Optimizing Complex Residential HVAC Systems

October 31, 2012
November/December 2012
A version of this article appears in the November/December 2012 issue of Home Energy Magazine.
Click here to read more articles about HVAC

With today’s escalating energy costs, the residential construction industry is seeing the advantage of automated building controls as a means to help its customers save energy. Home builders, home performance contractors, and HVAC technicians are becoming increasingly adept at including numerous energy sources, such as solar thermal and solar PV, in their residential HVAC designs. The challenge with these controls is to maximize the function of each individual system within the overall HVAC/mechanical design, and to ensure that all systems work in concert with one another. A number of control systems have recently been developed to meet this challenge. The most effective control systems “understand” the sophisticated nature of today’s hybrid residential HVAC designs, and fully leverage the capabilities of each component to work in an efficient, effective manner—while delivering as much comfort as possible for the homeowner.

Smart controls integrate the renewable energy technologies that are part of today’s hybrid HVAC systems designs. (Bestway Mechanical, Colorado)

HVAC Controls—We’ve Come a Long Way

Not too long ago, getting multiple systems—like a boiler, forced-air system, and solar thermal—to work together was accomplished with a combination of thermostats, set point controllers, relays, and wires from control box to control box. Although this way of setting up controls for the HVAC system worked, it was clumsy and difficult to troubleshoot, and it provided no means of system reporting. Take, for example, a home built 10 or 15 years ago. This home was built with a forced-air cooling system and a hot-air furnace. The ductwork was used for both cooling and heating. The control system for this type of house is a thermostat on the wall that is set to either “heat” or “cool,” and to reach a temperature set point. Maybe this home has a natural gas water heater as well, with an aquastat mechanically set to achieve the proper water temperature.

Today’s Home—A Picture of Efficiency and Comfort

In today’s residential construction market, hybrid HVAC systems are becoming standard. A hybrid system combines numerous HVAC devices to increase efficiency and comfort, and can include both traditional and renewable energy sources. For example, a house today can be built with an air-to-air heat pump able to meet 70% of the home’s heating and cooling requirements. The home builder or retrofitter, the homeowner, and the HVAC contractor need to work together to identify one or more additional HVAC mechanical systems to meet the remaining 30% of that home’s needs. They may want to add a boiler to help the air-to-air system during the heating season, when the heat loss of the house is at its greatest and the efficiency of the heat pump is at its lowest. They may also consider installing a solar-thermal system that can produce hot water, with excess heat stored in buffer tanks for later use in a radiant heating system.

You need to integrate six mechanical devices into one HVAC system. These devices are (1) an air-to-air heat pump, (2) a boiler, (3) a solar-thermal system, (4) a water heater, (5) a buffer storage tank, and (6) a radiant heating system. Each of these devices will have its own control system, and all six control systems must be tied into a hub of some sort, which can register what each of the other devices is doing at any given time. This is achieved by employing a home automation system strategy, which is where an integrated HVAC control system starts to make sense. Home automation is a much more commonplace concept than it was even 10 years ago. Homeowners can now control their lights from their web-enabled phone, and can lock their doors with a TV remote control. And now HVAC systems can be controlled in much the same way. A well-designed HVAC control system can save the homeowner both energy and money.

Choosing the Best HVAC Control System

When deciding to incorporate a smart HVAC control system into a home, we should start by asking, What do we want to control? Typically, the answer is temperature, humidity, and indoor air quality. There are many sensors on the market today that can detect some or all of these variables, as well as CO and CO2 levels. Readings from the sensors are sent to a computer interface that makes calculations based on the current conditions, and the computer adjusts the HVAC system to achieve the best balance between occupant comfort and energy-efficient operation (see Figure 1).

In selecting an HVAC control system, we need to evaluate five things. These are the computer, the software, the data collection and control devices, the means of communication, and the means of reporting.


A computer—desktop, laptop, or Programmed Logic Control—is at least part of the driver behind virtually every electronic or mechanical device we use today. In the realm of HVAC control systems the computer is needed to process large amounts of data.


Software is the driving force for integrated HVAC controls. It determines the way in which a home’s hybrid HVAC system is run. A good software program can greatly simplify this process for the homeowner.

Software for HVAC controls is written in general terms. Installing contractors can then customize these terms for each building. Installers establish the rules that govern how a system should operate to meet the customer’s comfort requirements most efficiently.

The best software is flexible and easy to use. Flexibility and ease of use have become such a challenge that at one point Energy Star considered dropping its rating on programmable thermostats, and since then has spent a lot of time and money teaching homeowners how to program their thermostats properly. Thus, an HVAC control system that is as easy to use as, for example, an iPhone (which people typically use without ever reading the instructions) will prove the most useful in the home.

Figure 1. This schematic of the REHAU Smart Controls system illustrates the components of an HVAC control system: (1) computer, (2) configuration and user-interface software, (3) data collection and control devices, (4) inputs and outputs, and (5) communication devices and Internet accessibility.

Data Collection and Control Devices

Data collection devices, otherwise known as input/output controllers, are used to bring data from the HVAC system into the control system and then out of the control system to the various mechanical devices, with instructions on how to operate. Outputs can be analog or digital. Analog devices vary the output along a predetermined scale. For example, if the output from the control to a pump is zero volts, it means that the pump is not working; if the output from the control is 10 volts, the pump is 100% operational; if the output from the control is 5 volts, the pump is working at 50% of its capacity. Digital outputs are equivalent to on-off switches. They are usually installed in a series electrical loop with a power source and the device they are intended to control.

Inputs also can be analog or digital. Analog inputs can be Percentage (for Humidity) Parts per Million (for CO or CO2 Levels), or Fahrenheit or Celsius (for Temperature). The system collects and analyzes these values to adjust the climate conditions of the space in question.

Digital inputs are on/off signals from an output device that allows the system to know the status of the device. Zone valves may have a digital input to a control system to let the control system know that the valve is fully open. The control system can use this information to turn on a pump, to prevent it from deadheading into a partially opened valve (which could damage the pump and other devices in the system).


Communication comes in many different forms in an HVAC control system. There is communication between the input and output devices to the computer. There is communication between the HVAC controls and the Internet. Communication between devices can be achieved using many protocols, or languages, that are common to the computer industry. Protocols like ModBus, TCP/IP, BacNet, and LonWorks provide a way for all the devices to communicate over a network where information and commands are exchanged to control an HVAC system.

The HVAC industry has tried hard to adopt a protocol that all component manufacturers can use to make their devices automatically discoverable by an HVAC control system. This would reduce or eliminate upfront programming to—for example—control a boiler or a pump.

There has also been a push in the marketplace for HVAC control systems that make possible off-site interaction via the Internet. The homeowner or service contractor gains access to these systems through a web-enabled device like a smart phone or another computer. The homeowner can adjust set points for individual zones in a house, turn domestic hot-water production on and off, or switch between heating and cooling modes, all by remote access.

For the installer, remote access provides the means to find out how a system is operating, how to diagnose a problem before scheduling a service call, or even how to make off-site changes to the system to render it more efficient. This is especially useful for contractors, since it allows them to visit only those sites where a larger issue needs to be addressed.


There are many government incentives to install renewable and energy-saving technologies in today’s homes, so reporting has become an important issue in the control systems industry. To ensure that these technologies are being installed and operated correctly and to develop the best means of integrating multiple HVAC systems, DOE, for example, requires reporting as part of the controls package.

Given the new remote-access HVAC control systems and wireless technology, reporting requirements can be easily met. Computers can store historical data on HVAC control system operation, and these data can be used to determine the efficiency of the system. Raw data can also be extracted into spreadsheets for analysis and simulation.

learn more

For more information on Reality LLC, visit www.reality-llc.com.

Find out more about REHAU Smart Controls at www.na.rehau.com/controls.

HVAC Enters the Digital Age

Just as forced-air ductwork and thermostats were once considered the wave of the future in residential HVAC design, so HVAC control systems are the wave of the future in today’s homes. As we continue to advance in the design and integration of individual HVAC components, these control systems will help us to achieve the ideal balance between energy efficiency and occupant comfort.

Oliver Carman is an applications engineer for the North American building technology business unit at REHAU.

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