can lead to
increased maintenance.
Electric tankless heaters also raise concerns
at the electric utilities, which worry that these
large-capacity heaters will cause problems with
the distribution system. Common concerns
include dimming lights due to high-amperage
In my opinion, the best
technical match for solar
preheating is a water heater
that will heat the water
only when necessary
(on-demand) and only as
much as needed to reach
the set-point temperature.
draws when units turn on; flicker due to rapid
modulation of large-capacity devices; and overheating distribution transformers, particularly
if several households served by that transformer have electric tankless units. These are valid
concerns, but if the wiring to and in the house
and the transformer serving the house are all
sized correctly and the electric tankless has a
good modulation strategy, they are just that,
concerns. (By the way, I have heard similar
concerns from gas utilities when multiple gas
tankless water heaters are installed at the end
of a distribution line.)
Since we plan to have a supplemental water
heater, it makes sense to size the solar water-heating system to be more strictly a preheater.
In addition, since we implemented all of the efficiency measures discussed in part one of this article and our demand for heat has been reduced
by 25 to 50 percent, the panel sizing needs to be
reduced proportionately. Further, if our system
was only able to heat the water halfway from
incoming cold water temperature to the desired
hot water temperature, we would still have a 50
percent solar fraction, wouldn’t we?
The typical recommendation for the relationship between panel size and storage volume seems to be 2 gallons of storage per square
foot (81.5 liters per square meter) of panel.
We should look more carefully at increasing
this to 3 gallons per square foot (122 liters per
square meter) or maybe even 4 gallons per
square foot (162 liters per square meter). Perhaps this simpler solar system would be much
more affordable, and we would see greater market penetration, too. ST
