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Info on Solar and Radiant Heating:
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Solar Heating - A solar heat producing system that utilizes solar
collector panels, tanks, and circulators to transfer solar energy from
source to final destination only when available/needed (as opposed to
passive solar heating or thermosiphoning).
Circulators - often referred to (incorrectly) as "pumps", are used to
move the heat transfer fluid around the system. They come in two
"flavors": DC (can be powered with solar electric, or PV panels) and AC
powered (power provided by house AC panel).
Collector Efficiency - Percentage of sun's heat impinging on collector
that is turned into actual heat that can be transferred to storage or
heat load. Collectors operate most efficiently when the heat storage
medium is cool. Efficiency of collection actually drops as the solar
collection portion of the day proceeds(!). As a result, heat storage
should be drawn down (used) as far as possible daily. This is one
reason why low temperature radiant heating systems work so well with
solar.
Drainback System - A solar collection system based upon circulating
pure water from a storage tank up through the collectors, where it is
heated and returned to storage. This circulation continues throughout
the day while the sun is shining, and eventually heats the water in the
tank to (generally speaking) 130-180 degrees. When the system stops
actively collecting heat energy from the sun, the circulator turns off,
and all water "drains back" into the storage tank (leaving none in the
panels to freeze on cold nights). Drainback systems offer some
advantages over glycol based systems in terms of efficiency and
complexity, but are often constrained by where the pipes can be run
(they must have a continual downward slope, and may have to be
contained in a "conditioned" space in some cases).
Evacuated Tube Collector - A more recent innovation that uses absorber
plates encased inside an evacuated glass tube. The vacuum prevents
convection heat losses inside the collector (the most prevalent heat
loss mechanism in flat panel type collectors), thus giving higher
performance under certain conditions (colder ambient temps and cloudy
conditions).
Flat Panel Collector - The "traditional" collector that looks like a
"large skylight". Generally, copper tubing is pressed into a black
absorber plate, and the assembly enclosed in an insulated box. Heat
transfer fluid is circulated between storage system and collector,
slowly heating the storage system throughout the solar collection
period. Flat panel collectors are actually more efficient than
evacuated tube collectors under certain conditions (milder conditions,
such as in relatively warmer/sunny desert)
Glycol - Either propylene (used in most heating systems today because
it is non-toxic) or Ethylene Glycol (toxic, used in automotive
radiators). Glycol protects the system from freezing, but cuts down
somewhat on heat transfer compared to using pure water as the heat
transfer fluid.
Heat transfer fluid - In the case of solar and boiler subsystems,
either water or water/propylene glycol mix.
Indirect Sidearm Tank - A domestic hot water tank designed with an
internal heat exchanger coil. This type of tank relies on a boiler (or
solar storage!)to heat the domestic hot water thru the internal heat
exchanger. These tanks, though more expensive than standard tanks, are
more efficient, and are a good match for solar heat systems.
Modulating/Condensing Boiler- A newer type of boiler that extracts more
heat from the fuel by absorbing more heat from flue gases than a
traditional cast iron "non-condensing" type boiler. Mod/cons also have
the ability to "modulate" the burner downward (lower flame) as the
season gets warmer (springtime) when used in conjunction with "outdoor
reset".
Outdoor Reset - A feature in most newer boilers/ control units that
measures outside temperatures, and modulates the boiler (either flame
intensity, in a mod/con, or firing time with a cast iron model) to keep
boiler firing to the minimum level necessary to provide adequate heat
to the load. Outdoor reset is a very desirable feature for the energy
conscious homeowner!
Passive Solar Heating - See FAQ page for an explanation of passive
solar heating.
PEX Cross-Linked Polyethylene pipe - A long lasting, flexible tubing
used for both heat tranfer and potable water plumbing. Generally much
less expensive than copper. Beware of using in the solar colection loop
though! PEX is only designed to witstand a maximum of 180 degrees, and
the solar collection loop can rise above 300 degrees on a sunny day!
Primary/Secondary Heating Loops - A method of plumbing the heating
system that allows greater flexibility in trading off between solar
heating and the backup boiler while offering finer control of heat
distribution from either type of heat source.
Pump Head - The fluid preessure that a pump or circulator develops to
overcome system head (defined below). A "high head" pump develops more
pressure, and therefore more fluid flow, than a low head pump under the
same conditions of system head. Pump head can also be expressed in psi
(2.34 psi = 1 foot of head). "Pump Curves" are a graph for a particular
pump (or circulator) pumping against various conditions of system head,
and are used to pick the appropriate pump or circulator for a given
system.
Radiant Heating Panels - Generally, PEX tubing placed under the surface
of the floor creates a "radiant panel". Lower temperatures are required
because of the large area of the radiant panel when compared to
radiators, convectors, or baseboard heating elements. Many consider
radiant floors to offer superior comfort to the other types of heating
elements. The low temperature requirement allows for much greater solar
and boiler heating subsystem efficiency, thus translating to lower fuel
costs.
Solar Storage Tank - A large, usually non-potable, hot water storage
tank, either pressurized or non-pressurized, that stores heat for later
transfer to the home heating system. Separate and distinct from the
Domestic Hot Water (DHW) tank used to store hot potable water (for
showers/washing/etc.).
Solar System Efficiency - Percentage of heat gathered by the collectors
that is actually delivered into the heating subsystem.
System Head - The resistance to fluid flow through the heating system
piping caused by friction (dynamic head) and vertical lift (static
head). Measured in "feet of head". Note that their is no "static head"
in a closed loop glycol system, since the loop is fully filled with
fluid (and therefore no fluid needs to be "lifted" against gravity).
Thermal Buffer Tank - Used in the boiler subsystem to both increase
boiler efficiency, and stop boiler from "short cycling", or turning
on/off too frequently (which decreases efficiency and increases
wear/tear on the boiler). Also can be used to store heat from an
electric or fossil fueled boiler for later release.
Thermosiphon System - transfer of heat from collector to load or
storage destination utilizing the different densities of hot and cold
water to set up flow of the heat transfer fluid in the pipes.
Thin Slab Radiant - A PEX based system where pipes are snaked thru a
thin layer of masonry (usually concrete or gypcrete) in the floor.
Particularly appropriate for use in solar heating due to lower
temperatures needed to heat the floor.
Thick Slab (or high mass) Radiant - The same as above, but with a
thicker concrete (or similar substance) pour. A thick slab has more
thermal mass than thin slab, and so stores more heat. This extra heat
storage can augment or even replace the solar hot water storage tank.
The temperature of a high mass floor varies more slowly with
addition/disipation of heat than a thin slab system. This is both a
good and possibly bad feature for some homemowners. For those who
prefer turning down the heat at night to sleep more cozily, a thin slab
system may be a better match.
Water - The BEST commonly available heat transfer and storage medium.
Many people think that rocks, masonry, steel, etc are better at storing
heat, but this is not the case. Water is the standard to which all
other forms of heat storage are compared. Water also allows for
efficent transfer of heat from heat source to where it's needed. This
does not mean it should be the ONLY heat storage medium to be designed
into the heating system, however!
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