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Air conditioner specs: watts vs Btu/hr

by softport
Tags: btu or hr, conditioner, specs, watts
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softport
#1
Apr5-12, 10:41 AM
P: 13
Hello, I am confused about air conditioner Btu specs. When I try to
match the power input in watts, to power out in Btu/hr, the numbers
don't come out even close.

Example:
This 6000 Btu window unit has the following specs:
Volts 115
Amps 5.4

So the power input in watts is: 115 x 5.4 = 621w,
and 1w = 3.42 Btu/hr

The maximum cooling should be 621w x 3.42 = 2124 Btu/hr, not 6000.

I have tried this with a couple of different ac specs, and always come up
short: (0.3~0.4) x stated Btu spec. What am I missing/doing wrong?

Thanks!
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huntoon
#2
Apr5-12, 01:30 PM
P: 145
Found this:

"A conversion exists for BTU to watts, but it does not apply in this instance. Air conditioners are rated by how much heat they can remove from a space in one hour. This rating does not imply they operate at 100 percent efficiency. Power usage requires calculating watts from voltage and amperes. The number of BTUs an air conditioner moves will convert directly to watts if you multiply by 0.293071, but the result is not the same as the power in watts used by the air conditioner.

Read more: How Many Watts Does a 5,000-BTU Air Conditioner Use? | eHow.com http://www.ehow.com/how_8746259_many-5000btu-air-conditioner-use.html#ixzz1rByT12eE"
huntoon
#3
Apr5-12, 01:34 PM
P: 145
And this:

What is TON?

Tonnage is used to express the one hour cooling capacity of air conditioners. One ton is 12,000 Btu's of cooling/hour. Residential capacities are offered in one half ton increments between 1.5 tons up to 5.0 tons. On average, one ton will cool between 400-700 square feet in a residential application. Homes with poor insulation will reside on the lower end of this scale while homes with excellent insulation qualities will reside on the upper end.

Homes requiring more than 5 tons are "zoned", using two or more systems to meet the necessary capacity demand.

http://www.acdirect.com/hvaclingo.php

softport
#4
Apr6-12, 12:19 AM
P: 13
Air conditioner specs: watts vs Btu/hr

Quote Quote by huntoon View Post
Found this:

"A conversion exists for BTU to watts, but it does not apply in this instance. Air conditioners are rated by how much heat they can remove from a space in one hour. This rating does not imply they operate at 100 percent efficiency. Power usage requires calculating watts from voltage and amperes. The number of BTUs an air conditioner moves will convert directly to watts if you multiply by 0.293071, but the result is not the same as the power in watts used by the air conditioner.
Thanks huntoon, but according to the above you should get the following:

6000btu x 0.293071 = 1758 watts (!)

The cooling and efficiency specs get even worse if you include the power used
by the fans and electronics:

From the specs in the example link:
Total power 115v x 5.4A = 621W
Cooling watts = 560

Btu = 560 x (1/0.293071) = 1911 (specs: 6000)

EER = Btu/power = 1911/621 = 3.1 (specs: 10.7)
huntoon
#5
Apr6-12, 01:21 AM
P: 145
SOFTPORT:

Here is the money quote:

"A conversion exists for BTU to watts, but it does not apply in this instance. Air conditioners are rated by how much heat they can remove from a space in one hour. "


The industry has made a rather obscure standard that 12,000 BTUs are required for a 400-700 sq. ft. room.

It's not complicated. Just arbitrary.

Notice on you AC specification page it can cool a 200 sq. ft. room. Therefore, it gets a 6000 BTU rating...


That's my take anyway.
softport
#6
Apr6-12, 03:15 AM
P: 13
Thanks huntoon, that does clear thing up a bit. I didn't know
about the 12000 btu standard.

I did spend some time trying to understand the 'money quote', but
I interpreted it to mean that simply converting the Btu rating to watts
would underestimate the power consumed.

In light of the 12000 btu/(400-600ft) standard, is the last sentence
of the quote correct? From all the discussion so far, at least for the
small units, the rated Btu doesn't seem to correspond to how much
heat it can remove in an hour, which cannot be any higher than the
power input to the unit (minus fans and electronics).

A few days ago I was looking at power amplifier specs, and reading
reviews from customers that were very disappointed after buying a
cheap amp, based on the output wattage rating. It turned out that
the manufacturer was giving peak watts, not rms. Based on this, I
just now tried to convert the 6000 btu to rms (assuming that 6000 btu
is a peak value), and lo and behold I get 2121 Btu, and everything
seems to fall into place:

2121 Btu x 0.2931 w/btu = 621.7W = 115v * 5.4 amps

Not claiming a great discovery here, just that in this case the spec
numbers now fit together. Coincidence?

I'm not sure I used the correct formula for rms:

rms Btu = 6000/ ( 2 sqrt(2) ) = 2121
russ_watters
#7
Apr6-12, 06:30 AM
Mentor
P: 22,243
Quote Quote by softport View Post
From all the discussion so far, at least for the
small units, the rated Btu doesn't seem to correspond to how much
heat it can remove in an hour, which cannot be any higher than the
power input to the unit (minus fans and electronics).
This assumption forms the basis of your calculations and it is wrong. A typical air conditioner puts out about 3x as much "cooling" as it takes in in input electrical power. This is not an "efficiency" of >100%, it is a coefficient of performance and the two are not the same thing:
http://en.wikipedia.org/wiki/Coefficient_of_performance


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