# BTU conversion and standard home outlets



## wahoowad (Jun 7, 2007)

I see Lowes sells a 10,000 BTU portable A/C unit that plugs into a standard 120V wall outlet. I believe a modern home wall outlet (and overall circuit) is rated for 20 amps and usually has a 20 amp breaker. Assuming my conversions and calculations are correct, how does a 10,000 BTU unit run off a 20 amp circuit?

a 10,000 btu a/c unit will (mathematically) use 25.4 amps if you use the 
conversion of 1000W = 3413 BTU. 

10,000 BTU * ((1000 W / 3413 BTU) / 115 V) = 25.4 amps 

That is more than any standard home outlet is rated for (or extension 
cord), yet I see Lowes sells 120V 10,000 BTU units? Are the BTU 
ratings BS? Am I using an incorrect conversion factor?


----------



## BrotherBart (Jun 7, 2007)

Lowe's ad for the Frigidaire 10,000 BTU window unit says it draws 8.4 amps.


----------



## wahoowad (Jun 7, 2007)

then that is only 3440 BTU's, not 10,000.

is it me or their marketing folks?


----------



## BrotherBart (Jun 7, 2007)

The BTU number is heat exchanged. Not generated like a stove. The air conditioner is moving heat from the inside to the outside. Not generating heat. Or generating cold for that matter. There actually is no such thing as cold. It is an absense of heat.


----------



## TMonter (Jun 7, 2007)

The AC unit is a heat pump in reverse. Like BB said it's moving the heat from inside the house to outside which you take a smaller penalty for. This is why heat pumps are more efficient than heating with a furnace, they take advantage of existing heat outside and move it inside your house.


----------



## webbie (Jun 7, 2007)

Exactly!

Air conditioners and heat pumps create much more energy than the electrical current that they use. It is a sort of magic, and evidence of the abundance of energy in the universe! The small amount that your air conditioner heats the outside could never, in any case, cause a problem with climate. So, in effect, you are using the giant "flywheel" effect of the relatively stable outdoor (or ground) temperatures.

The electric used is for compression of the refrigerant and fans, controls, etc.

As you know, technology has made these air conditioners and heat pumps vastly more efficient over the years. 
Given your figures, we can guess that your machine is 3X as efficient as the electric it takes on input, although I do get a bit confused with cooling BTU as opposed to heating ones.

SEER is best simply compared with other SEER - for instance, an air conditioner with a SEER of 13 is 30% more efficient than one of 10.

But the real number we are looking at is what is called COP - Coefficient of Performance. This is what you get out compared to what you put in, and a typical number is exactly as I mentioned - 3X as much out as in.

There is a relationship between SEER and COP, but COP depends more on where you live (humidity, temp, etc.)....

For discussion sake, it could be said that a SEER of 11 is a COP of 3 (3x) and a SEER of 16 could be as high as 4(4x) or more.

The highest SEER available now is about 23, which means it would use less than 1/2 the electric of a normal (10 SEER) window air conditioner to do that same job! When you look at numbers like this, it's easy to see what it is a joke when folks claim that conservation is not the best fuel source!


----------



## GVA (Jun 7, 2007)

A cool little calculator
http://www.mhi-inc.com/Converter/watt_calculator.htm
Yeah what they said :cheese: 

You can't go by the listed BTU value you must use what creates the transfer of energy, or the watts needed to run the compressor etc.  That is the wattage

Look at a hairdryer you will never find one over 1800 watts (anymore)
1800watts / 120volts = 15 amp
even though people have 20 amp recepticles, people still have 15 amp recepticles.  Unfortunatly most people don't know which one is which just by looking at them resulting in tripped breakers and overheated wiring.
Also people have been known to put a 20 amp rect on  15 amp circuit.
Wow i'm getting off topic so I'll stop..... :zip:


----------



## DiscoInferno (Jun 8, 2007)

TMonter said:
			
		

> The AC unit is a heat pump in reverse. Like BB said it's moving the heat from inside the house to outside which you take a smaller penalty for. This is why heat pumps are more efficient than heating with a furnace, that take advantage of existing heat outside and move it inside your house.



Heat pumps are fine for mild climates, but you won't find many up north.  The efficiency drops to 1 as the temperature differential rises, and so at cold temps you have an expensive resistive heater.  I don't know what the magic temperature is where they stop being effective, I've heard around freezing.  I never saw a heat pump until I left Michigan.


----------



## Sandor (Jun 8, 2007)

I just installed one of these in the womens house. It replaced an old Fridigaire slide in unit.

http://www.ajmadison.com/cgi-bin/ajmadison/AH093A35MA.html

It says maximum cooling watts is 990 at 4.6 amps. I suppose that is on high with a 9300 BTU capacity. Energy Star rated.

Ran this thing yesterday on the low setting and it had absolutely no problem keeping this 1400 sq/ft ranch at 75 degrees at 50 percent humidity. It was 90 outside.

Wahoo, even though your math is correct, it simply does not apply to heat pumps or ac units. ( Would work fine for electric baseboard units). As others have said, these devices move heat, they do not create it.

And BrotherBart, nice line about no-such-thing as cold, scientifically speaking. When I tell that to people, they look at me like an idiot. Then I tell them about Absolute Zero, then they kinda get it.


----------



## TMonter (Jun 8, 2007)

> Heat pumps are fine for mild climates, but you won’t find many up north.  The efficiency drops to 1 as the temperature differential rises, and so at cold temps you have an expensive resistive heater.  I don’t know what the magic temperature is where they stop being effective, I’ve heard around freezing.  I never saw a heat pump until I left Michigan.



You don't see many air-air ones, but I have seen a fair amount of ground source pumps up north.


----------



## DiscoInferno (Jun 8, 2007)

TMonter said:
			
		

> > Heat pumps are fine for mild climates, but you won’t find many up north.  The efficiency drops to 1 as the temperature differential rises, and so at cold temps you have an expensive resistive heater.  I don’t know what the magic temperature is where they stop being effective, I’ve heard around freezing.  I never saw a heat pump until I left Michigan.
> 
> 
> 
> You don't see many air-air ones, but I have seen a fair amount of ground source pumps up north.



Makes sense, although I didn't see any in the UP.  How deep do they go?  Frost depth in the UP is 4'+.


----------

