there is one system in the house that can benefit the most from automation,
the HVAC system is it. Almost all systems rely on a simple, often
mechanical thermostat that simply turns the system on or off. Almost
anything you do can improve its ability to provide increased comfort
Like lighting, HVAC subsystem
automation can be implemented as a stand alone system or as part
of a whole-house automation system. Almost any type of heating,
cooling, or ventilation equipment can be part of an automated HVAC
system. Since the heating and cooling system consumes about 50%
of the energy costs and is responsible for most of the occupant's
comfort, an automation system can enhance comfort and reduce energy
costs by setting the thermostat properly based on whether the owner
is home, asleep or away. In addition, sensors used for security
purposes can be used to adjust the HVAC system for maximum efficiency,
(for example, setting back the heat when there is no one in a room).
Finally, most automation systems offer access by telephone or Internet,
allowing the thermostat(s) to be adjusted remotely.
Terms and Definitions
HVAC products and systems have their own unique set of terms. The
following terms are used throughout this section of the Guide and
will also be found in HVAC and HVAC control system documentation
HVAC equipment - Heating, Ventilating and Air-Conditioning equipment - A generic
term for any equipment that provides heating or cooling for the
home as well as the ventilation and air handling equipment.
HVAC Zone (or
Environmental Zone) - An area of a home whose environmental
conditions (temperature, humidity, airflow, etc.) are individually
controllable. A home always has one or more zones.
HVAC System (or
Environmental System) - A group of one or more environmental
zones operated by a single source of heating/cooling/air-handling
equipment. A system may have multiple control devices. If there
is only one zone in a home, the zone and system are the same. There
may also be more than one system in a home.
Hydronic System - A type of heating system that uses hot water instead
of air, either in tubing in the floor, walls, or ceiling, or in
room radiators. A Hydronic system usually relies on heat radiation
or air convection rather than forced air movement.
Stages - heating and cooling systems are said to have stages of heating
and cooling to mean more than one source of heating or cooling in
the same system. For example, a heat pump compressor provides the
first stage of heat and a gas heating element provides the second
SEER - (Season Energy Efficiency Ratio) a measure of cooling efficiency
for air conditioning products.. The higher the SEER rating number,
the more energy efficient the unit. The minimum standard rating
established by the government for air conditioning is 10. The technical
definition is the total cooling of a central unitary air conditioner
or unitary heat pump in BTU's during its normal annual usage period
for cooling divided by the total electric energy input in watt-hours
during the same period.
BTU - (British Thermal Unit) is the amount of heat that will raise or
lower one pound of water by one degree Fahrenheit. A BTU is how
many BTUs are used per hour. The amount of heat added or removed
by a heating or air-conditioning system is measured in BTUs.
Ton - Heat pumps and air conditioners are generally sized in tons. Typical
sizes for single family residences are between two and five tons.
Each ton equals 12,000 BTU.
Hysteresis - used in HVAC control devices such as thermostats to mean how much
the actual temperature is allowed to vary around the desired set
point temperature. For example, a thermostat with a 2 degree hysteresis
that is set to 72 degrees for cooling will call for cooling when
the temperature rises to 73, and stop calling for cooling when the
temperature drops to 71. Too small a hysteresis can cause the HVAC
system to cycle on and off too quickly, and to large a hysteresis
can cause the temperature to vary too greatly. Hysteresis is set
based on each individual installations size and load.
MRT (Mean Radiant
Temperature) is a measure of the radiation temperatures
from all the surfaces in a room or space arriving at an occupant.
MRT is independent from the air temperature in a room and usually
is more influential in determining if a room "feels" warm
or cool. It is a more accurate measure of the perception of temperature
than the air temperature. For example, if a room has a several large
glass windows and it's very cold outside, this can cause the room
to feel cold even though its air temperature is warm. Likewise,
radiant floor heating will make a room feel warm long before the
air temperature in the room is warm. Unfortunately, conventional
thermostats only measure air temperature.
HVAC System Concepts
Heating, ventilating and air-conditioning equipment is designed
to provide a relatively constant and comfortable temperature in
the home. Secondary goals are to provide fresh and filtered air,
and a comfortable humidity level.
While there are many
different types of equipment that provide heating and cooling from
forced air systems, to in-floor radiant heating, they all work on
the same basic principles. There is a source of heat or cold, and
a way to distribute it throughout the home.
The control of HVAC systems
hasn't changed much over the last half century. When the temperature
is below a desired temperature (for heating), a control device (thermostat)
"calls" for heating. Heat is applied to the area where
the control device is located until the temperature is reached,
then the control device turns off the heat. Control equipment does
not, in general, make a distinction about the source of heating
or cooling energy (electric, gas, oil, etc.) or type of equipment
in the home.
The term "call"
for heating or cooling simply means that the thermostat does something
to turn the heat source or cooling source on, usually closing a
contact in a relay circuit.
In homes up to about
2500 sq. ft. there is usually only a single zone serviced by a single
heating or cooling device. In this case the heating and/or air conditioning
equipment comprise a single zone system. In larger homes, especially
homes over 4000 sq. ft., it is common to have multiple zones. A
multi-zone home may have each zone serviced by a single heating
or cooling device, or two or more zones may be serviced by a single
heating or cooling device. The equipment services different zones
by means of dampers, mixing boxes, or other devices to individually
heat or cool the zones independently.
In many parts of the
country where humidity levels reach extremes (over 90% or less than
20%) it is common for the HVAC system to include humidification
or dehumidification equipment. Humidification equipment, like other
HVAC equipment, can also be zoned. Humidification zones may not
be the same as environmental (heating/cooling) zones. There may,
for example, be several heating/cooling zones but only one humidification
zone (the whole house).
The HVAC house subsystem consists of one or more HVAC systems. HVAC
systems consist of a source of heat generation such as electric
resistive heating, gas, or oil, optionally a source of cooling,
almost always a freon gas compressor, and a way to deliver it to
the conditioned space. In the case of heating, this is usually done
by forced air, but can be hot water through in-floor tubing or in
radiators. For cooling, delivery is almost always forced air through
an air handling system. Combinations of different types of delivery
for heating and cooling are possible.
systems are controlled by a thermostat. A thermostat measures
the air temperature in the conditioned space and turns heating
or cooling on or off based on the measured vs. the desired temperature.
This forms a simple feedback system. The comfort and efficiency
of the system is determined by the efficiency of the heating and
cooling source, how well the delivery system distributes the heat
or cool, and to a great extent, how well the thermostat operates.
Better thermostats can make a significant difference in how well
the overall system operates.
While there are many different types of heating and cooling, some
more popular in different parts of the country, this Section will
concentrate on forced air heating and cooling systems either from
separate sources or via a heat-pump. It will also assume the equipment
operates by on/off control of one or more stages of heating or cooling.
Heating only systems, prevalent in northern climates, have a single
source of heat generation such as gas, oil, or electricity and typically
used force air to deliver the heat to the conditioned space although
hydronic systems (radiator and in-floor coils) are popular. They
use a simple on/off thermostat.
A conventional heating/cooling system is just a heating only system
usually with the addition of a compressor unit located on the outside
and a cooling coil in an air-handler (Figure 4.2.2). Cooling systems
rely exclusively on forced air to deliver cooling to a zone. Therefore
the heating part of the system also uses forced air. Some homes
are equipped with forced air cooling and radiator or floor hot water
heating. The two systems operate independently.
blower motor circulates air from the return air duct and filter
through the heating coils or element, then through a set of cooling
coils (if equipped for cooling), then out the supply air ducts to
each room. An outside compressor is used for cooling. The compressor
compresses a freon (or equivalent) gas that passes through a set
of heat exchanger coils where a fan draws outside air over the coils
to cool the gas. The gas then goes to the cooling coils inside the
air-handler where it expands, cools, and cools the air passing through
Two Stage Heating and Cooling systems
Some higher-end furnaces now have two stage gas valves, allowing
a low setting appropriate for most weather conditions, and a high
setting that can boost output for the coldest weather. A multi-speed
blower is also used. This may be used with a two speed or dual compressor
air conditioner, to provide two stages of cooling as well.
Single-stage Heat Pump
A single stage heat pump provides cooling in the conventional way
by using a compressor and an air handler, but they heat by reversing
the flow of freon using a reversing valve, causing it to "pump"
heat from outside to inside. The role of the inside coils in the
air handler is also reversed, providing heat by cooling the compressed
gas. Since the compressor only has one speed, it is referred to
as a single-stage heat pump.
heat generated in these systems may not be adequate for the coldest
part of the year, they are usually equipped with some form of auxiliary
heat, typically a gas or electric heating element in the air handler.
When the compressor is not able to maintain a constant temperature,
the thermostat will call for auxiliary heat as well.
Multi-stage Heat Pump
A multi-stage heat pump is the same as a single-stage heat pump
with either a dual-speed compressor or two compressors. Since the
compressor can run at two speeds it can cool at two rates and heat
at three rates (including the auxiliary heat).
With the ever increasing air-tightness of new homes, some means
of ventilating the air of the home is usually required, especially
in the winter when the homes tend to be kept "tight" by
the occupants. In many parts of the country, building codes mandate
some form of outside air exchange. This usually takes the form of
a small duct from the outside to one of the return air ducts. The
low pressure in the return air ducts will draw in a small amount
of outside air as inside air is lost from open doors or windows.
air for heating and cooling can be a big energy saver when it is
cooler than inside air when the system is calling for cooling, and
warmer than the inside air when calling for heating. This requires
an HVAC controller capable of measuring both inside and outside
air temperatures and controlling a damper from the outside to the
return air duct of the air blower.
and Dehumidification Equipment
Humidification and/or dehumidification equipment may be added to
any forced air heating or cooling system. In the summer in humid
climates, central cooling equipment (conventional or heat-pump)
provides its own dehumidification of the air. In the winter, dehumidification
is usually not needed. It can be accomplished in more sophisticated
systems by running the cooling equipment to condense the moisture
then reheating the air to the desired temperature (this is how many
car A/C systems operate in the defrost/defog mode). The cooling
coils must be placed before the heating elements.
systems have a humidistat input designed to increase moisture removal
in periods of high humidity. It slows the indoor blower during operation
of the air conditioner. This allows the indoor coil to operate at
a colder temperature for a longer period of time, thus removing
equipment is typically used in the winter and summer in dry climates.
The humidifier is attached to a forced air heating system to provide
added moisture to the air passing through the air handler. The equipment
is usually attached to the main air handler. A fine water mist is
injected into the air as it leaves the heating element.
is controlled by a hydrostat or humidistat ; a type of humidity
level thermostat. It is set for a desired humidity level and works
independently from any thermostat. In a multi-zone forced air heating/cooling
system, there is usually only one humidifier serving all zones.
In a zoned system serviced by separate equipment there may be humidifiers
on each system.
It is possible
to measure humidity electronically and use the value to control
humidification equipment from a home automation system.
The best way to achieve better temperature control throughout a
home is by zoning the HVAC system. Multiple zones may be incorporated
into a home HVAC system in one of two ways. In some systems, a single
set of heating/cooling equipment can service more than one zone
in the home by using dampers to adjust airflow, or other means to
regulate the amount of heating or cooling in each zone. A zone may
be a house, a floor, part of a floor, or a room. Each zone must
have its own thermostat or equivalent control device. In other systems,
each zone has its own heating/cooling source equipment, and is operated
independently. Again, each zone has its own thermostat. Combinations
of these two approaches are also possible.
the home's room configuration, homes under 3000 square feet can
generally be properly air conditioned using two or three zones.
Homes between 3000 and 5000 square feet can generally be properly
conditioned with 4 to 6 zones. Larger homes generally use separate
HVAC systems to divide the home (main living area, guest quarters,
master suite, etc.)
In order to
maintain the rooms of a zone within 1 degree of each other, a supply
and return air balance is required in each room so they will receive
the proper air flow rate (in cubic feet of air/min or CFM) when
the control damper is positioned open.
RCS Communicating Thermostats function as standard thermostats
and in addition allow remote commands to set specific temperature
setpoints and HVAC modes. Models control standard gas/electric,
single stage heat and cool, heat pump, 2-stage heat and single-stage
controlled thermostat connection. This shows the interface on the
but several thermostats are available with the interface electronics
Zone Control Systems
A zone control system allows a single HVAC system to independently
control the heating and cooling in more than one zone serviced by
the system. It allows multiple thermostats to be connected for each
zone. The most common residential zone control systems operate a
series of motor controlled air dampers to regulate the flow of air
into a zone, thus controlling the temperature in that zone.
example of the components of a typical zoned
Controllers are typically housed in a stand-alone box usually attached
to the side of the HVAC air handler. While the operation of the
controller is straightforward, its firmware operation can get complex.
It must take into account the varying demands of each zone and make
sure the system is operated correctly. It must make sure that airflow
is not blocked and may need to bleed air into zones that are not
calling for heating or cooling to maintain a minimum airflow.
If any zone calls for heat, it turns
the heating system on and opens the damper to that zone. If additional
zones call for heat, the system opens their dampers as well. When
no zone is calling for heat, the controller turns the system off.
If one or more zones call for heat and another zone calls for cooling,
the controller must decide-based on the temperature in each zone
as well as the outside temperature-what function to perform (ignore
the call for cooling, or shut off the heating zones and operate
cooling only in the zone calling for cooling). Some controllers
have an optimization function when one or more zones are divided
between an upstairs and downstairs. Since hot air rises and cool
air falls it is more difficult to balance upstairs and downstairs
temperatures. For example, more airflow is usually needed in a downstairs
zone when heating and in an upstairs zone when cooling.
typical wiring diagram of a 4 zone controller
for a heat-pump HVAC system. Connections are
provided for each zone thermostat, four dampers, and connection
to the HVAC equipment.
Designing a Zoned HVAC system
Designing a zoned HVAC system in new construction should be done
with coordination with the HVAC contractor. He will need to help
you determine how many HVAC systems will be needed verses zoning
of each system. He will also need to help determine duct sizes and
Determine how the home will be zoned and the total
number of zones. A multistory dwelling will normally be zoned by
stories. Make each zone handle approximately the same heating/cooling
load (the HVAC contractor can help determine this). Avoid zones
that have a small physical area.
Zones should bear some relationship to living patterns.
Make guest quarters, the master suite, living areas, and areas that
are seldom used into separate zones.
Have the HVAC contractor plan the ducting that connects
each zone to a separate branch of the main supply duct. Duct branching
usually occurs near the air handler but some branching may occur
in distant parts of the house or floors. Dampers are placed at the
branch point to each zone.
The ducting will usually be larger than for non-zoned
systems to handle the extra airflow in the zone ducts due to the
fact that some ducts will be closed. When the zone controller closes
one or more dampers, the back pressure should not increase so much
as to damage the HVAC system.
Select the type of controller that will handle the
number of zones and has the features needed for the home, then select
appropriate thermostats for the controller.
Plan the location of each thermostat in each zone
(see below) and run the necessary cable from the location to the
Run the necessary cable from the controller to the
damper locations and to the HVAC equipment.
Once the HVAC contractor has the system installed, he can test the
airflow (since the dampers are normally open) by just connecting
a regular thermostat to the HVAC system. Once air flow is working
correctly, the zone controller can be installed.
Typical ducting design for a zoned forced
air HVAC system. Dampers are installed
in the supply air ducts for each zone.
zoned HVAC will usually contain not only dampers for each zone air
duct, but unless the system can modulate the damper positions to
prevent excessive supply side air pressure, it will need to be equipped
with a bypass air duct from the supply side to the return side.
This is equipped with a barometric bypass damper that opens when
sufficient pressure builds up on the supply side. An outside air
duct can also be installed to utilize outside air to save energy.
The outside air duct damper can be controlled by some zone controllers.