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
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
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,
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),
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
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.
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.)
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