# Air Conditioner Running Costs ?

• posted on April 28, 2005, 12:26 pm
Hello:
Have moved into a 30 yr old house that has central air conditioning. Condenser is outside, and evaporator is part of the gas, forced hot air furnace.
Very curious about what it might cost to run the A/C.
Realize there are a zillion caveats involved, but would like to come up with a very approximate number. Data from previous owner is unavailable.
Tried looking up A/C Cost Calculators on the web, but couldn't find anything appropriate. All seemed to want to know house insullation characteristics, etc.
So:
The unit is a 3 Ton Installed in early 1984 (assume SEER of 9. Is this reasonable ?) Cost of electric is 12 cents per KW Hr.
If the unit is running continuously, what would the cost be per hour ?
Questions, and thoughts:
a. is it simply a matter of converting 3 tons to BTU/HR (6,000 BTU/HR), and converting this to KW HR (not sure how to) ?
b. Does the SEER rating, house characteristics, etc. come into play at all, if one assumes continuous operation ?
Seems to me, and I really don't know for sure, that the Tons of A/C being put out, and the electrical cost rate are really all one needs ? Is this so ?
Thanks, Bob
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• posted on April 28, 2005, 12:47 pm
Robert11 wrote:

Unless it was a "high efficiency" system, 9 SEER sounds a little high for mid-80's. 8.0 would be more likely, and even that is probably degraded by the wear and tear on the compressor valves, etc. In any case at 8.0 SEER, 3T (36K BTU) is 36,000/8 or 4.5KW/h. That should be close enough to give you an idea of operating costs.
HTH.
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• posted on April 28, 2005, 2:39 pm
Hi Travis:
Thanks for help; appreciate it.
On clarification, please, regarding the units:
Should, perhaps, the 4.5 KW/h you derived be 4.5 KW-h ?
Thanks, Bob
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• posted on April 28, 2005, 3:03 pm
Robert11 wrote:

I should have said 4.5 kWh.
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• posted on April 28, 2005, 5:15 pm

No. Just 4.5 kW.
Nick
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• posted on April 28, 2005, 5:42 pm
snipped-for-privacy@ece.villanova.edu wrote:

Right. I'm going to quit now while you are ahead.
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• posted on April 28, 2005, 2:59 pm

That's just kW, vs "KW/h," ie 4.5 kW of power, vs 9 kWh of energy if it runs 2 hours, but 8.0 may be the best case...
I actually measured 92 cfm of 45.3 F airflow cooled from 63.3 yesterday with 537 watts and calculated 1802 Btu/h of cooling with a 0.98 COP, vs the 9.7SEER/3.412 = 2.84 I expected from the \$98 Haier window AC box. I'll measure the heating COP today, and the \$69 "10.2 SEER" Daiwoo... Room ACs are tested according to DOE 10 CFR 430 Subpart B, Appendix F, at 26.7C [80 F] db/19.4C wb indoors and 35C [95 F] db/23.9 C wb outdoors. I suppose they are designed to meet those conditions vs my 63 F room temp and hot fin airflow restriction. This could heat water in summertime, but it doesn't seem likely to work all year in a basement, which may kill the water heating economics, where I live. More basement humidity might help, eg a damp basement floor. We might dampen the basement floor in wintertime and let it dry in summertime. We only need AC for 1-2 weeks/year.
A 50 F wet basement floor has Pw = e^(17.863-9621/(50+460)) = 0.367 "Hg, vs Pa = e^(17.863-9621/(40+460)) = 0.252" Hg near a 40 F AC fin, so a 1000 ft^2 basement might provide 100x1000(Pw-Pa) = 11.5K Btu/h of latent heat, according to one ASHRAE swimming pool formula, with a 100x0.252/0.367 = 69% basement RH.
I started measuring airflow by moving the Testo stick across a 2"x12" slot in a cardboard box that collected the cool air, but the readings bounced around a lot, even with no change in position, with 2:1 differences within the slot. Next I tried a \$400 Dwyer thermo-anemometer, which won't read as low as the Testo but has more damping. Then I foil-taped a 12"x4" to 6" round right-angle register boot to the cool outlet and added 5' of 6" pipe, which reduced the airflow swirling a lot, while warming it a little. There was some condensation on the outside of the pipe and more inside the AC, which ran out when I tipped it later. Counting that would improved the COP (as would disconnecting the fan motor.)
Nick
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• posted on May 4, 2005, 11:17 pm
Use EER not SEER
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• posted on April 28, 2005, 1:57 pm
An ac calculator..
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• posted on April 28, 2005, 7:56 pm
About \$15.00 day running blower and compressor and fans full time.
wrote:

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<%-name%>
• posted on April 28, 2005, 8:19 pm

Properly sized, there should be some cycling. Heat load during hte sunny part of the day if different than night time.