The best way to know what it will cost is to run it during a couple of hot
days and then project the run-time over the number of estimated days you
will be using it.
If you are in a high humidity area, the run-time will be a lot longer until
all the latent moisture is removed from the interior materials in your home.
Also, " high rates of air infiltration will cost you plenty "in a high
humidity climate" &/or hot climate." Check the insulation, windows, etc.
Electric power companies will sometimes do a heatload and recommend measures
to reduce A/C operating costs, (Contact your Power Co., and ask!). Do what
you can before you run the old power guzzler!

Considering the amp draw of that old unit and the indoor blower, --you do
NOT want continuous operation even during the daytime! Do everything you can
to reduce the heatload and increase the efficient operation of the system.
(That is usually, a lot!) (Read, Study, and Learn all you can, it can pay
big dividends.)

Before you ever use the system, it would be wise to brush up on a number of
things that can have a large impact on the amount of run time to cool and
dehumidify your home.
The ductwork and especially the Return Air system needs checking. You would
be wise to call the best tech in your area to check everything out, but
first learn what all needs to be thoroughly checked and asked him/her how
they check those things! Read and study and apply what you learn to save on
operating costs.
- udarrell - Darrell
--
Air-Conditioning Manufacturing Companies - Adding Marketing Values to A/C
Equipment
http://www.udarrell.com/aircondition...companies.html
===============================================
"Robert11" wrote in message
...
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 insulation 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 (=36,000 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