Solar
Heating / The Whole Story & More.
Hydronic,
or radiant floor heating is a method of heating a home,
shop, or other building with the heat concentrated in
the floor. It works by embedding special tubing in a
concrete foundation or in a thin concrete mixture or
wooded track system on top or below of a wood-framed
floor. Heated water (or a food-grade antifreeze mixture)
flows through this tubing, warming the thermal mass
of the concrete.
Conventional
forced-air systems, wood stoves, or other heating methods
produce uneven heat, with the highest air temperatures
near the ceilings. Hydronic heating puts the heat in
the floor under your feet, gently warming a room or
a complete structure. This results in similar heating
levels with superior comfort without wasting energy
and money in monthly fuel bills. The warm water circulated
through the tubing in a radiant floor may come from
solar collectors, water heaters, demand water heaters,
wood stoves, or heat pumps.
I
asked Stephen Heckeroth to describe the technology,
design issues, and construction and installation techniques
related to hydronic heating in general, and heating
radiant floors with solar collectors specifically.
MH:
I first heard of running heated water through tubing
in the floor a few decades ago. I think I heard about
the ones that didn’t work. Copper tubing that
leaked or corroded. Water that froze and cracked the
concrete. What’s the situation today? What kind
of tubing have you used?
Above:
Forced-air heating versus the ideal warming curve for
a person.
Below:
Radiant floor heating is a good match for human comfort.
Stephen:
The technology, materials, and techniques have come
a long way in the past decades. I use PEX tubing from
Wirsbo. It is specifically designed to withstand the
rigors of being embedded in concrete  and
exposed to water at high or low temperatures. It’s
available in a variety of diameters — 3/8-inch,
1⁄2-inch, 5/8-inch, 3⁄4-inch and 1-inch.
The 5/8-inch diameter tubing is popular because it offers
a good balance between cost and pressure drop. The 3⁄4-inch
and 1-inch tubing are relatively expensive. The 3/8-inch
and 1⁄2-inch offer too much resistance, which
means more energy consumption to pump the liquid through
the pipe. The 5/8-inch tubing is the minimum size needed
for thermosiphon. The tubing comes in 300-foot and 1000-foot
rolls.
MH:
We should explain that thermosiphon is a natural flow
of water. It is a result of water being heated and allowed
to rise convectively as part of a circulation plan in
a closed-loop system. For example, water heated in a
solar collector will naturally want to rise, effectively
both pushing and pulling at cooler water in a circulation
pattern. It’s a low-tech way to move heat from
a collector to storage and use.
Stephen,
will you describe the layout pattern for the tubing?
Stephen:
The PEX tubing is laid in patterns called zones in the
pad area to be poured with concrete. A zone might be
one room. A larger room might need two zones. These
zones terminate in a header pipe that is connected to
the source of heated fluid. The length of the zone determines
the diameter of the tubing. A small zone of 3/8-inch
tubing will need the same pump effort as 5/8-inch tubing
of a longer length. Since there is resistance in any
tubing, 280 feet is the largest distance recommended
by the manufacturer for 5/8-inch tubing.
The
tubing is laid out in an exaggerated S-pattern, with
many variations. It may be as tight as six inches on
center (distance apart) or up to 11⁄2 feet apart.
A 12-inch on center pattern is common. The zones should
be placed wherever there is foot traffic. Position tubing
in front of the toilet, near the tub, and in front of
the sink in the bathroom. Use the same strategy for
the stove, the kitchen sink, and around the dining table.
If you’re working from a detailed plan, avoid
areas like under cabinets or in closets. Increase the
spacing of the tubing to 11⁄2 feet apart in areas
that are less traveled. The average size of a zone is
about 250-400 sq. ft.
Wirsbo
has created a manual (CDAM, 185 pages, $5 from Wirsbo)
that lays out additional patterns that address specific
issues or preferences. The manual is extremely useful
for understanding the hardware, issues, layouts, options,
and methods of heating from virtually any energy source
in any climate. In Western Europe, 50% of all new construction
uses radiant floor heating systems.
MH:
Is there any difference in strategy with a system that
will depend on solar versus one that is dependent on
propane or wood heat?
Stephen:
Generally, yes. With solar heating, you’re counting
on the concrete to act as thermal mass. Slow to heat,
slow to cool. With propane heating, the mass isn’t
really necessary. The thinner slab, maybe as little
as 2 inches thick on an existing floor, will heat faster
than a big slab but it won’t hold the heat long.
MH:
Is this garden-variety concrete you’re talking
about?
Stephen:
Yes and no. Regular concrete works for thick slabs (4
inches plus) and solar-assisted heating. For thin slabs,
use Gypcrete and Flowcrete. They’re like concrete
but not as hard. Using them doesn’t result in
a finished floor. You must finish the floor with tile,
linoleum, or some other cover.
MH:
Radiant-floor heating seems a perfect application for
solar heating. In your experience, is this true?
 Stephen:
If you’re investing in a concrete foundation and
slab, it makes sense to have it work for you in another
way, as thermal mass. A thin layer of insulation under
a concrete slab will serve to keep the ground from acting
as a heat sink. At the same time, the ground serves
to help regulate the slab temperature because any extreme
will be tempered by the earth’s relatively constant
temperature.
For
solar heating, you will want a 4-6 inch thick slab.
It will take a long time to change the temperature of
that much thermal mass and its earth connection. It
will tend to be cool in summer and vertically-mounted
solar collectors will keep it warm in winter.
MH:
Can you give me a ballpark figure for cost of the Wirsbo
tubing?
Stephen:
Retail, a 1000-foot roll of 1⁄2-inch tubing is
about 70 cents a foot. It’s about 80 cents a foot
for 5/8-inch tubing. The tubing comes with or without
an oxygen barrier. I prefer the non-barrier because
it is less expensive and I’m careful not to use
fittings that will oxidize. A system designed to use
solar-heated water that circulates by thermosiphon is
susceptible to blockage by air bubbles. It’s hard
to avoid them where the tubing lies so flat or may have
high spots. Bubbles in the water accumulate in the smallest
high spots, finally blocking the flow. A small in-line
centrifugal pump, 1/20th of a HP in rating, can be used
for purging. It will circulate water through the tubing
fast enough to dislodge an air bubble. The purge pump
only comes on when the system stagnates and the collectors
overheat. When circulation is restored, the pump shuts
off.
How
do you know when a bubble blocks a thermosiphon flow?
Install temperature sensors at various points in the
system and connect them to a differential control. Use
the kind of sensor that fits in a tee off the plumbing
and accepts a probe from a digital meter. When the difference
in temperature between two points, i.e., at the top
of the collector and at some point in the concrete,
reaches a preset value, it will run the purge pump until
the thermosiphon flow is restored.
 MH:
In one hydronic heating installation I saw ball-cock
valves on each tube that led away from the manifold
to a zone. Presumably, this gave the owner control of
the individual zones, which room was heated, which was
not. How well do these work in a solar-heated system?
Stephen:
I don’t use zones in a solar-heated system. There
may be many loops but the whole floor is treated as
one zone. The system is always on. With vertically-mounted
collectors, the floor is heated by the sun through three
seasons and cooled to earth temperature in the summer.
The thermal mass is a huge thermal flywheel. You dump
heat into it in the winter and take it out in summer.
MH:
Does this system also handle domestic hot water for
showers, dishwater, and laundry?
Stephen:
Solar panels for a radiant-floor heating system are
angled to intercept the rays of the winter sun, which
sits 20-35 degrees above the southern horizon at noon.
Domestic hot water usage must be angled to optimize
heat gain year-round, so the collector must be pointed
toward a mid-point, roughly 45-60 degrees above the
horizon in the continental USA. Of course, these collectors
circulate this water through a storage tank for later
use. In the McMillan house, additional collectors were
added at the west end of the building and tilted to
utilize summer sun for domestic hot water.
MH:
What other plumbing is needed for the radiant floor
system?
 Stephen:
I’ve already mentioned the in-line pump which
is used primarily for purging the system of air bubbles.
It must be centrifugal or the water will not flow through
it during thermosiphon. An air-bleeder valve is needed,
as is an expansion tank and purge valves. This is standard
equipment.
MH:
Will you describe the requirements of the insulation
under a concrete slab that will act as thermal mass?
Stephen:
The insulation works only as a thermal break. It shouldn’t
have a very high R-value because we want the slab to
act as a heat sink in summer. I used foil-faced bubble
wrap material which is made specifically for under-slab
use. It doubles as a thermal break and radiant barrier.
And it’s inexpensive. Rigid foam, like foil-faced
technifoam or blueboard, also works. Around here, the
ground under a slab remains at a constant 58 degrees
F. Further north, the ground temperature is colder and
more insulation is required. Further south, little or
no insulation is required. Carlsbad Caverns stays at
a constant 70 degrees F. while the surface temperature
outside varies between zero and 115 degrees F.
MH:
Can you describe the site preparation for pouring the
foundation for a radiant floor?
Stephen:
The overall depth of the “floor” is about
8 inches thick. The process?
Cover
the excavation with two inches of dry sand. The ground
will tend to be damp so it must be dried up, and then
covered evenly with sand.
Lay
in one inch of foam or 1⁄4-inch bubble wrap. Don’t
scrimp; it’s cheap.
Spread
dry sand over the insulation to hold the insulation
in place and to keep bubbles from rising up through
the poured concrete and spoiling the finish.
Add
the wire mesh. I use 6-6-10-10 wire. This is #10 wire
in both directions, 6 inches on center. Bending back
the corners will tend to flatten the wire perfectly.
Lay
out the pattern of radiant tubing and tie it to the
mesh. Run the tubing from each zone up into the manifold.
The manifold is a pipe header, 3⁄4-inch to 1-inch
in diameter and made of brass, with tees to accept the
tubing.
Pour
the concrete. This should be 4-6 inches deep.
MH:
Will you describe the radiant floor system of the McMillan
house?
Stephen:
The McMillan house has a total of eight loops. The house
is open plan, so there are four loops in the great room
(kitchen, dining area, living room), two in a family/guest
room, and one each for the two upstair bathrooms. The
design called for direct solar gain on the south-facing
side, solar thermosiphon to a back-up propane tank on
the east end, and direct thermosiphon with a purge pump
on the west end. Propane is the backup heating source.
Thermosiphoning
tips
1. Use thermosiphon only in areas where freezing temperatures
are rare.
2. Cold pipe from bottom of tank to bottom of heat source
should slope down so as not to trap air.
3. Use a tee off the tank drain as the cold pipe returning
to collector so all the tank water is heated. (Avoid
using the standard cold water inlet in water heaters
as part of a thermosiphon loop.)
4. Hot pipe should slope up from top of heat source
to 1⁄2 to 3⁄4 up the side of the tank to
allow room for heat and air bubbles to rise in tank.
5. Locate an air-release valve and an expansion tank
at the highest point in the system.
6. All pipes should be insulated.
7. Avoid the use of L’s and reducers as much as
possible.
8. If a heat source is added to back up the collectors,
the sensor to control it should be located near the
top of the tank.
9. Use timers or other sensors to ensure that backup
heating cannot operate until the sun has had sufficient
time to heat the water.
 A
direct-vent, 80-gallon propane water tank is used on
the east end with a simple timer. The timer won’t
heat water for the floor until afternoon, giving solar
energy a chance to heat the system. If it hasn’t,
the timer engages the fan on the propane water tank
which, in the unit I use, will permit the heater to
switch on. A small pump circulates water through a heat-exchanger
in the tank and then through the radiant floor tubing.
I
like to minimize controls in systems because they don’t
last, and the system performs erratically or fails.
I will use differential temperature sensors. When the
floor is cooler than the water in the tank, the pump
turns on. This pump motor draws 80-watts.
MH:
We haven’t talked about solar-thermal panels yet.
Stephen:
The solar-water-heating collectors in this installation
are mounted vertically against the south-facing outside
wall. This maximizes the winter heat gain and impedes
any significant heating effect in summer. There are
many brands available, new and used.
The
panels in the McMillan house came used from Triple A
Solar in New Mexico. They have 1-inch diameter headers
and 1⁄2-inch risers in a 10-foot by 4-foot, bronze-iodized
aluminum case, 5 inches thick. The riser tubes and fins
are black-chromed copper to capture and channel the
heat converted from sunlight. The only plumbing requirement
is to use only similar metals in all parts to avoid
premature corrosion. The collector’s glazing is
tempered glass, with a roughened surface to minimize
reflection.
MH:
I understand there are quite a few used solar water-heating
modules out there. When tax rebates and write-off legislation
spurred a boom in the solar water-heating industry a
few decades back, a lot of different companies got involved.
Earlier, you mentioned designing a system with few controls.
A major failing of the industry years ago was the control
system. It was too complicated, too varied, too prone
to malfunction. On the other hand, many of the collector
designs of that period were solid. It was some other
part of the system that failed, not the collector. These
systems are still being stripped from buildings or replaced
by newer designs.
Stephen:
Used water-heating collectors are widely available.
Used collectors from Triple A Solar were $150 each.
New, these collectors would be over $500 each. Panels
that have been removed from a system may prove to be
a good investment. A simple pressure check will find
any leaks.
MH:
Let’s talk about freezing climates, solar water-heating
modules, and radiant floor heating systems. The danger
in any solar water-heating system is that water may
freeze in the collector and burst a pipe. At the least,
a mess. Certainly inconvenient. Likely expensive. This
is a challenge in solar collectors in systems for domestic
water heating. What about solar collectors for radiant
floor heating systems?
A
radiant floor system using a water heater as an energy
source
 Stephen:
There are two ways to approach this problem in cold
climates or warm climates that get an occasional freeze.
The first uses ordinary tap water and relies upon a
thermal-bleed valve, or Dole valve. This valve is designed
to start dripping when the water at the valve drops
to a preset temperature, either 38°F or 43°F.
Moving water freezes at a much lower temperature than
water which is stationary. A drip valve acts like a
leak in the system, letting water out, bringing in new
water warmed from the slab or tank. As it gets colder,
the Dole valve drips even more. I’ve found the
Dole valve to be reliable on the northern California
coast where freezing temperatures are rare. It needs
to be checked and cleaned annually, but it is perfect
for a mild climate.
The
other technique to avoid freezing the collector is to
add polypropolene glycol to the water. This is a food-grade
anti-freeze used as a dough extender in the baking industry.
It’s about $10 a gallon but you don’t need
much. A 10% solution will protect the collectors down
to 20-25 degrees F. Use a higher percentage for correspondingly
lower temperatures.
MH:
Stephen, thank you for taking the time to share your
experiences with BHM’s readers. Any closing thoughts?
Stephen:
Orientation is 80% of solar design. Good orientation
means choosing a building site with unobstructed solar
access, making maximum use of the south-facing roof
and walls, and using a lot of insulation in the north
walls and roof. The south-facing roof is the place for
solar-electric modules and collectors for domestic hot
water.
Most
of the window area (7-10% of the building’s floor
area) should be located on the south-facing walls for
daylighting and direct solar gain in winter. The building
plan should be designed to accommodate vertically-mounted
solar collectors for radiant-floor heating here, too.
Add overhangs to thwart solar gain in the summer. The
north and west walls should have minimum window area,
less than 2% of the floor area for north windows to
avoid heat loss and west windows to avoid afternoon
overheating. The east wall should have windows of 4-6%
of the floor area for early morning warmup.
The
ideal building site slopes down to the south, increasing
solar exposure and facilitating convection and thermosiphon
loops. The north side of the building should be dug
into the slope to prevent heat loss and increase the
earth connection. In my experience, the owners of a
well-designed solar home will pay little or nothing
for electricity or heat for the life of the building.
|
Radiant
floor heating is simple. It works by using the floor
as a giant radiator. Plastic tubes are laid out on the
wood sub- floor and then embedded in a light weight
concrete, or in the case of concrete slab-on-grade construction,
the tubes are embedded in the concrete. Warm water supplied
from a boiler circulates through the network of tubes
gently warming the floor. The warm floor then radiates
to all objects in the room.