HVAC Systems

The majority of systems installed in residences today are single speed, gas or electric heating and electric air conditioning, covering about 60 percent of the market. However, heat pumps are becoming increasingly popular as efficiencies and reliability have improved. Heat pumps now make up almost 30 percent of the market. Two speed or two stage systems make up the remaining 10 percent.

This section of the Guide will cover heating, ventilating, and air-conditioning (HVAC) subsystem automation. Since the 1950's when forced air heating and cooling systems first began to be installed in homes, heating and air-conditioning systems have changed very little. While the basic system operation is the same, there have been significant improvements in operating efficiency.

HVAC Systems
Home H.V.A.C

If 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 more efficiently.

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.

HVAC Systems
Typical conventional heating/cooling system

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 and literature.

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 stage.

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).

HVAC systems
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.

HVAC Systems

Basic HVAC system design

All 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.

Typical Systems
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
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.

Heating/Cooling (conventional)
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.

A 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 the coils.

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.

Heating/Cooling 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.

Because the 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.

Heating/Cooling 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).

Ventilation Equipment
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.

Using outside 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.

Humidification 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.

Some cooling 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 more moisture.

Humidification 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.

Humidification 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.

Depending on 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 cool.

HVAC Systems
Typical remote controlled thermostat connection. This shows the interface on the HVAC controller,
but several thermostats are available with the interface electronics built in.

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.

HVAC Systems
example of the components of a typical zoned control system

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.

HVAC Systems
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 placement.

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 controller.

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.

HVAC Systems
Typical ducting design for a zoned forced air HVAC system. Dampers are installed
in the supply air ducts for each zone.

A 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.

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