In this article we cover some of the main issues that continually come up in conversations with our customers regarding the type of equipment to select when it is time to replace their existing HVAC equipment. We cover issues that are shared for gas furnaces, air conditioners, heat pumps, package units, dual fuel units and mini splits. Our goal is to share some of the insights we have gained over the years helping homeowners make prudent choices regarding their HVAC system and homes.
Purchasing a new HVAC system can be a daunting task because there are so many different types of equipment and related issues to take into consideration. The answer to the question, “what is the best HVAC equipment” can change depending on your house, climate, utility bill rates, fuel types available, budget and goals. As a result of this you will find conflicting information on the web. Our hope is not to muddy the water, but to help you make more informed decisions.
We encourage you to read our articles “HVAC Contractor Selection” and “Quality HVAC System Installation” because your contractor’s commitment to delivering a quality installation is as important, if not more, than the actual HVAC equipment itself. Your new equipment is simply part of the HVAC system. The ductwork, air barrier, insulation, windows, solar control, etc. all have an impact on your HVAC system performance. Recognition that the equipment alone does not make the system is the first step to improving the comfort, health and efficiency of your home.
The starting point for HVAC equipment selection
A good starting point for equipment selection should be to create a criterion for how you would like your new HVAC system to improve your indoor environment. We think you are making a mistake if you abdicate this responsibility to an HVAC salesman. Ask yourself, “what would I improve about my existing HVAC system?” Here are some comments we often hear from our customers.
- More even temperatures throughout the home
- A much quieter system – one that is hard to hear at all
- Lower utility bills
- Less dust
- Reduce the impact of allergies
Our hope is that by thinking about your home’s performance and doing a little research that you may have a significantly greater opportunity to really improve the comfort and efficiency of your home. There is a significant difference between a system that puts out hot or cold air and one that performs at the efficiency, comfort and safety levels intended by the equipment manufacturers.
Don’t get hung up on equipment manufacturer
In the process of equipment selection we advise homeowners not to get fixated on using a specific equipment manufacturer. Any good HVAC unit, such as Trane, Carrier, Lennox, etc. can have its performance undermined by poor installation. The integrity of your contractor and the quality of your new system installation will have more bearing on the performance of your new HVAC system than the equipment itself. Proper sizing of the unit, duct design, correct airflow and refrigerant charge are key elements of the installation process that are critical to achieve the benefits of your new HVAC equipment.
The most efficient HVAC unit on the market
The most efficient HVAC system on the market is the one that runs the least amount of time because the house is well insulated, has ducts with minimal leakage, utilizes solar control strategies … No matter how efficient your new HVAC unit is, if the ducts and your home leak, the house is riddled with insulation defects or has no solar control strategy any new HVAC unit will only have minimal impact on your utility bills and comfort levels.
We also tell homeowners not to get fooled into thinking that the equipment efficiency is going to be the biggest determinant of the size of their utility bill. You can actually increase your utility bill by putting high SEER equipment on a leaky and poorly designed duct system. Two stage and modulating equipment have much longer run times than single stage units and if there are duct leaks, there is the potential for duct leakage to have a greater impact on the utility bill.
Bring a healthy skepticism to claims of energy savings
We bring a healthy level of skepticism to the big energy savings claims made from government agencies, utilities, equipment manufactures and many of our competitors about big energy savings as a result of equipment replacement. If you don’t address duct leakage, duct modifications, air infiltration, insulation defects and solar control simply replacing a unit is not going to reduce your utility bill by 25% to 50%. This is logical; a new unit is only part of the HVAC system.
Equipment installation also plays a significant role in realizing energy savings. Equipment efficiency numbers are measured in a controlled laboratory setting. A lot can and does go wrong when a contractor takes the equipment out into the real world and installs it in your home. Quality installation is as important, if not more, than the equipment itself.
The most neglected part of your HVAC system – the duct system
The most neglected part of the HVAC system is the duct system or air distribution system. Home owners often do not realize that the HVAC equipment is only one part of the whole “system.” Your new equipment is going to be attached to an air distribution system / duct system and this is where a major disconnect occurs in most homeowners understanding. If you attach a high performance HVAC unit to a poorly designed, restricted and leaky duct system you end up with mediocre performance. This is why we encourage homeowners to think in terms of an “HVAC system” and not just think “HVAC equipment.”
Duct modification & duct sealing
Duct modifications and duct sealing should be part of every HVAC quote. Our experience informs us that at least 70% of residential homes need both duct modifications and duct sealing. The APS website states that, “many systems also have an undersized return. This limits the airflow of the system and makes the unit work harder to cool or heat your home. Right sizing and sealing the return will help your system run more efficiently.” We can perform the right sizing and modifications for you. We need to do this in order to make sure your new equipment will perform the way the manufacturer intended it to perform.
Duct modifications and duct sealing are often the key to making a system perform to a customer’s expectation. By interviewing homeowners about comfort issues in their homes, measuring static pressures and air flows we can quickly tell if there are duct design issues, restrictions, disconnects, leaks or other problems. We find situations where ducts are incorrectly sized and or duct runs that are so long with so many connections they can’t possibly deliver the proper air flow. We often reconfigure duct work for homes that have comfort problems.
Our goal is to make only essential modifications and not to do full duct system replacements. In other words, we start with what you have and with an eye on budget constraints make the necessary corrections to the air distribution system. We have a featured article on duct sealing because it is such a widespread problem with systems installed prior to 2005.
Properly sized equipment
A very important part of the equipment selection process is to properly size an HVAC system. HVAC equipment performs more efficiently and lasts longer if it is properly sized and makes for a more comfortable home. Unfortunately we routinely see systems that are as much as twice the size they need to be, which creates several problems to the comfort and energy efficiency of the home.
At Advantage Home Performance we use an engineering software program to perform the Manual J load calculation to properly size heating and cooling systems. To generate a load calculation we have to enter all the parameters of the house into the design software which requires areas and R-values of all walls floor and ceilings, U-vales and shading coefficients of windows, overhang projections over windows, infiltration values, etc. In existing homes your salesman must go into the attic and confirm R-values of insulation in the attic, R-value of duct work, quality of air barrier etc. When done correctly performing a load calculation is a fairly involved process.
In Phoenix we are required by code to size an AC system in the summer at a design temperature of 108˚F and in the Prescott Tri-city area at 94˚F. In Phoenix the outdoor temperature goes over 108˚F only 1% of the time. This is important to note because we are required to size equipment for extreme weather conditions. When done in a disciplined and professional manner load calculations are extremely reliable.
Single stage equipment
The majority of tract homes have single stage heating and cooling equipment because it costs less than two stage equipment. Single stage equipment has one speed and one output. It is either on or off and its output is sized for the severe summer and winter weather temperatures.
The downside of single stage equipment is that it short cycles. When the outdoor temperature is below the summer design temperature of 108˚F in Phoenix and 94˚F Prescott Tri-city area, which it is most of the time, the run time of the air conditioning is shorter. For example when it is 98˚F in Phoenix your unit comes on, quickly satisfies the demand because it was sized for 108˚F, and then shuts off. This is a short cycle. Short cycling is a problem. The short cycle is like driving around town and not on a highway. During local driving your efficiency is low and the wear and tear is greater. These short cycles often don’t mix air as well which leads to uneven temperatures in the house. Short cycling also shortens the life of the equipment.
The benefit of single stage equipment is economy. Single stage equipment costs less than two stage or modulating equipment. We also council some customers who are on a budget that they might be actually better off investing in single-stage equipment and combining it with an insulation, air barrier and duct sealing package. A single stage unit with an envelope upgrade may actually be the ultimate ticket to enhanced comfort and efficiency.
Two stage Equipment
At Advantage Home Performance we like to move air through a home at a low fan speed because more mixing of air occurs which creates more “even” temperatures throughout the home. It also helps to heat or cool the sheetrock and objects in the house instead of just the air which in turn makes the space “feel” more comfortable. A typical two stage unit will run on low speed approximately 75% of the time. Low speed also equates to low noise levels. Two stage equipment costs more, but in terms of comfort, in most cases, it is worth the investment.
Two-stage gas furnaces feature burners that can fire at two different levels, depending on the heating needs of the moment. On low capacity, most 2-stage models run at 65% to 70% capacity. An 80,000 Btu furnace would run at 52,000 Btu. During the course of a normal heating day, the furnace will run on low heat as much as 80% of the time. It will usually only shift to full capacity if outside temperatures drop quickly or if the thermostat is turned up. We recommend that your two stage gas furnace have a variable speed blower.
Two-stage air conditioners are similar to two stage gas furnaces. On low capacity, most two -stage models run at 65% to 70% capacity. It will usually only shift to full capacity if outside temperatures rise quickly or if the thermostat is turned down. The benefits are more even temperatures and quieter operation.
Modulating / Variable Capacity Equipment
The premium HVAC equipment on the market is modulating / variable capacity gas furnaces, air conditioners and heat pumps. This is truly multi-speed equipment. The result is a system that is constantly adjusting to deliver just the right amount of heating and cooling. The result for the homeowner is a unit that provides greater comfort and is extremely quiet and energy efficient.
The gas valves of the modulating gas furnaces open anywhere from 40% to 100%, depending on the precise need for heat in the home. Some of them adjust in increments as little as .5%. Modulating gas furnaces often begin on very low capacity and ramp up during a heating cycle to meet the demand for heat. They run at less than full capacity almost all the time. It is remarkable equipment.
A four ton modulating air conditioning system can run at 25% of its capacity meaning it can quietly run like a 1 ton system. Modulation adjusts the compressor capacity to closely match cooling demand. A modulated compressor will operate at a reduced compression ratio reducing energy consumption. Also, because a modulating compressor tracks the load more closely, it will cycle on and off less frequently.
The high SEER ratings for modulating air conditioners are the result of those longer cycles. A compressor cycling on and off frequently wastes a significant amount of energy and doesn’t create the same premium level of climate control.
This equipment is impressive, but also expensive. This equipment is for people who want the highest efficiency and most comfortable equipment on the market. We also recommend this type of equipment on custom homes that have unique architectural challenges. The amount of glazing in a room or the number of stories often determines if we recommend zoning. Finally we recommend this type of equipment for zoned systems.
Mini Splits Systems
Mini splits are small, don’t require a duct system and some of them have very high efficiency ratings. The heat pump models provide both cooling and heating. The technology has evolved to the point where the heat pumps can produce significant amounts of heat at very low ambient outdoor temperatures. We’ve had great success with them in many different types of applications.
Since mini splits have no ducts, they avoid the energy losses associated with ductwork of central forced air systems. Duct losses can account for more than 30% of energy consumption for space conditioning, especially if the ducts are in an unconditioned space such as an attic. Single head mini splits are some of the most efficient units on the market and some of the smaller units have SEER rating of 33 (seasonal energy efficiency ratio), which is remarkable.
Mini splits also work well for zoning or heating and cooling individual rooms. Some models can have as many as four indoor air handling units (for four zones or rooms) connected to one outdoor unit. The number depends on how much heating or cooling is required for the building or each zone (which in turn is affected by how well the building is insulated). Each of the zones will have its own thermostat, so you only need to condition that space when it is occupied therefore saving energy and money.
We have used heat pump mini splits extensively in some of our new construction projects where the architecture did not permit the installation of a larger conventional forced air system and have extremely positive feedback from our customers. On some projects we also use small ducted mini splits. Ducted mini splits are not as efficient as single head systems, but we have had great success with them.
Zoning typically means taking a single heating and cooling system and making it control multiple groups of room / zones that have similar characteristics. In other words you could keep the temperature lower in the kitchen and higher in the bedrooms with a zoned system. When done correctly a zoning system will be expensive.
Our philosophy is that if you are going to do zoning, do it right or don’t do it at all. Start with a Manual J Load calculation and figure the loads for individual zones. The next step is to do the Manual D Duct Design. You will also have to anticipate making extensive duct modifications to your existing duct system to make your new zoned system work. Ideally you would select modulating equipment with modulating zone dampers and forego bypass ducts to ensure a great end result.
We’ve seen plenty of poorly designed and executed zoning systems. We know from firsthand experience that zoning is simply a band aid for other issues. These zoning systems often only address symptoms and not the core problem. Bad duct design, crushed and restricted duct work, missing insulation and lack of solar control are a few of the problems we routinely find. Often it is much easier and less expensive to simply address the system’s and house’s problems, than to install zoning.
Before you commit to zoning an HVAC system in your home, have an audit performed. Rule out poor duct design and envelope issues before you invest in zoning. The solution may be to solve the problems driving comfort issues. Also consider using a mini split to solve your problem.
Indoor Air Quality
Many HVAC contractors simply want to address an indoor air quality problem by selling an expensive high tech filtration system. It has been our experience that many homeowners are satisfied with these systems. The HVAC salesperson that sold the expensive filtration system often didn’t try to diagnose the cause of the problem. We believe in not simply addressing symptoms, but trying to make a determination as to what is causing a problem and only then developing a strategy to solve it. We work to come up with a strategy based on the customer’s health and the condition of the house and HVAC system.
We have a dedicated article on indoor air quality in our home performance section. This article focusses on how source control should be the first line of defense on any indoor air quality problem. In our new construction projects our focus for enhanced indoor air quality has always has been duct sealing, filtration and fresh air ventilation. This seems to almost always work.