Furnace Efficiency—the DOE label only tells part of the story


The “AFUE” (Annual Fuel Utilization Efficiency) rating that DOE requires on a furnace indicates how efficiently the equipment is burning natural gas, propane, or oil and turning it into usable heat.  This rating takes into account heat normal lost up the chimney, through the cover of the equipment, and as the unit cycles on and off.  (See “Scorched Air” and sizing.)

Recommendation:  95 AFUE in cold and mixed climates; 90 AFUE in hot climates

Checking the efficiency of an older furnace.

For starters, we generally recommend at least an ENERGY STAR labeled furnace.  For gas or propane, in mixed and cold climates, we recommend 95 AFUE or better—and most of our customers agree.  95% efficient is much better than the older furnace we often find operating at less than 75% efficiency—and better than a “standard” 80% efficient furnace.   Even in hot climates we recommend at least 90 AFUE, or 90% efficient.  We prefer these in hot climates not just because of efficiency but also because they are “sealed combustion”, meaning the combustion air isn’t drawn from the house and they’re much less affected by pressure changes within the house.  

Recommendation:  Variable speed, or “ECM”, motor

The AFUE rating is just the starting point.  This number does not consider electrical energy.  And furnace with conventional blower motors can use more electricity than even an older refrigerator (let alone a newer ENERGY STAR refrigerator). 

A new 95 AFUE, two stage furnace, with an efficient motor. New higher-efficiency furnaces--with more efficient motors--can help you avoid wasting money on utility bills.

We see furnace fans which draw between 500 and 1,000 watts of electricity.  And depending on both the wattage draw of the fan, and the run times, electricity use can be 600-700 kilowatt hours (kwh) per year.  If you run the furnace fan continuously, either to improve air mixing or in conjunction with advanced filtration or air cleaners,  annual electricity use will be much higher.  [Note, if you run your fan continuously, any duct or house leakage gets greatly magnified and the energy and money you’re wasting.]

Since inefficient fans generate waste heat, this is even worse in cooling climates since the air-conditioner battles the fan and you pay twice!

When you buying a new furnace, follow our AFUE recommendations above.  But also look for one with a fan that’s run an electronically commutated motor (or “ECM”). Such motors use significantly less electricity than conventional motors. 

Even better news.  These ECMs are typically used in multistage units which can better match the output of the furnace to the needs—like feathering the gas pedal rather than stomping on it.  This increases comfort. 

Improve your comfort and stop wasting money—that makes sense to me!



Tags: , , , , ,

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

%d bloggers like this: