Choosing the Correct Efficiency System for Your Home

AFUE is the acronym used to indicate fuel burning heating equipment efficiency.  It means, Annual Fuel Utilization Efficiency.

All central heating systems that burn fuel (oil, natural or LP gas) vent the combustion byproducts to the outside of the home.  Part of the heat generated by the combustion process is vented to the outside as well.

Think of AFUE as the percentage of heat that stays in your home as opposed to being vented to the outside.

An furnace rated at 80% AFUE simply means that 80% of the heat generated by burning the fuel stays in your home in the form of useful heat and 20% is lost to the outside.

If you have a 12 to 15 year old fossil fuel burning furnace, chances are that 35% or more of your heating dollar is lost to the outside through the vent.

When selecting a new heating system select the highest AFUE possible.  You must keep in mind that in milder climates the very highest efficiency equipment may not make economic sense.  See,  “Efficiency and Practicality”.

In order for this definition to make sense it will be helpful for you to know a little of how a heat pump works.

The term heat pump is inaccurate.  The correct terminology for this type of system is, Reverse Cycle Air Conditioner.  That’s because this system actually produces heat by removing heat from the outdoor air in the wintertime.  It air conditions the outdoors in winter!

How can that be?  Well, even though the air at 40 degrees is cold to you and I it still has a lot of heat in it.  It has a much more heat in it than air at 10 degrees. 

Since we are removing heat from the outdoor air, a “heat pump” is much more efficient at 40 degrees than at 10 degrees.  HSPF takes this into consideration by predicting how many hours a heat pump will run at various winter outdoor temperatures and coming up with an average efficiency or, Heating Seasonal Performance Factor.

HSPF is much like AFUE in that the higher the number the more efficient the unit.

SEER is the acronym used to indicate cooling efficiency.  It means, Seasonal Energy Efficiency Ratio.

Since the majority of cooling is done with electricity, this ratio compares the number of Btu’s (British Thermal Units) of cooling produced to the amount of electricity (in watts) consumed by the equipment in the process of producing those Btu’s.

Simply, this efficiency measurement is stated in Btu’s per watt and again, more is better.  For example, if you have a 12-year-old central cooling system, chances are it’s approximately a 6 SEER.  The minimum efficiency allowed by law today is a 10 SEER unit.  That means that today’s minimum efficiency unit will produce 4 more Btu’s per watt used than a 12-year-old system.

In dollars and sense terms, a 12 SEER system uses about half as much electricity to produce the same cooling as a 6 SEER system.

A couple of more definitions for you to consider:

British Thermal Unit (Btu) is an arbitrary unit of heat measurement.  Someone decided on it long ago just like someone decided at some point that 12 inches equals one foot.

A Btu is the amount of heat required to raise one pound of water one degree Fahrenheit at sea level.  Someone once compared that to the amount of heat produced by completely burning one of those large, old-fashioned kitchen matches.

A Ton Of Cooling is an old expression that goes back to the very early days of the heating and cooling industry.  This was a time when ice was used to cool theatres and opera halls. 

Consequently, one ton of refrigeration effect is measured as the amount of heat required to change one ton of ice from 32 degree (F) ice into 32 degree (F) water in a 24 hour period of time.

288,000 Btu’s of heat is required for this change to take place.  If you break that down it’s 12,000 Btu’s per hour for a 24 hour period.

If you look at a nominal three ton system you will find it’s capacity to be three times the hourly rate or 36,000 Btu’s per hour.

You need to understand the impact of efficiency on comfort when choosing the correct efficiency for your home.

Heating Efficiency and Comfort

In heating, for example, many older furnaces brought the burners on and let them heat for a while before the indoor fan started to run.  This provided a nice burst of warm, comforting air into the home.  Unfortunately that was not very efficient since many of your fuel dollars were going up the vent and  to the outside during that warming up process.

Today’s furnaces only allow the burners to come on for a brief period of time before starting the indoor fan.  This is much more fuel-efficient but, if you were used to the old blast of hot air, you may be disappointed in today’s newer technology.  This type of operation is mandated by minimum efficiency required by law so there is not much you can do about this performance difference.  (If this change in operation really bothers you, consider gas logs.  One advantage to gas logs is that you can back up to them to warm up.)

One thing to consider for heating comfort, especially with fuel burning heating systems, is that heating systems dry out the interior of your home in the winter.

This drying can have an effect on your comfort because it can cause discomfort when you breathe.  It can also have some tangible affect on your home such as causing molding to separate and pianos to lose their tune.

A simple humidifier installed in your heating system can reverse this.  Just remember not to set the desired relative humidity higher than 45% or lower than 35%.  (See the Health and Home Comfort section for a more detailed explanation.)

Cooling efficiency can have a large impact on comfort.  This is especially true if you live in a southern or coastal region of the country. 

When manufacturers began making higher efficiency equipment their goal was to lower the amount of electricity used in the production of cooling capacity. 

One of the things they did to accomplish this was to make the heat transfer surfaces of the equipment larger, which in turn reduced the work the compressor had to do.  While electricity consumption was reduced a real trade off resulted in newer systems that removed less moisture from the air than the older systems.  

Many homeowners have added insulation over the years and perhaps storm windows and doors.  Shade trees surrounding the home may have grown as well.  All of these things in combination reduce the required size of the cooling system.

While all of these energy efficiency improvements were being made in the home, the existing cooling system was aging and, as all mechanical things, losing capacity.  In a sense it was keeping pace with the energy improvements.

The ability of a central cooling system to remove moisture depends on run time.  A properly sized unit should be sized so that it runs all the time when it’s 95 degrees (F) outside.  That is in order for it to run enough to remove moisture the 80 to 85% of the time when it is not 95 degrees (F) outside.

This is very important:  Oversized equipment will not run an adequate amount of time to remove moisture except during the peak of the cooling season.

Whether or not your cooling system removes the correct amount of moisture can have a big impact on your comfort.  If a system removes too little moisture the result can go from moisture damage and rotting of wood to serious health problems caused by mold, mildew and dust mites.  (For more information see, Health and Home Comfort.) This is in addition to the clammy feeling you may feel as a result of the excessive moisture. If the moisture is properly controlled you may feel very comfortable at a thermostat setting of 78 degrees (F) and your overall operating cost would be less.

When selecting a new, high efficiency system to replace an older system, make sure it is correctly sized to your home.  A contractor can accurately estimate the size required by performing a heat gain / heat loss estimate.

If you own an older home you may be surprised to find that all the energy improvements you have made have reduced the size of the system you need to heat and cool your home.