The
Crestone Eagle, September 2007:
A simple, elegant solar hot water drain-back system (for
home heating and domestic hot water)
story & photos
by Paul Shippee
 We
live in a climate perfectly suited for solar heating of homes:
cold winters with lots of sunshine. Yet most of us, including
those in new homes, still use propane or electric for home
heating and hot water needs. That large propane tank sitting
in the yard must soon give way to a heating source that is
more stable, cheaper in the long run, and less vulnerable
to world market fluctuations and distant resource wars.
To help with the transition to a more reliable, cheaper,
distributed, cleaner and saner energy source people are waking
up to solar energy. Using today’s sun today, rather
than drawing down our fossil fuel bank account, makes wonderful
sense—especially for low temperature applications.
In addition to uncertain fossil fuel energy prices, interruptible
supplies and immoral resource wars, consider the ecological
impact of the peak oil era we are now entering. Using high
energy sources of energy (like propane and electricity) for
home heat is a sin (i.e., misses the mark). This is because
the capacity to do high energy work is permanently lost by
burning up our high energy finite fossil fuel supply to do
low temperature jobs like heating homes and domestic hot water.
We can no longer afford such extravagant mistakes.
One economic difference between using fossil fuels and using
solar energy is that one is an ongoing cost whereas the latter
is an investment, a hedge against future uncertainties.
There are simple technologies that have existed for a long
time that have proven to be effective and a good investment
for heating homes and domestic hot water with the sun. The
simplest and most efficient active solar system I have found
for heating homes, after thirty years of designing them, is
to combine regular flat plate solar thermal collectors with
a thermal mass radiant floor heat distribution system. The
marriage of these two simple technologies yields a low temperature
operating system that produces the highest solar collection
efficiency of any alternative. It also provides a comfortable
and silent heat distribution floor system.
What I am talking about here is an active solar system that
utilizes a pump to circulate water through outdoor thermal
solar collectors mounted, let’s say, on the roof. The
water is pumped up to the solar collectors, where it collects
heat from the sun during the day, and is circulated directly
through pipes embedded in the thermal mass floor, where the
solar heat is released and stored for night time distribution
off the surface of the floor as needed. The stored heat will
only come out of the floor as the room cools, thus the term
“as needed”.
The high efficiency factor in this simple system is due to
the fact that since floors naturally have large areas, the
temperatures needed to heat a room to 65-70 can be quite low.
Solar heat collection efficiency then is quite high because
the system runs cooler and less solar heat is lost to the
outdoor cold winter air through the collector glass during
sunny daytime hours.
Passive solar systems, where south windows admit sunshine
directly into the house interior are, of course, much simpler—and
these can be used in conjunction with an active system to
provide attractive views and sunlight on winter days. In our
climate, passive south facing windows require night time thermal
shades for comfort and increased efficiency. This prevents
the heat from going out those cold south windows on long winter
nights.
When using solar thermal collectors in our climate the freeze
hazard must be addressed. The liquid used to transfer solar
heat from the collectors to the radiant floor is subject to
freezing. One method is to mix antifreeze into the water just
like in your car engine. Antifreeze is toxic, requires replacement
maintenance with aging, corrodes copper piping causing leaks,
and transfers less heat than plain water.
For all these reasons, it is preferable to use plain water
as the heat transfer liquid. But plain water will freeze when
left in the solar collectors on cold winter nights. In this
case, the freeze proofing method is to design the system plumbing
so that the water circulating between the solar collectors
and the radiant floor tubing “drains back” into
the warm home interior at the end of the day when the pump
turns off. Thus the name: drain-back system. This system requires,
of course, some simple electronic controls which are beyond
the scope of this article.
 The
diagram shows a schematic layout of the plumbing aspects of
a simple, elegant drain-back solar heating system for home
heating and domestic hot water. Notice in this configuration
that only one pump is required to operate the entire system
for solar heat collection, space heating distribution, as
well as domestic hot water. This is one aspect of why it is
simple and elegant when compared to some antifreeze systems
that use up to five pumps to accomplish the same function.
The system requirements to achieve this simplicity are the
following:
1. Solar collectors are mounted higher than the radiant floor
and domestic hot water thermal storage tank.
2. The radiant floor has high thermal mass of earth or concrete
materials.
3. The hot water back-up temperature booster (electric or
gas) is located downstream from the domestic hot water storage
tank. That is, the back-up energy should not be used to heat
the solar thermal storage.
4. The storage tank is a passive heat exchanger consisting
of a tank within a tank design (commercially available) of
about 80 gallons.
5. Two zone valves are used in conjunction with a timer to
direct water to the radiant floor thermal mass OR to the domestic
hot water tank according to a specified control schedule.
6. A small drain-back accumulator tank is provided high up
in the circulation line. It contains enough air to replace
the water in all the collectors when the system drains back
each time the pump turns off.
7. All piping, including the collectors, are tilted slightly
to facilitate the drain-back operation.
The cost of such an installation might range somewhere in
the neighborhood of $100 per square foot of collector area.
However, a cost estimate to suit your specific situation is
best obtained from a solar designer, installer, or contractor.
This cost may be substantially offset by various tax and rebate
incentives offered by state and federal agencies. To keep
up to date on these changing subsidies visit http://www.crestonesolarschool.com
where you will find a list of websites that monitor and report
on tax credits.
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