In most US households, air conditioning and heating systems are the biggest users of energy, so once we have maximized our envelope efficiency, the next big target is an efficient HVAC system sized correctly to match our great envelope. This will give us the most improvement toward our overall energy reduction goals. The important considerations for heating and cooling equipment are quality of installation, correct sizing of the system and, finally, the efficiency ratings of the equipment, in that order. Before building science looked at and started recognizing system-wide equipment efficiencies, it was standard practice in the industry to oversize everything. Bigger is better right? Not if you want high performance, comfort and efficiency.
When equipment sizing meant installing the largest system that you could afford, the thought was that if you cool or heat your home as fast as possible, the equipment would run less and therefore use less energy. But HVAC systems, like cars, need to warm up before they reach their top efficiency. A 13-SEER system starts each cycle out at about 7 SEER and needs to run for at least ten minutes to reach its rated 13 SEER 1 Oversized systems can cool the house so quickly that they cycle off before they even reach their rated efficiency. This is called short cycling. Boilers and other heating equipment can also suffer from short cycling and lost efficiency. That means they don’t have time to remove any indoor humidity. Back to the car analogy, this is like rush-hour, start-and-stop driving. In testing conditions, the system is often allowed to run for hours so that it can achieve its most efficient operation. This is like long-distance highway driving, where our cars get their best miles per gallon efficiency.
To determine what equipment efficiency makes the most sense for your project, research the minimum efficiency ratings required by residential energy codes in your area, and then research the rating requirements for ENERGY STAR or green building programs in your area. Also, check into what rebates are available through your local utility service provider or if any federal tax credits are available. These programs will usually offer tiered incentives based on increased efficiency ratings on equipment and other criteria. Then you should model (using a building energy simulation model) the different systems that you are considering to determine which contributes best to your overall total home performance goals. The software can also estimate your utility costs for each system option and your simple payback using the various prices of the options.
Right-Sizing
The one and only way to accurately determine what size cooling and heating systems you need is by performing an Air Conditioning Contractors of America (ACCA) Manual J protocol. This is often called a heat gain/heat loss calculation or a heat load/cooling load calculation. It is a very detailed software calculation that builds a model of the home based on heating and cooling loads generated by all of the external exposed areas of the home. Each fractional square foot input is listed in terms of its orientation according to the basic eight compass directions. Every wall, floor, ceiling, window and door, along with their individual R-values and U-factors, as well as the tested airtightness of the building envelope and the heating and air ducts and any overhangs, appliances and the number of occupants in the house are included in the computer model. The program then calculates exactly how many BTUs of heating and cooling the home will require on a room-by-room basis using weather data for the last 30 years in your city. National building codes require that every newly installed heating and cooling system be sized strictly to an ACCA Manual J. If your contractor balks at this code requirement, you might want to think about finding another contractor. This software actually includes a pie chart that breaks down the loads by building component on both the heating and cooling sides. The mechanical contractor, energy auditor or engineer creating the report will usually need to modify any default values to comply with variations that are known to exist on your project and local climate conditions. However, the devil is in the details, in that the inputs for well-sealed ducts or envelope are seldom considered, even though the software has inputs for them.
Of course, if your Manual J shows that your home is near a breakpoint in sizing, further improvements to the building envelope will result in downsizing your system size. You should note if there appear to be large loads on wall or glazing assemblies. If so, you might consider it a good investment to add some outdoor shading structures to reduce those loads. And finally, review the contractor’s design for the system installation to make certain there are no obstacles in the structure that will inhibit it from delivering the premium performance that you are paying for.
Only a small percentage of residential contractors are competent in doing Manual Js, and doing them well takes experience. As with any software, the GIGO (Garbage In/Garbage Out) rule applies. You should always have the Manual J report reviewed by an independent commissioning agent, to assure that the inputs correctly reflect the climate, construction and system capacity inputs specific to your project. Or better yet, have the Manual J performed by an independent third party who is ACCA-certified to do this calculation. You can find firms online to whom you can e-mail your house plan or the CAD file and specifications and receive a cooling/heating load in a few days. You then can include it in your bid package to assure that all contractors are on the same page.
Next, the contractor should calculate the Manual S, which analyzes your load and then matches equipment selections to your home. Finally, Manual D will correctly determine the size of ductwork required to deliver the volume of air needed for comfort to each room of your home, quietly and without drafts. Ideally you want your systems sized to run constantly for extended periods of time on the hottest and coldest days and still be able to hold the set point (the temperature you select on the thermostat) with maximum efficiency and minimum wear. An oversized system that is constantly turning on and off will wear out more rapidly and never operate at its peak efficiency.
We promised you that achieving high-performance green won’t cost an arm and a leg. One of the key ways to achieve this is found in the interactions between the envelope of the home and the HVAC system required. If you follow our advice to spend what is required to achieve a truly high-efficiency home envelope, here is one of the places where you will recover some of that investment.
ACCA studied sizing a few years ago and concluded that the average air conditioner in America is 150-200 percent the tonnage required by the house. High-efficiency envelopes require far smaller heating and cooling units to maintain the indoor temperature. Green envelopes employing sound building science principles often need only one-third to one-half the air conditioner tonnage and heating system BTUs of traditionally built homes. This means that the air ducts are a smaller diameter and when designed correctly, each run is shorter. This means that you can budget for smaller heating and cooling units and a less expensive duct system.
Almost as important as the efficiency rating of the HVAC equipment selected is achieving the right airflow and this requires the correct Total External Static Pressure (TESP). TESP measures the resistance to airflow that your HVAC fan sees. The fan can only push so hard, and high TESP results in low airflow and reduced comfort. This happens when ducts or registers are undersized and is why we so often see rooms near our units getting enough or too much air and rooms farther away starving for air. Higher utility bills and, often, a rushing noise at the vents are other results. We measure TESP in inches of water column (wc) or Pascals (Pa) of pressure. All residential HVAC fans work most efficiently at 0.50 inches wc which equals 125 Pa of pressure. Unfortunately, field testing very often finds that they are working against one and one-half to twice that much resistance.
Your mechanical contractor should measure the resistance to airflow to ensure that it does not exceed the recommended levels, and this should be confirmed by the performance testing consultant at system start-up. This assures the correct pressure to deliver the air to the rooms in the amount needed for comfort. Having enough cooling or heating capacity at the unit, but not being able to get it to each room is an exercise in futility. This situation is very common and has been found to be the cause of comfort complaints and high bills far more often than undersized equipment.
Utility studies across the nation have documented the fact that half of all residential HVAC units suffer from highly inadequate airflows. To be honest, HVAC equipment is almost never undersized, but it is often too starved for sufficient airflow to deliver real comfort. Don’t let this happen to your new home. It’s easy to measure and fix now, but very hard and expensive to do after the drywall is up. Many HVAC contractors are afraid of right-sizing air conditioners and furnaces because they have grown up in a world of poorly built, leaky, drafty, badly insulated homes.
You may have to stand your ground and insist on an HVAC system that isn’t oversized and is tested and shown to have the right airflow at the right TESP. It’s important to your utility costs that you do this. It is also critical if you expect your home to be comfortable. Do not let yourself be sold the old bill of goods that a bigger HVAC system than your home really needs will improve your comfort! Another consideration for right-sizing air conditioners is humidity control. Our comfort is defined as much by the humidity level as by the temperature of the air around us. In humid summer climates, dehumidification (which is accomplished by cooling systems) requires long run cycles.
This is accomplished by right-sizing the equipment, so that it runs long enough to remove the excess moisture from the air. Most modern air conditioners must run for at least eight or nine minutes before they begin to lower indoor humidity levels.? When equipment is oversized, the resulting short-cycling does not manage humidity effectively. At higher humidity, even at a colder setting on the air conditioning thermostat, we are still uncomfortable. On the flip side, we are more comfortable at a higher temperature and lower humidity. We can actually raise our thermostat setting a couple of degrees if our air conditioner is doing a good job of managing humidity, which means long-term utility cost savings.
In humid summer climates, it is also important that the contractor select an evaporator coil and variable speed fan to maximize the system’s ability to remove indoor humidity. Often these systems are controlled by a combination unit called a thermidistat. A typical thermostat only measures and controls the temperature in a home. A thermidistat includes a humidity gauge or humidistat and varies the fan speed to allow improved control of both humidity and temperature for a real comfort boost.
