June 17, 2010

Comparing Furnace and AC Efficiency

Have you considered getting a new furnace or air conditioner?    There are some good tax incentives right now which can help you pay for the purchase.   We've been thinking about getting a heat pump installed in our home.   When you look at furnaces or air conditioners they have efficiency ratings.   You'll see specifications like SEER 14 or 90% efficient.   But these numbers don't really tell us how much money you'll actually save.   It is not really obvious how a SEER 14 air conditioner compares to a SEER 16.   Here I'll tell you how to tell how much more efficient one HVAC unit is compared to another.    I'll compare the units in terms of % savings.    You can then use that figure to see how much actual money one unit might save you versus another.


Note: Keep in mind that all the examples I give below are just arbitrary examples.   Electricity and natural gas costs will vary depending on where you live.   The calculations are not super exact either.   The efficiency figures are meant for estimation purposes and theres not guarantee your energy costs will be exactly the same.  You should also bear in mind that weather changes daily.   Some winters are colder than others so changes from year to year can have more to do with how good or bad a season you have more than how efficient your furnace is.


Air Conditioner efficiency

AC units are rated by a SEER number.   EESI explains what SEER is :
"The SEER is defined as the total cooling output (in British thermal units or Btu) provided by the unit during its normal annual usage period divided by its total energy input (in watt-hours) during the same period."

So the SEER is a direct measure of cold air / energy used.   Higher SEER is better because it means more cold air for the same energy.

Lets say you have a unit with SEER 10 and another unit with SEER 15.   The SEER 15 unit is more efficient since you get 1.5 times as much cold air for the same amount of energy used.   The SEER 15 unit will use 33% less energy to give you the same amount of cooling.  If your current summer air conditioning costs run you about $600 for the season then the more efficient unit will cut about 33% of that off to save you around $200.

cost savings = 1- (current SEER / New SEER )

So for example if your current AC system is SEER 12 and you're upgrading to an SEER 13 unit then the cost savings would be = 1- (12 / 13 ) - 1 = 0.923 - 1 = 7.7%

Gas Furnace Efficiency

Gas furnaces are given a % efficiency number like 80%, 90%, 92.3% etc.   This rating is a basic measure of how much of the energy from the natural actually gets converted into heat in your home.   The higher the % the more heat you get.

cost savings = 1 - (current AFUE / new AFUE)

So for example if you currently have an 80% AFUE furnace and are looking at updating to a high efficient 95% AFUE model then that would mean your savings are = 1 - (80% / 95%) = 1 - 0.84 = 0.16 = 16%

The ACEEE site has an easy table showing you how much you can save if you upgrade from various level AFUE furnaces.  


Heat Pumps


Heat pumps have two different numbers for their efficiency ratings.    They have a SEER that is the same as the value used on air conditioners.   You can compare the cooling efficiency of a heat pump by comparing the SEER the same way you do when comparing air conditioners.   For heating the heat pump has a rating called the HSPF or Heating Season Performance Factor.   The dept. of Energy defines that as

"Heating efficiency for air-source electric heat pumps is indicated by the heating season performance factor (HSPF), which is the total space heating required during the heating season, expressed in Btu, divided by the total electrical energy consumed by the heat pump system during the same season, expressed in watt-hours."

In other words: HSPF = BTU / Watt - hours

Higher HSPF is better.   

The HSPF is  the same kind of measure as a SEER so we can compare HSPF the same way as comparing SEER.

cost savings = 1- (current HSPF / New HSPF ) 

When you are looking at a heat pump you have two different numbers to compare.

For example:  Lets say you are looking at a new heat pump with SEER of 14 and HSPF of 8.2.   Another model is a bit more expensive but it has better SEER 13 and HSPF of 8.0.   

cooling cost comparison = 1 - ( current SEER / new SEER ) = 1 - ( 13/ 14 ) = 7.1%
heat cost savings = 1 - ( current HSPF / new HSPF ) = 1 - (8 / 8.2) = 2.4%

The SEER 13 / HSPF 8.0 unit is 7.1% cheaper to cool in the summer and 2.4% cheaper to heat in the winter.



Standard Electrical Furnaces 

A basic electric furnace is effectively 100% efficient.    DOE explains that "Electric resistance heating converts nearly 100% of the energy in the electricity to heat."   So there aren't really more or less efficient electric furnaces.  

Comparing Gas Furnace to Electric furnace to Heat pump

Its straight forward to compare a gas furnace to a gas furnace since they have the same ratings and its a straight forward question of math.   But comparing a gas furnace to a heat pump is a more complicated question.   When comparing across gas, electric and heat pumps you're comparing not only the different efficiency levels but also the different costs for gas or electricity.    We can use this handy calculator to estimate heating costs for different forms of energy it is from a previous article I wrote talking about wood pellet stoves.

If you go to the calculator page you can see a form that lets you compare heating costs for wood pellets, fuel oil, electricity, natural gas, propane, wood and coal.    For our purposes we can use it to compare electric and gas furnaces.   You have to know how much natural gas and electricity cost where you live to compare them.  If you start with that and then put in the efficiency rating for the gas furnace and use the assumption that the electric furnace is 100% then the calculator will show you how much each type of furnace will cost to heat 1 million BTU.

So for example lets say that you currently have an electric furnace and electricity costs you 10¢ per kWH.  You're considering replacing it with a 95% efficient gas furnace and natural gas costs $1.20 per therm.  If you plug in those figures and hit the 'Calculate' button it will tell you that 1 million BTU with the electric furnace will cost $29.31 and 1 million BTU from the gas furnace is just $12.32.     The 95% efficient gas furnace would cut your heating bills by about 58%   [ 1- (12.32/ 29.31) = 58% ]

The calculator doesn't have an entry for a heat pump but we can  use the definition of the HSPF to figure out an equation to find out what 1 million BTU's cost.    The HSPF is the BTU / watt hours.  

HSPF is BTU / watt hours.  
The cost of 1 million BTU with a heat pump
= 1,000,000 / HSPF * 1 kWhr / 1000 watt-hr * cost per kWh
= cost of 1 million BTU = 1000 / HSPF * cost of kWh

1 million BTU from a heat pump = (1000 / heat pump HSPF ) * cost per kWh

So if you know the HSPF and the cost of a kWH then you can figure out what the cost of 1 million BTUs are for your heat pump.    For example looking at the 8.2 HSPF heat pump we talked about previously, if electricity costs you 10¢ per kWH then that gives us : cost of 1 million BTU = 1000 / HSPF * cost of kWh = 1000 / 8.2 * 10¢ = 121.95 * $0.1 = $12.19

Using the calculator page and the formula above for cost of 1 million BTUs for a heat pump we can do apples to apples comparison between gas furnace, electric furnace and heat pump:

So the end result of the three options from the last two examples would be: 
Electric furnace 100% efficient, electricity cost 10¢ = 1 million BTU costs $29.31
Gas furnace 95% AFUE, gas cost $1.20 per therm = 1 million BTU costs $12.32
Heat pump with HSPF 8.2, electricity cost 10¢ = 1 million BTU costs $12.19

Important Note: The cost for 1 million BTU isn't the complete picture of heating costs with a heat pump.   The cost to heat with a heat pump is based on when the heat pump is in operation.  If you live in a very cold climate then the heat pump will need supplementary heat from a standard furnace for the coldest days.   When temperatures get below a threshold of 40F or so then less efficient electric heating coils will kick in to supplement the heat.   The cost for 1 million BTU figured above is based on the heat pump providing all the heat.   To know how much the heat pump is working all alone and howmuch it has supplemented heat from traditional furnace will depend on the climate.   Thats a little more detailed than I'd like to get in this already lengthy article.  So suffice it to say if you have a fairly mild climate then the 1 million BTU calculation is closer to accurate but if your region is particularly cold then heat pumps become much less cost effective.

If you know the annual costs to heat your home with one form of furnace you can use the above information to estimate the cost to use the other forms of heat.

Lets say that you have the electric furnace right now and you spend about $800 a year in heat.   If you're spending $29.31 per 1 million BTU then that means you're using about 27.29 million BTU a year.   [ $800/29.31]

Electric furnace = $800
Gas furnace 95% AFUE= 27.29 * $12.32 = $336
Heat pump HSPF 8.2 = 27.29 * $12.19 = $332    (assuming milder climate)

If you replaced your electric furnace which currently costs you $800 a year with the gas furnace you'd save approximately $464 a year and if you replaced it with the heatpump then you'd save approximately $368 per year.

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