I have searched over the Net, but couldn't find what I have been
looking for. AFAIK, all AC systems are measured using BTUs (or a
variant of). Now, what BTU means? Here is the most common definition:
"amount of heat required to raise the temperature of one pound of
water by one degree Fahrenheit" (from Wikipedia). Yep, I understand
this and it is the most useful thing in the world. What does this mean
to me? Absolutely nothing. I understand that someone making ACs must
know this, so he can compare or test or whatever, but to a normal
user, this is absolutely useless. Are you going to cool your pool with
What the normal user might need is the answer to this question: How
much time do I need to cool my A x B sized room from X to Y degrees
Celsius? Yes, there are other things to worry about (for example:
building insulation, geographic location, north-south orientation,
etc.), but is there some rough measurement for a common domestic
building to be relatively sure that XXXX BTU AC will do it job or not.
I don't care if I miss for 10%, but I care if I don't know if I missed
at all (because I cannot "translate" the above BTU definition).
I didn't find something similar - all the calculators are saying: you
will "adequately" cool your XXX m2 room if your AC has this many BTUs.
What is adequately? Does it mean that a 30m2 room can be cooled from
30*C to 20*C in 20 minutes or 2 hours or 20 hours? Will it run for the
whole day if the outside temperature is 30*C or will it stop after 1
In fact, the real question would be - how BTUs (or, to be precise, day-
tons) project into "degrees Celsius per hour"? What is the margin that
one should be always holding in order for AC not to run the whole day,
but to run reasonably (e.g. 30% of the time - is 30% a good measure)?
If you have any good answers or links to sites explaining this, that
would help many people on this world, I presume.
kool, thanks for the link. The page is informative, but contrary to
what Noon-Air said later - this is rocket science, but shouldn't be. I
read some of the text on the page, but:
a) I couldn't find the answer to my question,
b) Even if there is an answer on that page, why is everyone talking
about BTUs, kJ, calories, etc. and not something much more familiar
like degrees per m2 per hour? Why don't we start expressing the speed
of our cars in light year/picosecond or measure time as "duration of
9,192,631,770 periods of the radiation corresponding to the transition
between the two hyperfine levels of the ground state of the
caesium-133 atom at zero kelvins" (which actually is the definition of
c) Again, even if there is an answer, it's hidden in tons of
specialized text - is there an FAQ, for God's sake? Come on, people,
we should be making our lives easier, not more complex. Whoever needs
to know of BTUs (e.g. HVAC people) - let them know. Anyone who doesn't
need - don't let them even near BTUs!
d) I spent 15 minutes just reading this. If I spend 15 minutes reading
all kinds of rocket-science-type texts, I'll probably die pretty soon.
Hey, I cannot be an expert for everything, so there are many things
pretty clear to someone from the field, but not to an average AC
buyer. Can anyone recommend a clear, average-human-readable answer or
text on this?
e) "OK, I'm stupid".
I don't think there is a more basic answer to your question or anything laid
out in layman's terms better than the one you just read. Read again about
sensible and latent heat and how temperature in degrees is qualitative vs.
BTU's which are quantitative. Or don't waste any more time worrying about it
and hire someone who knows.
"In fact, the real question would be - how BTUs (or, to be precise, day-
tons) project into "degrees Celsius per hour"?
Answer: They don't ... directly.The A/C system has to remove moisture
(latent heat) as well as reduce temperature (sensible heat).
Paul, I disagree that a-d are whining. I am trying to be ignorant
about all things I can, since trying to know everything in this world
just doesn't work. http://en.wikipedia.org/wiki/KISS_principle. I try
to concentrate on the things I need every day. I was trying to get the
answer from the people who know what they are doing. I assume people
on this group are - at least kool seems to be good at it and, at the
end, not trying to analyze/judge my personality, but answer the
question, which really is the right thing to do.
kool, thanks for the answer. You are right about hiring someone who
knows, but what "someone who knows" told me was - you have type A AC
system which is "this, this and this" and type B which is "that, that
and that" - choose (one is 2x more expensive, that is everything
meaningful I can see). Tried few of them. That's how it's where I
live. Wouldn't waste time at all if I could, but seems inevitable.
That's why I was looking for a simple transformation from things
commonly used to describe AC systems (BTUs, for example) to something
I (and all people I know) commonly use to describe their feeling about
how fast the room is getting cooled - degrees C per hour. I don't care
about being 100% precise or so - I just need enough info to make a
There must be a way, since HVAC people are doing it. The question is -
is it simple or complicated? If there is a simple method which makes <
10% error, that's good. From what you have said:
there doesn't seem to be. Thanks again, I'll consider the options.
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