Use two-stage equipment and zone dampers to increase
efficiency, comfort, and customer satisfaction
Hydronic, or hot water, heating has been standard for years
in many parts of the U.S. It is seeing a surge in popularity at
present, mainly because of the increasing use of radiant floor
heating, which is known for providing even, comfortable heat.
Yet contrary to popular belief, not every home in the free
world needs hydronic heat to achieve this level of comfort.
Apples to Oranges
Hydronic heat is sometimes touted as more comfortable than
forced-air heat. But since the typical hydronic system is
significantly more expensive than the typical hot-air system --
especially if cooling is included -- this is an
apples-to-oranges comparison. Customers willing to invest in a
quality hot-air system, rather than a bare-bones package at the
lowest price, will find that forced hot air can be as
comfortable as hydronic heating.
The least expensive forced-air system usually includes a
single-stage furnace with a single-speed blower motor. The
entire house is ducted as a single zone, and therefore has just
one thermostat. If the system is sized by a contractor who uses
a rule-of-thumb formula to estimate heat loss and heat gain,
the homeowner can end up paying high energy bills for a noisy,
inefficient system that provides uneven temperatures from room
A quality forced-air system would probably include a
two-stage furnace with a variable-speed blower motor. The house
would be separated into several zones with separate
thermostats, and the air would be distributed through
well-sealed, insulated ducts. In many cases, such an upgraded
hot-air system will still cost less than a hydronic system.
Ask the Right Questions
One of the most important steps to designing a quality heating
and cooling system is to take the time for a long talk with the
homeowner. Don't assume that you know what the customer wants
and is willing to pay for. Most homeowners are not aware of all
the available options. Here are some of the questions you need
What is your budget for this
work? This is a tough one to get answered. Often the
answer is, "Gee, I really have no idea." However, someone
building a 2,500-square-foot home with a $7,000 budget is not
looking at the same system as a person with a $12,000
What type of system do you have now, and what do you
like and dislike about it? The answer to this question
will tell you what the customers expect from their new system.
Different customers have different priorities when it comes to
efficiency, comfort, noise, and ease of operation.
Does anyone in the home have allergies? If the
answer is yes, the customer may want to consider a high-
performance air filter. There are three basic types of
high-performance air filters: electrostatic air cleaners (about
$100 to $150 installed), pleated media filters ($275 to $325),
and electronic air cleaners (about $625 to $700).
What type of fuel is available at your site, and what
fuel do you prefer? If a client has a phobia about gas
or an aversion to oil or heat pumps, you should know about it
before you design their system.
Finally, you need to explain to the homeowner what your
standard design temperatures are -- for example, 70°F
inside on a 0°F day, and 75°F inside on a 95°F day.
Make sure you're in agreement on these parameters up front, and
if they have other ideas, incorporate them into your design, as
long as their ideas are reasonable.
Choosing the Right Sub
Heating contractors vary in their attention to detail. Ask
your prospective heating sub how load calculations and duct
design are performed; the answers will help you evaluate the
Load calculations. Does
your heating contractor calculate accurate room-by-room heating
and cooling loads? In order to perform these calculations, your
sub needs to know the insulation values of the floor, walls and
ceiling; the R-value of the windows; and the orientation and
measurements of any skylights (see "Trouble-Free Forced Air
Heat," 12/98). Many heating contractors still use rule-of-thumb
square foot formulas for calculating heating and cooling loads.
But since glass-to-wall ratios can differ significantly from
one floor plan to the next, "square-footing it" is a dangerous
Duct design. Heating subs vary in their level
of attention to duct design (see "Duct Design Basics," 12/95).
The standard duct design manual is Manual D -- Residential Duct
Systems from the Air Conditioning Contractors of America (ACCA,
1712 New Hampshire Ave. N.W., Washington, D.C. 20009;
One basic duct design error is inadequate return ductwork. A
system with multiple return grilles is preferable to a system
with a single, central return grille. Another basic error is
supply ductwork that is not matched to the output of the
furnace. In extreme cases, undersized ductwork is unable to
remove the furnace's heat fast enough, causing the heat
exchanger to overheat and crack.
Supply air vents should be placed where they can deliver air
along the exterior perimeter walls, where the greatest heat
loss and gain occurs. Avoid low sidewall supplies, which can
cause drafts and result in dissatisfied customers.
The calculated loads will show the Btus per hour (Btu/h)
required at peak load conditions (the coldest outdoor
temperatures in winter and the hottest outdoor temperatures in
summer). In most areas, peak conditions are reached only for a
few hours in a typical season. So most of the time, a correctly
selected unit is oversized for moderate conditions. That's why
it's important to offer the customer a two-stage system.
1. Two-stage gas furnaces, like this model from
Goettl Air Conditioning, have two levels of Btu output
and airflow. Since low stage operation is adequate to
meet the heating demand most of the time, such units
are quieter and more efficient than single-stage
Two-stage models are available in mid-efficiency and
high-efficiency gas furnaces, as well as air-source and
geothermal heat pumps (see Figure 1). These units offer "Btu
staging" -- for example, a two-stage gas furnace might have a
65,000 Btu/h input on low, and a 100,000 Btu/h input on
If you burn oil, you probably will be limited to choosing a
single-stage furnace. While gas burners can be equipped with a
two-position gas valve, an oil burner is equipped with a unique
nozzle that is optimized for a single firing rate. Two-stage
oil pumps are usually not available for residential
In heating mode, two-stage units operate at low speed for
80% of the time. Two-stage units quietly deliver consistent
indoor comfort through longer run cycles at lower speed than
conventional single-speed systems. A typical furnace allows the
air temperature in a space to fluctuate up to 4°F, while a
two-stage system reduces the temperature fluctuation to less
than 2°F, while improving air circulation at the same
This has several benefits to the owner. First, two-stage
units offer improved comfort, because of consistent
temperatures throughout the zone. Second, two-stage units are
extremely quiet, because their two-speed fans are usually
operating at low speed. Last and most important, operating
costs are lower, because the Btu per hour output is matched
more closely to the actual load, and because the unit fires for
longer cycles, reducing the start-up and shut-down cycles
experienced with a single-speed system.
Variable-speed blower motors. A furnace with a
variable-speed blower motor provides improved comfort and
efficiency. A variable-speed motor, which is available for
either a gas or oil furnace, slowly ramps up on the initial
call for heat, so that the air-speed increase more closely
follows the increase of heat available at the heat exchanger.
Conversely, on the shut-down cycle, the fan will slowly ramp
down, extracting the maximum Btus from the heat exchanger
surface. In contrast, the blower in a standard furnace usually
has a timed on/off control, which can cause an objectionable
"cold blow" on start-up and shut-down.
A variable-speed blower can also be set up to operate at
very low "fan-only" speeds. The fan-only feature, which is
controlled by a manual on/off switch at the thermostat, is
especially important for systems with high-performance air
filters, because the only time the air filter will work is when
the air is moving through it.
Fan-only operation may also be useful for a room that is not
on a dedicated zone and is located far away from the
thermostat. Constantly introducing new air into the room will
bring the temperature more into line with the temperature in
the rest of the home. The cost of running an efficient
variable-speed blower on fan-only for a year is less than $20,
while a standard motor running for the same year would cost
more than $150 to operate.
Two-speed condenser. For the cooling side of
the system, consider upgrading to a two-speed condenser. Homes
with skylights and large window areas will often have very high
heat gains on sunny days, dictating the size of the cooling
unit. But on warm, humid days without a bright sun, the house
will not have as much heat gain, and a single-speed condenser
will not run long enough to remove the high humidity.
A two-stage condenser (which usually includes both a
two-stage compressor and a two-speed condenser fan motor)
coupled with a variable-speed blower fan can remove up to 30
times more moisture than a standard fixed-speed system, because
it will not be short-cycling as often.
Some manufacturers use microprocessor technology and
humidity sensors to control the cooling unit. One example is
Carrier's Thermidistat, which looks like any electronic
thermostat, but is capable of displaying the outdoor
temperature and indoor relative humidity. It will precisely
operate the system based on desired indoor temperature and
humidity set points.
When this control is coupled with matching heat-pump
components, it will regulate the variable-speed fan motor to
maintain consistent air discharge temperatures.
Such systems can provide a minimum air temperature delivery
of 100°F, up to a maximum of over 120°F -- about
20°F warmer than previous-generation equipment. Most users
report higher comfort levels with these warmer air delivery
Many contractors do not recommend or install zoning equipment,
due to the mistaken belief that zoning is complicated. But if
the homeowner's lifestyle requires different temperature levels
in different areas, zoning probably makes sense, especially in
Another reason to zone is to provide good temperature
control in any area of the house with much greater heat loss
(or gain) than other areas of the house. Typical examples are
areas with many windows or rooms that are oriented toward a
different direction than most of the other rooms in the house
(Figure 2). During your initial meeting with the owner, look
for such areas -- for example, a sunroom or a finished area
above a garage.
2. A room with an unusually large area of glass
tends to have high heat loss at night and high heat
gain on sunny days. Such a room is a good candidate for
a separate zone.
How do you decide whether an area needs its own furnace, or
just a zone off the main unit? Assuming you have sufficient
capacity, it is usually less expensive to install (and always
less expensive to operate) a zone off of the main system than
to install a separate dedicated unit. In some cases, though,
the location of the zone or the building's total heating and
cooling loads may dictate a separate unit.