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Thread: BTU's per square feet
06-01-2012, 02:32 AM #1Regular Guest
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- May 2012
BTU's per square feet
I know theres alot to consider (light,windows, ect) but in general what would u use?
06-01-2012, 03:43 PM #2
500 sq.ft. per ton
2000 sqft house 4 ton unit. In most cases after you do a study you will fall somewhere more or less close to that.
Did i just hear an ME curse?
06-01-2012, 04:49 PM #3
06-01-2012, 05:38 PM #4
I'm in a 2400 sq foot split level with a 35 yr old 8 SEER unit and I'd say it's sized just right.
By your R of T I'd have a 5 T unit in here which would blow my ductwork to bits.Question authority!
06-02-2012, 01:53 PM #5
06-01-2012, 05:52 PM #6
06-01-2012, 06:09 PM #7
You can take the same house and put it on 3 different lots facing different directions and you'll need 3 different sized A/Cs. Take the same house and move it north or south and you'll change the load some more. A rule of thumb gets you nowhere accurately. Better off standing across the street, holding you hand up and the number of fingers it takes to cover the house is the number of tons it needs
06-01-2012, 07:03 PM #8Regular Guest
- Join Date
- May 2012
rule of thumb
my boss lives by rule of thumb, thanks all you more "seasoned" techs for helping us newer ones know the right way. hacks come from bad teachers
06-01-2012, 07:56 PM #9
If someone, said hey i got a 2000sqft house, sits on a lot no shade, bad windows etc. built 1980.. I would tell them 4 ton for budget purposes. Of course you will need to do a study. But that was not his question.
If you told someone a budget # to install a 4 ton unit. regardless if they needed a 3 ton or 4 ton how much cost difference is there? not a whole heck of a lot. If they can't afford the 4 ton they sure as heck cant pay the 3 ton cost. Unless you pay a hack !
yes I'm old school. But people will waste your time, ask for quotes when they cant afford to install anything. When i give a rule of thumb based on their description and they don't cry at the cost. I then say would say "but lets do a study and see what you really need. could be more or could be less.
Everyone here has good points, but a rule of thumb has it place and will never go away. Sometimes that's called experience.
the next time you find yourself asking someone "hey what do you think that would cost" you will get a rule of thumb answer and you all done that!
when someone comes up to you with a quote to install an AC system you may say , yeah sounds right, or wow sounds high! you just gave your rule of thumb thought...
06-01-2012, 09:10 PM #10
Living room, family room, dining room, kitchen have a 100K 80 and 2.5 ton A/C. Calcs say with this insulation 60K furnace and 1.5 ton A/C
3 bedrooms and 2 baths, about 700 sq ft with no east or west glass and no doors, have 80K 60% and 2 ton A/C. Calcs say 40K furnace and 1 ton A/C.
So for this 2000 sq ft, they have 180K heat and 4.5 tons A/C where they need less than 100K heat and 3 tons cooling. With insulation added to ceiling, the whole house is barely over 1.5 tons A/C and they have 4.5 tons!
Oversized equipment isn't about affording, it's about comfort and efficiency. Short cycling equipment isn't efficient. Oversized A/Cs don't dehumidify and with today's stuff, that's even more of a problem. The 70s relics she has had a low sensible heat ratio and got to a cold coil faster than the high efficiency stuff of today.
06-01-2012, 11:10 PM #11
The point I was trying to make, which I failed to do by saying "OLD SCHOOL", was that a statement of "500 sqft/ton" is just as much of a WAG, as is "you have a 3 ton now, lets put in another 3 ton unit", or "from my vast experience we usually install this size unit for this type of home".
I just feel that giving the OP any answer other then "do a manual J", just encourages these Old School "rules of thumb" for sizing equipment.Instead of learning the tricks of the trade, learn the trade.
06-04-2012, 01:04 PM #12
This is what I got from ASHRAE Principles of HVAC 1994 (attached is a longer list of buildings).
Cooling: 17 BTUH/Sq. FT
Heating: 3.0 BTUH/Sq. FT
06-04-2012, 01:50 PM #13
Thats what im Talking about
I know it don't take much to do a calcs when the job is yours. But, how many times are you going to do a calculation or give a detail estimate than have the customer turn around and handed to the next guy, before you start doing round about estimates?.. call me when your serious...