Understanding Efficiency and Capacity

SEER Seasonal Energy Efficiency Rating Measures cooling performance on air conditioners, heat pumps and gas/electric package products. HSPF Heating Seasonal Performance Factor is a measure of the average number of Btu's of heat delivered for every Watt-hour of electricity used by the heat pump over the heating season. AFUE Annual Fuel Utilization Efficiency It measures the amount of heat actually delivered to your house compared to the amount of fuel that you supply to the furnace. Thus, a furnace that has an 80% AFUE rating converts 80% of the fuel that you supply to heat - the other 20% is lost out of the chimney.

As ratings increase, so does unit efficiency. In heating and air conditioning systems there are two types of efficiencies. The efficiency of the equipment and the efficiency of the system. Homeowners often make the assumption the efficiency is in the box. The box being the  furnace, heat pump, boiler or air handler where you find a yellow sticker that rates the unit efficiency. In older systems this efficiency was the operating efficiency. In today's system the efficiency is more complex and rates the equipment efficiency on a seasonal basis rather than only when it is operating. Furnaces and boilers have an operating efficiency rated in AFUE or Annual Fuel Utilization Ratio. This efficiency is calculated as the efficiency of the unit while it is operating, the efficiency of the unit as it warms up and cools off and the off cycle efficiency. During all these processes the unit is adding or taking away from the overall efficiency. The AFUE is the total and true overall efficiency. Older boilers and furnaces did not have AFUE ratings and efficiencies were determined by the operating efficiency only meaning heat output divided by energy or heat input. In this simplistic method only the energy losses that occurred up the chimney when the furnace was operated were considered. The problem was it took a lot of heat energy to warm up the older furnaces until they reached operating temperatures to deliver heat. When the furnace or boiler was shut off there would continue to be lost heat left in the heat exchanger as chimney continued to pull air over the heat exchanger cooling it and removing that heat to the outside air.

After the heat exchanger was cooled off warm air from the house was continually pulled out and up the chimney. An older boiler or furnace that may have had an operating efficiency of 75% may have only had a 55 or 60% total efficiency or AFUE when all the losses were taken into account. Today those off cycle and start up losses are at a minimum for newer higher efficient units as heat exchangers and control systems are more efficient at extracting the maximum amount of heat and little heat is lost during the off cycle. On furnaces today heat exchangers are tubular rather than the older traditional clam shell design that existed for many years. Tubular heat exchangers provide more surface area for heat to be extracted and are also less prone to failures which plagued clam shell designs from stress cracks at weld joints. Tubular heat exchangers have no weld joints. In addition there is a secondary stainless steel heat exchanger to finally condense moisture out of the burned gas which increases AFUE efficiencies to 92% and more.

Dollar amounts computed at $.092 ccf, for 2,000 full-load heating hours and a system rated at 60,000 Btuh. Actual saving may vary depending on climate conditions, fuel rates, and patterns of usage according to individual lifestyle. 3-Year Heating Savings 92.1% and 80% AFUE unit vs. 65% AFUE units

As we see from the examples on boilers and furnaces efficiency is rated as an overall seasonal usage and not just the operating efficiency.

Heat pumps and air conditioning efficiencies are also rated for overall seasonal performance. Years ago air conditioners were rated in EER or Energy Efficiency Ratio. That was simply a measurement of the energy output divided by the energy input. Today air conditioners are rated in Seer or Seasonal Energy Efficiency Ratio. Seer measures the total operating efficiency of an air conditioner including start up and off cycle losses as well as operating efficiency. Since EER only takes into account the operating efficiency it's rating will always be higher than the Seer rating which takes all losses into account.

Heat pumps measure air conditioning efficiency the same as air conditioners in Seer. When measuring the heating efficiency the HSPF method is used or Heating Seasonal Performance Factor which measures energy input plus start up and off cycles losses. COP or Coefficient of Performance was the original measurement of a heat pump's heating efficiency which again only took into account the operating heating efficiency. Today you will find HSPF the measurement of a heat pump's efficiency.

Annual savings based on 36.000 Btu unit, 1500 cooling load hours, and .08/kwh. Actual savings may vary depending on climate conditions, energy rates and patterns of usage.

ARI The American Refrigeration Institute establishes the method of measurement of heat pumps and air conditioners efficiencies. ARI also establishes how heat pumps and air conditioners are tested and efficiencies are determined. Each air conditioning and heat pump system has an ARI rating number which shows the actual efficiency rating and the tests performed. On split systems each outdoor unit is rated with a matching indoor coil and blower or furnace to establish the equipment used for the determination of the ARI rating. Packaged units of course are rated as they exist. In addition to efficiencies ARI also has established conditions for measuring the output capacity of each system while certifying the efficiency. The United States Department of Energy establishes minimum standards of efficiency for equipment that can be manufactured. In January 23, 2006 HVAC manufacturers will only be permitted to make 13 Seer or higher efficiency air conditioning systems including split systems, packaged units and window air conditioners.  And heat pumps must be a minimum 13 Seer and 7.7 HSPF. Again those ratings of efficiency are registered and certified with ARI and the ARI certification will be found on ARI's web site. The ARI certification is usually shown on the unit as well as the HVAC manufacturer's specifications which can also be found on the respective manufacturer's web site.

Understanding the basics of efficiencies you will also find more information on the manufacturer's specifications. For air conditioning systems specifications there is shown the total capacity. This capacity is shown in btus under the following conditions. The outside temperature or the ambient air over the outdoor coil is 95 degrees and the air entering the cooling coil from inside is 80 degrees and the wet bulb temperature is 67 degrees. To fully understand dry bulb and wet bulb temperatures it is necessary to understand how they are taken and what the relevance is. Dry bulb temperature is the temperature you would read on any thermometer. It is the daily temperature you see on the weather or on your thermostat. Wet bulb temperature is determined by placing a cotton sock on a thermometer and immerse the sock in distilled water. An air movement of 500 feet per minute as the ambient air is passed over the thermometer. As the ambient air passes over the wet bulb thermometer the temperature is read until it reaches it's lowest point. This temperature is the wet bulb temperature. There are also electronic thermometers that read wet bulb temperatures instantaneously without the need for socks and a fan and distilled water. However realizing the procedures of taking the wet bulb temperature provides a through understanding of the concept and it's importance. By taking the wet bulb and dry bulb temperature and using a relative humidity chart the relative humidity can be determined. Also the dry bulb thermometer is a measure of the specific heat of the air. The wet bulb thermometer is a measure of the latent heat of the air. The combined latent and sensible heats are called the total heat or enthalpy of the air.

Understand in air conditioning two important functions occur. Removing heat and removing moisture or dehumidifying the air. The heat of the air is called the sensible heat. The moisture in the air is referred to as latent heat. Air conditioning systems remove sensible and latent heat. Or in layman's terms air conditioners lower the room temperature and dehumidify the air or remove moisture. Stay with this because understanding this will also help you better understand how air conditioning systems are rated and selected.

When you fill out our online sizing form our Manual J software system analyzes the heating and air conditioning requirements of your home. The air conditioning requirement is calculated at the peak average summer temperature for your specific location. Then after calculating the thermal efficiency of your house the actual btus are calculated to remove sufficient heat and moisture from the indoor air to obtain 74 degrees at 50% relative humidity. Depending on your location the amount of moisture to be removed will vary. For example a house in Florida will have more moisture to be removed from the air then a house in southern California where the outside hot summer air is substantially drier.

After the cooling load for your house is determined the next process is to look at the selection of an air conditioning system that matches that same requirement. For example let's say a load calculation determines 48,700 btus are required to cool your house. If a contractor does do a load calculation here is where many of them will tend to make a mistake. The total btus required to cool the house is 48,700 btus. But that figure is a combination of the btus required to cool the air plus the btus required to dehumidify the air. In our example 44,700 btus are required for removing heat or lowering temperature. This is called the sensible btus. The remaining btus are those required to remove the moisture or the latent btus which is 3,000 btus.

Now a typical contractor may immediately determine there is a 4 ton system required. this determination was made by taking the amount of btus that make 1 ton of air conditioning and dividing the total btus required by that amount. 12,000 btus equals one ton of air conditioning. By taking 48,700 btus and diving this by 12,000 we come up with 4.06 tons of air conditioning. But remember this is a mistake so don't practice this mistake.

Instead what is required is to go to the manufacturer's specifications for the type and efficiency of the system required. In this case we are looking at a Goodman 14 Seer efficiency system or the model CLQ. In the specifications for the CLQ condensing units we find there is only one unit that has sufficient capacity for our example. The CLQ60-1 5 ton system has a total capacity output at 95 degree outside ambient temperature of 55,000 btus. The CLQ48-1 4 ton system doesn't have enough capacity because the largest output is only 45,000 btus. Now here is where this can get confusing to inexperience. For the CLQ60-1 the sensible output or the ability to lower actual temperature indicates only 39,600 btus of work is capable and the remainder of 15,400 btus is assumed to be the latent capacity. However if the unit is not doing that latent work of 15,400 btus the remainder will go towards the sensible requirement. In our example there is only 3,000 btus of sensible heat required to be removed for dehumidifying. Taking the 15,400 btus shown as the latent capacity and subtracting out the actual 3,000 btus of latent work required, the remaining 12,400 btus goes towards sensible heat removal. So the total sensible cooling btus available from the CLQ60-1 unit is 52,000 btus which is more than enough for our example. Remember in the original selection we only needed 44,700 btus of sensible cooling capacity. Since the next smaller system the CLQ48 has insufficient capacity at a grand total output of 45,000 btus the next size CLQ60-1 was selected.

Now if this example was for a house in Arizona, Nevada or southern Texas where outside design ambient temperatures can be 105 degrees the system selection shown above would have been wrong. As the outside air temperature increases beyond the ARI rating of 95 outside air temperature the capacity of the system decreases. So if this example were in an area where the outside ambient exceeded the ARI rating temperature of 95 degrees it would have been necessary to review the manufacturers specifications and ratings at that design outside temperature. Since we are showing this as a possible scenario the system capacity for such areas could easily exceed the highest output capacity of a standard residential system which is 5 tons but still less than 60,000 btus as you can see by the manufacturers specifications. In the case where a system requirement is greater than the largest residential system then it is necessary to use two smaller sized units to achieve our goals.

You will also see on the specifications the differences of capacity and efficiencies based on the matching coil or air handler. When the condensing unit is matched to the AEPT variable speed air handler the capacity and efficiencies are the highest. This is due to the variable speed blower, larger matching coil and TXV or thermal expansion valve. DESCO Energy will always match up the most efficient system for your application. In more humid climates such as Georgia or Florida it is better to use a slightly smaller coil to obtain colder coil temperatures for more dehumidification than to attempt to maximize efficiency with a larger coil and less dehumidification.

There are also other considerations that need to be taken into account in system sizing and selection. Relatively speaking houses that require 4 or 5 ton systems should consider using two smaller system for better control, comfort and longevity. Systems so large are used for large square foot houses. Any single story house with over 2,000 square feet should consider two systems. Houses with two stories and more than 700 square feet per floor should also consider using two systems. Again comfort, difference in load requirements from the first floor to the second floor will always make it difficult to achieve equal comfort and temperatures on both floors. Zoning can also be a consideration however we have found the cost of two stage heat pumps or air conditioners with zone dampers and controls and long lengths of ducts cost slightly more than two smaller systems.  Two smaller systems also require less labor for ducting and installation in comparison to a single large system.

For heat pumps the same procedure for selection applies because heat pumps are selected on the house air conditioning requirements. Only the electric back up heat is sized for the heating requirements of the house. The maximum heating capacity of heat pumps is 20kw of electric heat provided which is 68k btus at 240 volts and 59k btus at 208 volts. (Some manufacturers capacities do go to 30kw.) If the heating requirement of the house is greater than this amount either 2 smaller systems should be selected or use a propane gas furnace dual fuel system if natural gas in not available.

. In heating there is only one form of heating and that is sensible heating or the raising of temperature. Humidification of the air should be considered for increased comfort and energy efficiency but does not apply to the calculation. Gas furnaces are initially shown as heat input. A heating requirement for a furnace should not be considered on the basis of the heating input but the heating output. An oil furnace or boiler is rated at heat output. As mentioned heat output is the only way to select a furnace. So why are gas furnaces backwards providing only heating input? At one time all gas furnaces had the same operating efficiency. Initially that efficiency was 65% then 70% and then for the longest period all furnaces were 75% efficiency until the late 1970s with the introduction of solid state ignition and stack dampers. >From that point until today gas furnaces have continually climbed in efficiency to 98% today. But what hasn't changed is the identification of those gas furnaces. Today a 100k btu furnace can have anywhere from 78k to 98k btus output depending on the efficiency. That's a big difference yet each furnace would be called a 100k btu furnace. Seems screwy and it is very misleading and needs to be changed. What also hasn't changed and in fact in many ways has become worse is the aptitude and thinking on the part of most HVAC contractors. To many of them every house should have a 100k btu furnace unless the house is bigger than most. Then it requires a bigger furnace. Many contractors don't take into account the output capacity of the furnace nor the insulating values and energy efficiency of the house. Many years ago, 40 to be exact, it was okay to size furnaces the same to most houses. Why? Because most houses didn't have insulation and all furnaces had the same output. But the same dumb rules of thumb used back then are in use by many contractors today. This method does require less than thinking and makes the contractors head free of any stressful calculations. 

Other factors can increase the efficiency of a heating and /or air conditioning system. For example variable speed blower motors increase the efficiency of air conditioning systems by one Seer point and .4 points for heat pumps. Variable speed also increases the electrical efficiency of any furnace. Two stage increases the efficiency of both heating and air conditioning systems 6 to 12%. Modulation increases the efficiency of a boiler 6 to 14%. Hot water reset on boilers increases efficiency up to 16%. Putting real dollars to the increases in efficiencies makes increased efficiency systems with energy savings features very worthwhile when making an HVAC system purchase at wholesale levels on a do it yourself project. Typical paybacks for added features and higher efficiency systems have paybacks of less than two years. In addition there are high efficiency systems and features with Federal Tax Rebate Credits in the new Energy Act for 2006. Furnaces and boilers with efficiencies of 95% or more are eligible for $150 tax credits. Variable speed provides a $50 tax credit and air conditioning systems of 15 Seer or higher are eligible for $300 tax credits for split systems and 14 Seer for packaged systems. Heat pumps with the same qualifying Seer efficiency for air conditioning and 9.0 HSPF for heating are eligible for $300 tax credits. Not only are these products more energy efficient providing significant energy savings, they also provide a substantial tax credit. Tax credits are available not to exceed $500 for the next two years until 2008 for energy savings products for your HVAC system. For further information and details go to the Energy Star website. In addition the Department of Energy has mandated upgrades to efficiency standards for all air conditioning and heat pump systems manufactured after January 23, 2006. Air conditioners must have a minimum 13 Seer (Up from 10 Seer in 2005) and heat pumps in addition must have a minimum 7.7 HSPF heating efficiency. Since these 13 Seer systems now become the builder's models SEER - Solutions for Energy Efficient Results recommends purchasing a minimum air conditioning and heat pump system with 14 Seer and 7.8 HSPF and higher using variable speed blower motors. Forced air gas furnaces of no less than 95% AFUE and variable speed two stage. Gas boilers in addition should have modulation and hot water reset controllers. There are also factors which can reduce capacity and efficiency. High altitude applications require special accessories in most boilers and furnaces. Higher altitudes with a depletion in oxygen will also reduce capacity outputs and adjustments or correction factors for the variances in altitudes need to be made. Conversion of natural gas to propane requires a change of nozzles or gas valves and decreases capacity as well.

The most important factor affecting any system is the adjoining components. In a forced air system using the most efficient method of air distribution and involves proper sizing and layout. DESCO Energy will layout any duct system according for the most efficient design. Most importantly we supply the most efficient ducting materials precut to your system specifications to make the system complete. Fiberglass duct systems provide the most efficient materials with almost zero air leakage. Fiberglass duct systems are easy to work with and assemble. If you can assemble a cardboard box you can assemble fiberglass ducting. There are no razor sharp edges to deal with nor expensive tools and machinery required. In addition fiberglass duct systems provide maximum insulation keeping energy losses to an absolute minimum. Fiberglass ducting is quiet and attenuates noise from blowers and air movement. Fiberglass ducting is less expensive than equivalent sheet metal with wrapped insulation and requires substantially less labor to install. Typical sheet metal ducting losses a minimum of 25% of every dollar of energy spent through leakage and thermal losses. Fiberglass ducting losses are less than 3%. DESCO Energy is the only online source capable of providing a complete HVAC system with the complete ducting as well.

Remember the most lucrative investment you can make is not on Wall Street, it's on your street in your HVAC system.    

If this process of equipment selection and sizing seems too complex, don't worry. The technical support staff at DESCO Energy will select and size your system without difficulty. Our technical support staff is well trained with many years experience. A typical technical support associate will size and select more systems in one month than most contractors do in their entire careers. We guarantee our sizing and results. So take some time to do your own selection and see how close you come to what we recommend. Call us toll free 877-265-9764 or email info@descoenergy.com