# Energy efficiency - purchasing the right furnace

1. Jan 29, 2012

1. The problem statement, all variables and given/known data

Consider a homeowner who is replacing his 25-year-old natural gas furnace that has an efficiency of 55 percent. The homeowner is considering a conventional furnace that has an efficiency of 82 percent and costs $1600 and a high-efficiency furnace that has an efficiency of 95 percent and costs$2700.

The homeowner would like to buy the high-efficiency furnace if the savings from the natural gas pay for the additional cost in less than 8 years. If the homeowner presently pays $1200 a year for heating, determine if he should buy the conventional or high-efficiency model. 2. Relevant equations None. I just want to know if what I'm doing below is mathematically sound. The answer I got seems reasonable but I'd like somebody else to look through my calculations. 3. The attempt at a solution Cost per year of conventional furnace =$1,200*(.55/.82) = $805 Cost per year of high-efficiency furnace =$1,200*(.55/.95) = $695 Savings per year with conventional furnace =$1,200 - $805 =$395

Savings per year with high-efficiency furnace = $1,200 -$695 = 505

Difference in savings (high eff. vs. conventional furn.) = $505 -$395 = $110$110 * 8 = $880 Additional initial cost of high-efficiency furnace =$2,700 - $1,600 =$1,100

The additional initial cost of the high-efficiency furnace exceeds the savings difference over 8 years. Therefore, the homeowner should buy the conventional furnace.

2. Jan 29, 2012

### Redbelly98

Staff Emeritus
Looks good to me.

3. Jan 30, 2012