Zoning for Comfort
The ability to accurately control heat delivery among
several independent zones is a major advantage of hydronic
heating. The more zones there are, the greater the ability to
adjust the system to individual preferences.
Most clients instinctively like the idea of having lots of
zones until they realize the extra cost involved. Sometimes,
too, people will spend the money for elaborate zoning, then
seldom use it. Here are some guidelines for matching zoning
needs to the budget.
Zone controls are not always
Remember that in many cases baseboard
systems can be designed to keep different parts of a building
at different temperatures without adding zone controls. Simply
closing the damper on a baseboard can reduce heat output to a
room by about 50%.
that aren’t used much are obvious candidates for zoning.
Examples include workshops, guest rooms, and basements. Energy
savings will easily repay the extra cost of putting such areas
on a separate zone.
Bathrooms are good candidates for
With the bath zoned separately
from the bedroom, the homeowner can sleep in a cool room, then
step into a toasty-warm bathroom for a morning shower.
Consider other heat
Heat delivery to sunny rooms and areas
with fireplaces or woodstoves should be able to be interrupted
without affecting other areas of the house.
common way to build a zoned system is to use a separate piping
circuit to and from each zoned area, equipped with either a
circulator or a zone valve. Heat input is controlled by
individual thermostats in each zone. Hot water flows through
the zone circuit only when its thermostat is calling for
Although both circulators and zone valves have been used in
thousands of systems, I prefer circulators. The cost is
slightly higher, but so is the long-term reliability. If you
install a multi-circulator system, be absolutely sure a "flow
check," or spring-loaded check valve, is installed in each zone
circuit to prevent off-cycle heat migration and reverse
Monoflo piping. One
piping alternative is called a "monoflo" system. With this
approach, the baseboard in each room has its own thermostatic
valve and can be separately controlled. The piping is arranged
so that hot water is always flowing through the piping loop
from which all the baseboards are "tapped." This creates a
constantly circulating loop of heated water from which any of
the individual baseboards can extract heat when needed.
A thermostatic radiator valve (TRV) piped into each supply
riser regulates flow through its baseboard as necessary to
maintain the desired level of comfort in the room (Figure
3. Thermostatic radiator valves (TRVs) allow adjustment
of individual baseboards in a series loop. The plastic wrap is
to protect against splatters from drywall mud.
When a room is warm enough, the TRV closes, and flow through
the distribution circuit bypasses that baseboard altogether. As
the room begins to cool, the TRV slowly opens.
Home run piping.
Another zoning technique, relatively new in the U.S. but common
in Europe, is the "home run" manifold system. Each baseboard
gets its own supply and return line, usually of PEX or
PEX-AL-PEX tubing. All supply lines begin at a supply manifold
like that used in radiant floor systems, and all return lines
go back to a return manifold. Zone control is provided in one
of two ways: with low-voltage valve actuators mounted on the
manifold valves and wired to thermostats or with non-electric
TRVs on each baseboard.
Layout at the Boiler
arrangement of components near the boiler has undergone some
changes in the last few years. Figure 4 (See ) depicts a
typical arrangement for a system with three heating zones and a
separate zone for domestic water heating. One important change
is the placement of circulators on the supply side of the
boiler and downstream of the system’s expansion tank.
This makes air purging simple, often eliminating the need to
"bleed" air from the baseboards. The traditional air scoop used
in older systems is being replaced by a newer device called an
air separator, or deaerator, which can capture even microscopic
air bubbles and eject them from the system.
Notice, too, that all the wiring for the system’s
circulators and thermostats has been consolidated into a single
control panel called a multi-zone relay center. Several
manufacturers now offer a relay center (see ), which greatly
simplifies installation and reduces cost compared with systems
that use a number of single zone controls with a separate
24-volt thermostat for each zone.
Also disappearing is the traditional tankless coil for
domestic water heating. Taking its place is the
indirectly-fired storage water heater, of which dozens of
models are now available. In this type of system, the boiler
fires only when there’s a demand for space heating or
water heating rather than inefficiently maintaining a minimum
water temperature year-round. Most multi-zone controls have a
switch that can be set to provide "priority" domestic water
heating. In this mode, all space heating is temporarily
suspended during DHW heating, allowing full boiler output to
quickly heat the tank.
Although type-M copper tubing has long
been the standard for hydronic distribution circuits, it now
has some serious competition, in particular from crosslinked
polyethylene (PEX) tubing manufactured with an oxygen-diffusion
barrier (ASTM F876). A number of companies now market PEX
tubing along with brass fittings for making either soldered or
threaded connections. The tubing can easily be snaked through
joist cavities where installing rigid piping is all but
impossible — a tremendous advantage in retrofit jobs.
A variation is PEX-AL-PEX composite tubing, which has an
inner and outer layer of PEX bonded to a welded aluminum core.
PEX-AL-PEX manufactured to the ASTM F1281 standard is rated for
service conditions up to 210°F at 115 psi. After being
uncoiled, PEX-AL-PEX tends to retain its shape better than
standard PEX. Its thermal expansion is only about one-seventh
that of PEX, because it’s controlled by the aluminum core
rather than the plastic.
Although it’s possible to install baseboard almost
anywhere wall space is available, placement can affect room
comfort. The preferred location is always along exterior walls,
specifically under windows. The rising current of warm air from
the baseboard counteracts the draft effect of the cool window
and wall surfaces, and also helps prevent interior condensation
on the glass during very cold weather.
A careful baseboard layout should also consider furniture
arrangements, door swings, and obstacles such as wall columns
5. Baseboard layout should be carefully planned to avoid
labor-intensive installations and to ensure even, effective
Whenever possible, talk over baseboard placement with the
homeowner before doing a final layout, remembering that
compromises are inevitable. Here are some other points to
Avoid moist locations.
Baseboard is not well-suited to the moisture levels of heavily
used bathrooms, especially when placed next to or behind a
toilet. The enclosures will start to rust within a couple of
years. Since available wall space in most bathrooms is minimal,
a panel radiator is often a better choice, though it will cost
two to three times more for the same heat output. Another
option would be to use underfloor heating, which is also more
Leave an air space.
When baseboard is installed before finish flooring, remember to
leave at least a 1-inch space beneath the enclosure. This
ensures that the finish floor will not block air coming into
Locate floor framing.
Before mounting the baseboard enclosure to the wall, be sure to
locate the floor framing so you don’t have to butcher a
joist to make room for the riser pipe from below. Try to keep
riser pipes at least 2 inches away from any framing to make
In most cases you can get a measurement for the riser pipe
holes by sliding an elbow onto each end of the element and
measuring between the centers of the elbow sockets. If the
design requires a flow balancing valve or thermostatic radiator
valve on one end of the element, dry-assemble the components
Drill oversized holes.
After marking this center-to-center distance on the floor, use
a bit that’s at least 3/8 inch larger than the outside
diameter of the riser piping to drill the holes. This provides
space for the element to expand without jamming the riser
against the side of the hole. For single-piece elements, you
can solder the element and any fittings and valves together,
then lower the whole assembly into the enclosure. Be sure to
support all fittings so the assembly is not twisted when
soldered. When set into the enclosure, be sure the element
rests on the support cradles provided. Some manufacturers
supply plastic expansion cradles that prevent metal-to-metal
contact, and minimize expansion noise.
For situations where a baseboard must be supplied and
returned from the same end, install a vented 180-degree
"return" fitting at the far end, and route the return pipe back
through the enclosure above the element.
Preventing noise from thermal expansion is an important part of
piping installation. Where copper tubing is suspended beneath
floor joists, use the plastic-coated wire hangers with pointed
ends that drive into the joists. These hangers flex as the pipe
expands, preventing the noise you would get if the pipe were
expanding inside a rigid support.
With I-joists, use a filler block in the web space before
using this type of hanger. Don’t drive the points into
the I-joist flanges, or you’ll damage this key structural
Support 1/2-inch and 3/4-inch tubing at intervals not
exceeding 4 feet. Supports for 1-inch and 11/4-inch tubing
should be not more than 6 feet apart. PEX tubing should be
supported about every 30 inches.
Make sure all holes in the joists are aligned. Again, drill
holes at least 3/8 inch larger than the outside diameter of the
piping. Never rigidly fasten the copper tubing to any framing,
and don’t wedge a length of pipe tightly between any
rigid surfaces. Be sure hangers are rated to operate at
temperatures at least 20°F above the design water
temperature of the system — typically 180°F in
piping system is completed, it’s time to test for leaks.
This is best done with compressed air rather than water (unless
you like running through buildings desperately searching for
shutoff valves). Don’t get smug and "blow off" this step.
I know guys who could probably solder pipe in their sleep, but
still religiously test every system before the piping gets
Before you pressure test, make sure all air vents are
tightly sealed, and all inline valves are open. Add air to the
system with a Schrader valve (like the valve on a tire). Pump
the system up to 20 to 25 psi — any higher and air starts
leaking from the 30 psi relieve valve. If all’s well, the
pressure should remain stable for at least 12 hours. If the
pressure slowly decreases over a few hours, check all joints by
brushing on a solution of dish detergent or a commercially
available leak detection fluid, and look for bubbles. Threaded
joints tend to be more prone to leaks. Track down any leak and
fix it before covering the piping or adding water.
Charging the System
pressurized domestic water is available on site, the system can
be filled by closing the isolation flanges on all but one of
the zone circulators, opening the purging outlet valve, and
lifting the fast-fill lever on the pressure reducing valve.
Water flows into the boiler and up through the system piping.
As the boiler fills, air exits through the air separator. The
water continues up through the open zone circuit, pushing most
of the entrapped air ahead of it. Eventually the water-air
mixture makes it back to the open outlet valve. After a minute
or so the exiting stream will be relatively free of air
Open the next zone circuit and close the first. Repeat the
procedure, one zone circuit at a time, until all the zones are
purged. Reset the fast-fill lever and close the purging outlet
valve. Most of the air, other than that dissolved in system
water, has now been expelled from the system. The air separator
will get the rest during the first few days of operation.
When a system like the one in Figure 4 is first turned on,
what happens depends on the control settings. If they’re
set for DHW priority, the DHW zone will be the only one
operating. Assuming the DHW tank has water in it, set the tank
thermostat to 100°F and wait a few minutes for this load to
To check space heating, turn each zone thermostat all the
way up, one at a time. The boiler should fire up as each zone
is turned on. Don’t be surprised if you still hear slight
gurgling sounds in the piping. Expect to hear occasional air
hissing from the air separator as the water gets hotter. Make
sure the boiler stops firing when it gets up to the aquastat
setting (170°F to 180°F for a typical baseboard