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.
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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.