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More
Info on Solar and Radiant Heating:
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Radiant heating (PEX tubing embedded in the floor, wall, or ceiling),
solar heat, and
high efficiency modulating/condensing boilers are the perfect
combination to provide the highest efficiency heating systems available
at a reasonable cost. Radiant heating also happens to provide the
absolute highest comfort available in a modern heating system. In other
words, there are no "tradeoffs" involved between comfort and utility
costs for these systems. An explanation for why this is the case
follows.
Radiant panel systems operate on the principle of a large radiant area
(the floor/wall/ceiling surface) being heated to a comfortable
temperature of
approximately 80 degrees, give or take a few degrees. A heating fluid
temperature of approximately 85-120 degrees is needed, depending on
building heat load, flooring materials, etc. Given the large radiant
area, this lower temperature is adequate to supply the heat load. Other
types of heat supply devices, such as radiators, baseboard, and fan
coils, require the heating fluid to be supplied at a much higher
temperature, say, 160 - 190 degrees. Since the fluid temperature
required for radiant is
much lower than for radiators/baseboard, radiant is a great match for
the lower temperatures provided by solar and "modulating/condensing"
(mod/con) boilers.
Here's why mod/con boilers are so much more efficient: A traditional
cast iron boiler operates with an output temperature of approximately
165-195 degrees. This temperature is required for two reasons. First,
the heat radiating devices require the high temperatures because of
their relatively small area (as compared with a large radiant floor
surface). Secondly, and less well understood, the RETURN temperature
from the radiator/baseboard to the boiler is kept above 140 degrees to
prevent condensation of flue gases (the byproduct of natural
gas/propane combustion) on the exterior of the cast iron or steel heat
exchanger. If condensation of flue gases were to occur, then the heat
exchanger would rust out quickly, thus destroying the boiler. So what
we have with a traditional boiler (and why it's not such a good match
for radiant systems or solar), is a firing range of 165-195 degrees,
and the consequent higher usage of fuel. If used with a radiant floor
system, the conventional boiler's high temperature output must be
"mixed down" (mixed with cool return water from the radiant floor) to
the desired range of 85-120 degrees. This is not the most efficient use
of fuel.
By contrast, the mod/con boiler acheives it's high efficiency as
follows: First (the "con" part), it fires in a much lower temperature
range. They can be programmed to fire from 50 to 185 degrees, but
generally fire from 95-145 degrees (the perfect range for the radiant
floor system). When firing below approximately 130 degrees, the cool
returning fluid from the radiant floor intentionally causes flue gas
condenstation on the heat exchanger. In the case of the mod/con boiler,
the heat exchanger is made of stainless steel (not cast iron), and the
condensate drains out of the boiler into a special condensate drain.
The nice thing about this condensation is that the heat exchanger thus
captures the "latent heat of vaporization" from the condensation, and
extracts approximately 10% more heat from the flue gasses before they
exit the exhaust! This is why effciencies in the 90 percent range are
obtainable from mod/con boilers (when they are configured correctly!).
Secondly (the "mod" part), these boilers "modulate" the burner to match
the seasonal heat load. If it's fairly warm outside, say 40 degrees,
the boiler will fire at a lower rate than if it were10 degrees outside.
Operational efficiency of mod/con boilers can be increased by designing
the radiant floor so that it requires the lowest temperatures possible
to still satisfy the given heat load. That is why an in-depth heat
analysis must be performed to provide for optimal efficiency.
The solar storage tank also fits well in the radiant heating scenario,
since it "operates" in the range of, say, 90-170 degrees. Regardless of
how high the temperature in the tank rises, it can be "mixed down" to
the required range before being circulated through the floor. If, for
instance, the solar tank reaches 160 degrees, the radiant mixing device
will "moderate " the temperature (by mixing it with cooler fluid
returning from the floor) so that it will be , say, 115 degrees as it
is delivered to the floor. As the tank temperature is drawn down (loses
heat), a fairly constant heat supply can thus be provided to the floor.
Heat delivery occurs until such time as the usable heat is depleted
from the tank, and the boiler automatically kicks in to provide the
balance of the required heat load. The highest efficiency can be
obtained from solar storage if the temperature of the tank can be drawn
down as low as possible. In other words, if as much heat as possible
can be drawn from the tank before the heating system
switches over to the boiler. Although this makes common sense; perhaps
less well understood is that, if the tank is as cool as possible the
next morning when the sun starts shining again, it will "grab" more
heat from the collectors than if the water in the tank were warmer.
Another way of
saying this: the cooler the fluid temperatures going into the solar
thermal collector, the higher the collection efficiency.
Although radiant floor is the best match for solar and high efficiency
boilers, there are gradations in radiant floor design that should be
discussed: First of all, the type of finish floor placed on top of the
radiant floor has a very marked effect upon the supply temperature.
This is significant because any increase in the supply temperature
decreases the efficiency of both the solar and mod/con boiler
subsystems. To some extent, this can be avoided by proper choice of
finish surfaces. As with anything other technical issue, proper design
choices can maximize efficiency in a given situation.
From the efficiency standpoint, a poured concrete floor offers the best
efficiency for a radiant panel installation. The reason is that
concrete (either thin slab or high mass) has a good heat transfer
capability, and transmits the heat directly into the room. Concrete
floors can be stained and stamped to produce an aesthetically pleasing
surface. Poured gypsum (with a tile or other masonry covering) is a
close second in efficiency, and has it's own advantages in some
situations (especially when retrofitting in older houses). If a wood
floor is contemplated, a thin engineered flooring, such as bamboo or
hardwood, won't impede heat flow as much as as, say, a 3/4" tongue and
groove floor. When using a wood flooring, heat spreader plates can be
used to assist heat tranfer from the pex through the wood flooring. Of
course, carpeting/carpet padding have the highest "R" value of all of
the flooring surfaces discussed so far, and will require the highest
supply temperatures. Consider throw rugs instead of wall-to-wall
carpeting! Alternatively, consider radiant ceiling heat!
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