Cost per hour to run new 3 ton, 14 SEER , heat pump

garygAugust 31, 2007

I have been figuring the cost per hour to run my new 14 SEER Goodman heat pump based on the Rated Load Amps (RLA) on the condensing unit's data tag. I was using the 13.4 RLA because it was the only info that I had. I found out that Goodman has a "Technical Information Manual" that is not listed on their website but is available to pros. I contacted them and received a copy the other day. The manual lists technical info for heating and cooling based on a variety of outdoor and indoor conditions.

The only time in cooling mode that the condensing unit draws RLA is at conditions of 115 F outside air temperature.

On a 95 degree day, the unit draws between 10.5 and 12 amps depending on indoor temp and humidity. On an 85 degree day, between 9.7 and 11.2 amps.

If I use 11 amps as an average number, then the cost to run the new heat pump is:

(11 amps) x (220 volts) x (\$.138/kw-hr) / 1000 = \$.334 per hour = 33.4 cents/hour to run the system.

This includes the recent 65% increase in Maryland electric rates which are now 11.4 cents/kw-hr generation plus 2.4 cents/kw-hr transmission.

33 cents/hour to run the new unit is not bad.

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mr_havac

Garyg, you do realize those values they give are under lab conditions and not something we have in the real world don't you? They don't take into account things like cleanliness of coil, sun and shade factor, deviation from actual rated voltage, temperature range on different times of the day and things like that. Any of those will increase your amp draw and with higher amperage comes higher wattage.

August 31, 2007 at 4:34PM
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brickeyee

Amps x Volts only works for resistive loads.
Motors are ALWAYS inductive loads and have a power factor associated with them that is LESS than 1.0
Unless you want to connect a watt-hour meter into the lines feeding the unit you are NOT going to get anything useful without knowing the power factor.

Duty Cycle will also have a huge effect on the actual power dissipation.
Residential A/C systems should only reach 100% duty at the design temperature, and many are at least slightly over sized.

August 31, 2007 at 7:49PM
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garyg

I believe that my costs are a good ballpark figure based on info that I have recently gotten versus the data plate info that I was using previously. Not as good as having an ammeter hooked up 24 hrs/day w/data logger, but solid info that can be used get an idea of operating costs. I'm not really concerned if the amp draw and power numbers are not exact, I'm looking to be in the range of what they should be. If I use the data plate numbers, I get a max cost/hr to run the unit. If I use an amp draw thats in the range of the operating conditions, I get more typical operating costs.

August 31, 2007 at 9:06PM
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mr_havac

If you don't mind me asking, is there any reason in particular that you're monitoring you system so closely?

August 31, 2007 at 10:15PM
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garyg

Havac man:

I'm really looking at ballpark operating costs and not exact numbers. I replaced a functional, 21 year-old, 7 SEER heat pump with a new 14 SEER to combat a 65% increase in MD electric rates.

I'd like to know how much per hour that the new unit costs to run to see if I am getting the 40% savings that I estimated. I'm also an engineer so I like tech info and the effect on operating costs.

Finally, I want to know how loud I should yell at the wife for knocking down the t-stat when she has hot flashes. At about 33 cents/hour to run the new unit, looks like I won't be yelling that much.

Take care.

September 1, 2007 at 11:18AM
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don_b_1

Gary, you may come out better than you're estimating. We checked our 3T 13 SEER recip with VS air handler and it was pulling a total of 8.8a @ 250v. This, at 90Â°/75% outside and 78Â°/45% inside.

September 2, 2007 at 10:22AM
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mr_havac

"Finally, I want to know how loud I should yell at the wife for knocking down the t-stat when she has hot flashes.:
ROFLMAO, hey bud, I just got off of 3 ------- years of that hot flash stuff with my wife. A thousand plus nights of misery! She'd say "you have no idea what its like' and I'd reply, "no but you sure as hell are trying to give me some idea" After its over you'll understand that "for better or worse" thing! LOL Thanx for the chuckle.

September 2, 2007 at 10:40PM
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udarrell_2007

Goodman says the unit should be within 3 amps of their Performance Ratings.

I have the Expanded data on that Goodman 3-ton 13-SEER.
That 45% RH reduces load somewhat on the condenser.

At 80-IDB & 63-IWB or 44% RH it list 10.7 amps @ 350-cfm per ton, or 1050-cfm.

Looks like it is really taking care of the humidity.

I like the way they have more data available on the Net than other makes.

Near the bottom of the linked page I list some 1.5-ton 13-seer Goodman data. udarrell _ 2007

Here is a link that might be useful: udarrell.com

September 2, 2007 at 11:29PM
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garyg

Thanx Don and Darrell.

Havac Man:

Will they get "hot flashes" in the winter time so I can cut back the heat? Don't tell me, they get "cold flashes" in the winter so I have to raise the heat. I knew it. Dang women.

September 3, 2007 at 9:20AM
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don_b_1

Gary, you'll get a respite in the winter although I have heard "being too cold gives me hot flashes".

The summertime thermostat wars begin to fade after a year or two when they realize that even living in a refrigerator won't stop the flashes. Reinforcement training occurs by flipping a breaker when you notice the stat cranked down to 70. Once the temp reaches 80, you suddenly notice the AC has quit working and are miraculously able to revive it (after much laborious tinkering) and say "See, I told you quit goofing with that thermostat."

September 3, 2007 at 10:39AM
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mr_havac

"Will they get "hot flashes" in the winter time so I can cut back the heat?"
There's nothing like sleeping in a big drafty 200+ year old farmhouse in northern New England when the thermometer dips below zero and you're nice and toasty under a layer of 5 or 6 blankets when all of a sudden the covers go flying off the bed like someone just stuck a giant vacuum cleaner in the room! I drew the line when she would open the window a little. I'd tell her if she's that hot go sleep in the barn with the amimals! :-)

September 3, 2007 at 11:05AM
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brickeyee

"Goodman says the unit should be within 3 amps of their Performance Ratings."

You still need the power factor to get actual dissipated power in an inductive load.

September 3, 2007 at 3:05PM
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mr_havac

I think garyg has switched his concerns to how long he has to put up with his wifes hot flashes instead.
If he's lucky maybe he can figure out if he'll save enough over the next few years to construct an small living area for himself over the garage until his and his wifes lives get back to normal. ;-)

September 3, 2007 at 8:42PM
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egor_sb

Hey Gary,
I think MotherEarth is havin' one of them hot flashes. i got home from a week trip and the temp outside was/is 105 outside and over 115 inside!! I fired up the new Goodman 3ton and in about 1 hour it was a dryer tollerable 90, and after 4 hours we were in the high 70's. It took it a full 7 hours to bring the upstairs down to the set temp of 72 degrees, but once there, it held pretty good the whole next day! At the hottest time, around 5pm, it had creeped back to 77 upstairs where the t-stat is.
I have one of those "whole house cost monitor" electric meters and it said the heatpump and variable blower pulled an avg. of 2.3KWh's. Thats about 45cents an hour but was sooo worth it!! Besides, it doesnt get that hot that often, just the last few days is all.
I need to concentrate on better insulation and I think it will have a better time of it. Also some more returns will help the new girl as well!

September 4, 2007 at 9:03PM
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stm1

Your math is good except for the power factor. The power factor of the unit is likely .85 to .92. This makes your watts actually about 8% to 15% less than you had computed. Multiply measured amps times power factor times measured volts to get watts. Divide by 1000 to get kilowatts. The amps will be less when it is cooler outside and more when it is warmer outside. SEER is "Seasonal Energy Efficiency Ratio" It is the average efficiency and takes into account (somewhat) seasonal outside temperature variations. Your measurements are taking a snapshot of the EER (Energy Efficiency Ratio) EER = unit capacity divided by watts used. 3 tons =36,000 BTU/3000 watts = 12 EER "12 BTU's of cooling per watt used) " at 14 EER you would use about 2570 watts. Both unit capacity and watts used vary according to outside temperature. Hotter temp= more watts, slightly less BTU & thus lower EER. The rated capacity of the unit is probably at 95 degrees outside. 14 SEER= average EER for entire average cooling season.

April 9, 2008 at 11:26AM
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googe

Be sure to figure in the indoor fan Watts when doing your calc's.

January 20, 2009 at 12:29AM
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garyg

Wow. One of my old posts from a year-and-a-half ago.

The air handler fan amp draw is included in the calc.

And no, power factor is not a cost in residential electric.

Maryland electric rates have increased since my original post, now 16 cents per kw-hr delivered.

The new Goodman heat pump is running strong. Aux heat isn't needed until low 20's in winter. In summer, no problem keeping the house cool even in 100F humid summer days.

Power consumption is about 2000 kw-hr in winter. Average of 1000 kw-hrs/month in '08.

Not bad. Reduced some months by 40% compared to the 21 year-old unit that it replaced in '07.

January 20, 2009 at 6:37PM
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smallen

My rheem builders grade 4 ton 14 seer AC uses 3.6-3.8 kWh of electricity @ 85F, which includes the blower. I have a meter passively hooked up to my main power line which measures my power usage. I subtract the reading of the display before the AC starts from the reading about 30 seconds after it starts.

The monitor is called "the energy detective" and you don't have to be a electrician to hook it up...very easy and explained in the instructions. It also plugs into my computer via USB and provides lots of data, graphs, voltage readings, dollar amount spent far far in billing cycle, how much it's costing per hour to run everything, etc. I can't remember the site I picked mine up at last year but I'm sure something would come up if you google it. I think it was around \$150 which included the software for the computer (they have plain ones that just have the LCD screen with your power usage, no usb port.

PS I thought my rates were high...thanks to deregulation mine is \$0.129 per kWh. Just last year it was \$0.089, but it went up when Nat Gas went up, but now that Nat Gas is lower than it was last year, my rates are still high...go figure.

January 25, 2009 at 1:26AM
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brickeyee

"And no, power factor is not a cost in residential electric. "
While you are not billed by power factor for residential use, it does affect the power you use.

For a non-resistance load, power = amps * volts * power factor.

Volts * Amps is V-A, and is always higher for non-resistance loads than the actual power dissipated.

Without the power factor for an electric motor you cannot compute the actual power it is using.
The factor also changes depending on the motor load (though most hermetic compressors operate at a pretty constant load when they are turned on).

January 25, 2009 at 12:26PM
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zheng1975_comcast_net

I want to say I enjoy humor as much as the next guy - HMMM after reading this maybe NOT!!!
With a subject so interesting, especially since many are concerned in trying to make ends meet month after month - one guy jokes about his Wifes heat flashes and so many LOST THE SUBJECT here. And taking up so much valuable screen space - to do internet giggling!
What is even more lacking is - it is a fact heat pumps cost so much more to GIVE HEAT than A/C BUT where is that information???

February 5, 2011 at 5:47PM
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