Getting a more efficient heat pump

[Jud McCranie]

I'm trying to estimate how long it would take to recoup my investment if I buy a more efficient heat pump.

The one we have is 11 years old and has a SEER of 10.05. It is a

4-ton unit for about 1975 square feet and the real estate agent said that it was oversized, so me might could use a smaller one.

I got my monthly electric bills for the past 12 months. We used a total of 27,100 kWh, at a cost of $2,574. Our rate depends on the season and also on how much you use in a month. Summer rates are higher, and the more you use, the more it cost.

There are periods in the fall and spring where very little heating and cooling is used. Our smallest bill was in November, using 1600 kWh, at a cost of $129. If I assumed that this used no heat/cooling, that means that heating and cooling came to 7,900 kWh, or $1026.

Does that look like I could recoup my investment in a more efficient heat pump in, say, five years?

[Robert Allison]

Your real estate agent also does air conditioning?

Let the HVAC guys size your unit. That said, savings depend upon a lot of factors, far too many to review right here. You can get an idea of the savings based on the chart at this site:

Just upsize by 25% and that should tell you what you want to know.

[CJT]

From that page:

"The following data is based on a 3-ton system (36,000 BTUH) at 9 cents per KWH at 2800 annual hours of use."

2800 annual hours of use? I hope they don't mean that's how many hours they assume it runs in a year. Even on a hot day here in Texas, our unit only runs about 10 hours. And while it's warm here much of the year, the heavy AC season is only about 4 months long. Maybe if I have some spare time, I'll add up the minutes our AC ran last year (I have it instrumented); I'm pretty sure it's nowhere near 2800 hours.

[CJT]

I realized I could get at the number another way (I know about how many KWH I use for cooling each year, and about how many watts the AC and air handler consume while running). In a year, our AC runs about 1600 hours. 2800 seems way high to me.

Austin, Texas

[CJT]

FWIW, our peak day this year was August 12. Our AC ran for 11 hours 40 minutes.

[<kjpro]

Straight or in a 24 hr period?

[CJT]

In a 24 hour period, midnight to midnight.

[<kjpro]

I hope that wasn't at design temperature.

[CJT]

Whatever design parameters were used 25 years ago when the house was built have been forgotten long ago. And the original designer couldn't have known what temperature profile I would now set in my programmable thermostat (especially since I don't think such thermostats were even on the market yet back then), nor could (s)he have known that I would have installed a radiant barrier in my attic, nor many other factors that affect the AC load. So there's really not much sense in discussing whether the peak day this year was at "design temperature" or not. In fact, one might argue that design by rule can't hope to get very close to the "right" answer, particularly when load patterns can change over time (e.g. I could install more insulation or the insulation I have could compact and lose its effectiveness, I could throw more/fewer parties with lots of heat-exhaling guests, global warming could affect the climate, I could plant trees which over time could mature and shade the house, I could install blinds and draperies, I could paint the house a different color and install a different color roof, etc.).

[<kjpro]

Geez.... 25 years old, replace that old energy hog!

[CJT]

Replacing it would be uneconomical.

[<kjpro]

Not likely.

[CJT]

like you would have a clue

[<kjpro]

You sure don't.

[CJT]

???

You&#39;re talking some real nonsense, boy.

I know a lot more about what it costs to run my AC than you do.

And I question the value of any economic analysis you do _without_ that information.

[<kjpro]

programmable

Boy, if you see a boy, you must be looking in a mirror.

I know *a lot* more about properly sized units that save on operational costs. Which is what you lack.

A moron would question facts. Why, cause their not smart enough to understand the details.

[CJT]

Having read what you&#39;ve written, I&#39;m convinced you know squat.

[<kjpro]

Don&#39;t know her, but I do understand that you know very little about HVAC.