You’ve squeezed the water and energy inefficiencies from your hot water distribution system and determined that you can’t live without a backup for your solar water-heating system (see part one of this article, September/October issue: solartoday-digital.org/
solartoday/20100809). Now it’s time to size and
select your supplemental heater.
units, or Btu, per hour, or 15–30 kilowatts). The
latter is my preference.
Table 1 shows the capacity of different types
of water heaters needed to continuously heat
1 gallon per minute at various temperature differences between incoming cold water and the
set-point temperature. When the water is cold
— requiring a large temperature rise — such as
when it has been cloudy in the winter, the power
input needs to be large. Remember to proportion
the capacity of the heater up or down depending
on the actual flow rate of hot water.
The supplemental heater needs to be able to
accept or deal with very hot preheated water.
That is not a problem for storage water heaters,
as they accept whatever temperature water they
are given and turn on only when their thermostats register cold water. Standard gas-fueled tankless water heaters do not like high-temperature
inlet water. The manufacturers advise the use of
a tempering valve between the solar storage and
the cold water to ensure that the water entering
the tankless unit is cold enough for the water
heater to fire. That reduces the amount of solar-heated water used in any given hot water event,
keeping more of the sun’s energy stored in the
tank for future use. But the point of the solar
water-heating system is to use it, isn’t it? Electric
tankless water heaters appear to have no problem with high-temperature preheated water, they
don’t turn on when the temperature is above the
set point, and when it is below the set point, they
heat only the amount needed.
The supplemental heater needs to be able to
provide very small amounts of additional energy
for boosting. This requirement is a challenge for
what size supplemental heater
Do you Need?
Most solar water-heating systems are, in fact,
preheating systems. The solar portion of a solar
thermal system heats the water whenever the sun
is shining and stores the hot water in a tank so
that it can be used whenever the demand arises.
We want to avoid overheating the water in the
tank and the fluid in the panels as much as possible, and we don’t want the panels or exposed
piping to freeze.
The size of the supplemental heater depends
only partly on the amount and temperature of
the stored solar-heated water. The supplemental
water heater needs to do three things:
1. Be able to meet the entire load when the
sun hasn’t shined strongly in several days;
2. Accept or otherwise deal with very hot
sun-heated water, say, 180°F (82°C) or hotter;
3. Boost the temperature as little as 1°F at
a flow rate as small as 0.25 gallons per minute
(gpm) when the water temperature in the solar
storage tank is off by just a little bit and the
demand for hot water is small.
So, if we expect a “seamless” solar water-heating system, the supplemental water heater
must be able to do the entire job expected of the
water-heating system regardless of how much
energy has been provided by the sun. That effectively means that we have duplicate systems.
Hmmm.
The supplemental heater needs to be able
to meet the entire load. Customers want “
continuousness” from their hot water system, not
recovery rate, which implies that they have run
out of hot water. According to the people I have
interviewed, continuousness means never running out in their getting-ready-for-work shower.
This can be accomplished in any of three ways:
a large-enough storage tank with hot-enough
water; a burner or element large enough to sustain any desired flow rate; or a combination of
some stored volume and a more modest burner
or element ( 60,000–120,000 British thermal
1
5
10
20
30
40
50
60
70
80
90
Customers want “
continuousness” from their hot
water system. This can be
accomplished in any of three
ways: a large-enough storage tank with hot-enough
large enough to sustain
any desired flow rate; or a
combination of some stored
volume and a more modest
burner or element ( 60,000–
120,000 Btu per hour, or
15–30 kilowatts). The latter is
the author’s preference.
most tankless water heaters, particularly the
most commonly advertised gas units, those without any built-in storage. Although it is, as yet,
rare to see hot water outlets with such low flow
rates in residential applications, it is common
in the hot portion of a hands-free faucet found
in public lavatories. It is also common when a
Capacity of Water Heater Required to Heat 1 Gallon per Minute
Electric Power (k W) Fossil Fuel (Btu)
Coefficient of Performance (COP) Thermal Efficiency
Temp. Rise (˚F) Resistance Heat Pump Natural Gas, Propane, Oil
0.9 0.95 1 2 2. 5 3 70% 75% 80% 85% 90% 95%
0.16 0.15 0.15 0.07 0.06 0.05 714 666 625 588 555 526
0.81 0.77 0.73 0.37 0.29 0.24 3,570 3,332 3,124 2,940 2,777 2,631
1.63 1.54 1.46 0.73 0.59 0.49 7,140 6,664 6,248 5,880 5,553 5,261
3. 26 3.08 2.93 1.46 1.17 0.98 14,280 13,328 12,495 11,760 11,107 10,522
4.88 4. 63 4. 39 2. 20 1.76 1.46 21,420 19,992 18,743 17,640 16,660 15,783
6. 51 6. 17 5.86 2.93 2. 34 1.95 28,560 26,656 24,990 23,520 22,213 21,044
8. 14 7.71 7. 32 3. 66 2.93 2. 44 35,700 33,320 31,238 29,400 27,767 26,305
9.77 9. 25 8.79 4. 39 3. 52 2.93 42,840 39,984 37,485 35,280 33,320 31,566
11. 39 10.79 10. 25 5. 13 4. 10 3. 42 49,980 46,648 43,733 41,160 38,873 36,827
13.02 12. 34 11.72 5.86 4.69 3.91 57,120 53,312 49,980 47,040 44,427 42,088
14. 65 13.88 13. 18 6. 59 5. 27 4. 39 64,260 59,976 56,228 52,920 49,980 47,349
