Gas furnace and boiler thermal efficiency is measured with the Annual Fuel Utilization Efficiency or AFUE rating. It is defined as the ratio of output heat delivered to input heat produced from the combustion of a fossil fuel. The delivery point is considered to be the point at which the heated air, water or other medium exits the furnace or boiler.
The AFUE rating is similar in concept to the SEER rating but applies to heating equipment instead of air conditioning systems. AFUE ratings are used classify the thermal efficiency of the various furnaces and boilers on the market. If you are considering a gas furnace or boiler upgrade to a higher AFUE rating and want to know how to calculate the savings, read on...
Let's assume your current gas furnace has an AFUE rating of 80% and you are considering a new unit that is rated at 95%. The first step is to determine the percent of energy savings anticipated by using the following:
Next, determine the annual heating fuel cost from your utility bills. If you have a gas furnace dedicated to only heating your home just add up last winter's bills. If you have gas heat and use gas for cooking and/or hot water heating you'll need to look at your last 12 months of bills.
Graph out your cost with a simple spreadsheet. Notice that consumption has a big bubble in the colder months but remains relatively flat during the summer. Draw a horizontal line across the graph that best estimates average consumption during these summer months. All consumption above this line is used for heating.
Another way to quantify this is to average your June, July and August gas bills. Multiply this average monthly expense by 12 and subtract it from your total gas cost for the year. The result is your total heating cost as shown here:
Multiply this total heating cost by the percent efficiency increase to get your annual savings. Divide this amount into the cost of the new furnace to determine how many years it will take to recover your investment.
In 1992 the Department of Energy (DOE) determined the minimum allowable AFUE to be 78% for all furnaces sold in the U.S. except for mobile homes which were allowed a minimum AFUE of 75%. This AFUE, according to the DOE, takes into account the seasonal efficiency of a furnace or boiler based on the on/off cycling and the associated energy losses as it responds to the load which varies with weather and control settings.
This turns out to be a more accurate measurement for furnace efficiency than boiler efficiency. Furnaces heat air which is circulated to warm living spaces. Boilers heat water and circulate it for space heating or direct consumption in a production process.
Furnaces cool down during off cycles but some boilers maintain a standby mode where the water is kept warm when circulation is off. The fuel used to maintain boiler standby heat is not part of the AFUE calculation. Water tank insulation jacket losses, over sizing effects and actual draft regulator losses are also excluded for boilers.
The result is that AFUE efficiency ratings for boilers can be artificially higher than actual efficiency. If you are considering a boiler system to heat your home or business be sure to look beyond the AFUE rating alone. Compare features and real efficiency carefully before committing to a purchase.
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