Solar Domestic Hot Water
The sun has been heating water since the dawn of time. So,
it is probably not surprising that it is pretty easy to do. Not only is
heating water with the sun easy, it is both necessary and relatively
expensive. Every household in the United States, with few exceptions, uses
large amounts of hot water for cooking, cleaning dishes, personal care and
bathing, and other cleaning purposes. A widely accepted rule of thumb for
the last few years is that a family of four spends around $800 per year in
energy costs to heat water. This number goes up with the cost of fuel and
goes down with any efficiency improvements like insulated water tanks or
low flow showers and higher efficiency dish washing machines. Since most
efficiency improvements are at their end, the cost for Domestic Hot Water
(DHW) can be expected to rise with the cost of energy.
If you only consider one solar technology, DHW is the one
to consider. Simple evacuated tube systems can be purchased on line that
are very easy to mount and use for less than the cost of two years of DHW.
Thus, you can reduce your pollution from DHW to near zero immediately and
reduce your energy bill from DHW to near zero within two years.
The DHW portion of your energy bill is likely about 50%
down to 25%. This means you can reduce your energy costs by 50% to 25%
with an investment that is fully recovered in less than two years.
Compared to other popular energy improvements like upgrading a dish
washer, upgrading a water heater, upgrading you house windows or upgrading
your house insulation. Solar DHW is many times more effective and usually
costs less. In my opinion, from an energy perspective, only homes built
before 1980 should have insulation or window upgrades before solar DHW is
I have had a situation repeat several times in the last
few years that is worth mentioning here. Everyone has periodic need to
replace their tank based water heaters. About 12 years ago I remodeled a
house and put in an "on demand" water heater. This device has no
water tank but rather a large burner that is capable of heating a
significant flow of water to a desired temperature as you use hot water.
The selling points recommending on-demand heaters were these:
No water tank to corrode and fail (longer life of
No limit to the amount of hot water available.
No possibility of biological (Legionnaire's disease)
due to insufficient heat in stale water.
Reduced energy consumption by reducing heat loss of
Smaller space requirement.
Since I installed my system I have had many ask me how I
liked my on-demand system as they were considering converting when their
tanks failed. I have recommended they not use an on-demand system, but
instead put their money into a simple solar conversion. My advise has only
been heeded once and greatly appreciated. Once a person who asked me
stayed with his tank system. The other times people have not heeded my
advise and installed on-demand systems and observed the negative
characteristics I am listing now:
There is a delay from the time you open a hot water
spigot until you get any hot water that is significantly greater
than that of a hot water tank. This is due to two things. First, the
core of the on-demand unit has to heat up which usually takes about
20 seconds. Second, the on-demand unit has to sense water flow which
only takes about 1 second. So, you have to waste 21 seconds of
water, energy, and time whenever you use an on-demand system that
has been cool for any length of time.
On-demand systems know to turn on by sensing water
flow. If you want a low flow of hot water for rinsing dishes for
example, the On-demand system may not turn on or may not stay on.
This means you end up using more hot water than you need in many
On-demand systems are significantly more expensive
than tank heaters to install because the flue usually needs to be
Most On-demand systems are open-loop, which means
they do not regulate their output temperature, but rather put as
much heat in the water as possible. This means hot water temperature
varies based upon the temperature of incoming water and the amount
of hot water being used throughout the house. You must manually
adjust the "remix" valve of an on-demand system seasonally
as incoming water gets colder or hotter.
There are circumstances where an on-demand system is nice,
like if you use a big whirlpool bath. But generally speaking, if you are
thinking about an on-demand water heater conversion, instead think
about a solar DHW solution.
There are many differing technologies and many very
qualified installers of these systems throughout the country. In fact,
many probably are advertised to the right of this page. See what these
people have to offer. Or, search ebay for a tube based hot water system
for a quick and easy DIY project that really pays off.