Sizing Your Furnace - Doing the Job Correctly

Sizing a Furnace for Your Home

You are probably purchasing a new furnace for the first time. Like most people, your old furnace has been keeping you or the previous owners comfortable for some time . . .even during the worst of the cold weather seasons. In some instances, our customers wonder why the BTU size furnace we recommend for their home is less than their old one.  When selecting a new furnace, the size or BTUs (British Thermal Units) must be calculated accurately to prevent problems from both under-sizing and over-sizing!

Question1:  Will the new furnace be the same size as the old furnace?

The answer is probably not, furnaces are now more efficient than ever. This means that you get more bang for the buck; lowering your gas or oil and bills.  Another reason why the furnace won't be the same size is becasue the method of determining the BTU output is different. If a furnace had an output of 80,000 BTU, the actual BTU to the ductwork, as measured by standards set by the Dept. of Energy, would be between 60,000-64,000 BTUs. For a 100,000 BTU input furnace, this is 60%-64% AFUE (efficiency). Today and 80% (AFUE) efficient furnace would have a BTU input of between 80,000-88,000 BTU and an output into your ductwork of 64,000-71,000 BTU. In this case a new Carrier furnace would give you a little more BTU’s than your old furnace and save you between $211-$271 each year on your gas and electric bill.

Question2:  Why do you should not oversize when sizing a furnace?

Furnace manufacturers do not recommend oversizing furnaces because they will rust out and break down more often and cause the occupants more discomfort. An oversized furnace will usually not run long enough and have a long span between cycles. This causes cold drafts during the evening and comments like, “Should we turn up the thermostat, it is getting cold in here?” And sure enough, just before you get up to raise the temperature, the furnace kicks on. New technology in furnace designs has created a need for more accurate BTU furnace sizing. Furnace manufacturers have been warning that oversized furnaces will break down more often and cause more discomfort than in the past. Just recently, our service department has seen six homes between 11 and 15 years old with oversized furnaces with rust-through holes in the heat exchangers.  The technical reasons for this are lengthy but if you are interested in learning, please contact us for a detailed explanation.

Question3:  What does Energy Systems do that is different than most other heating contractors to arrive at the proper BTU size of your furnace?

A few firms like Energy Services keep up with the latest sizing methods and use the correct way to arrive at the proper BTU size of furnace for your home. Unfortunately, most heating contractors have not learned how new furnace technology affects BTU sizing and still adhere to the old methods.

Unlike many other contractors, Energy Services takes all the factors that affect heating into account, including;

• We ask questions, investigate and calculate!!!!
• Measuring the exact square footage of your home.
• Determining the amount of insulation in the walls and attic.
• Number of windows and doors, and their condition.
• Ask you about your level of comfort or discomfort at temperatures between 10 and 40 degrees outside.
• Ask you if there are rooms that are uncomfortable in the winter or summer.
• We do not first look at the old furnace BTU rating, because the old BTU sizing may be incorrect using today’s standards.

Another very important aspect of properly sizing a furnace that many contractors miss is at the chimney. For a full explanation on why paying attention to the chimney venting is SO important click here.