# How to calculate air-con BTU

• posted on May 13, 2004, 4:01 pm
Hi ,
Anyone know how to calculate total BTU require for a room space? Pls help with formula. Thanks
Teo

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• posted on May 13, 2004, 4:50 pm
Hi Teo,
Here's the 'ballpark' chart I've used for window units for many years:
http://www.DavesRepair.com/DIYhelp/DIYacsizingchart.htm
Hope that's of some help.
God bless,
Dave Harnish Dave's Repair Service New Albany, PA www.DavesRepair.com snipped-for-privacy@sosbbs.com 570-363-2404
I'm a 32-year pro appliance technician, and love sharing what I've learned - in a FREE Monthly Appliance Tips Newsletter. (Back issues now posted here too!) www.DavesRepair.com
John 3:3

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• posted on May 14, 2004, 9:17 pm

Assuming the windows are shaded, divide their area in ft^2 by their US R-value (or multiply by the U-value) to find the window conductance then add the exposed non-window wall area divided by its R-value, then add the air leakage in cfm to find an approximate thermal conductance G = Awindow/Rwindow + Awall/Rwall + cfm in Btu/h-F. A room with volume V and an air leakage rate of N Air Changes per Hour (ACH) has an air leakage rate of NV/60 cfm.
Multiply G by the indoor-outdoor temp diff to find a basic AC load in Btu/h.
Add solar heat (about 200 Btu/h-ft^2) if some windows are unshaded, and add heat gain (3.41W) if the room contains W watts of electrical stuff.

http://www.DavesRepair.com/DIYhelp/DIYacsizingchart.htm
which says a 400 ft^2 room that's occupied upstairs needs 7500-8000 Btu/h.
Consider a 400 ft^2 room in Minneapolis (NREL record max temp 84 F) and another in Phoenix (NREL record max temp 122 F), in a fixed font:
Minneapolis Phoenix
Awindow 32 64 ft^2 Rwindow R4 R2 Gwindow 8 32 Btu/h-F dimensions 20'x20' 10'x40' exposed walls 2 4 Awall 288 736 ft^2 Rwall R30 R20 Gwall 10 37 Btu/h-F ACH 0.5 2 cfm 27 107 G 45 176 Btu/h-F Tout 84 122 F Tin 75 70 F Basic AC load 405 9152 Btu/h
unshaded window area 0 32 ft^2 sun 0 6400 Btu/h electrical power 10 1000 watts heat gain 34 3410 Btu/h Total AC load 439 18962 Btu/h
It looks like the chart is accurate to within a factor of 43, if we ignore dehumidification :-)
Nick
Need an answer or a method for figuring which window, insulation, heating system, etc to use? Ask us.
Join solar guru Steve Baer and PE Drew Gillett and PhD Rich Komp and me for an all-day workshop on solar house heating and natural cooling strategies ("HVAC Nonsense") on July 9 in Portland, OR--see page 25 of http://www.ases.org/conferences/2004_call_for_papers/SOLAR2004_prelim_program.pdf

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• posted on May 13, 2004, 5:56 pm
TKL wrote:

The best you can do is a ballpark number. There are far too many factors to take into consideration to do it by room size.
If you are buying a one room A/C it might be OK to use ballpark. However if you are doing the whole house, get it done right. Have a professional come out and do the measurements and plug then into the formulas to give you an accurate answer. It will be cheaper in the long run.
--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math

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• posted on May 13, 2004, 11:17 pm
TKL wrote:

ACCA's MANUAL J is the most common method used to determine BTU load (and thus, the number of 'tons' of cooling and heating required) for residential buildings.

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• posted on May 14, 2004, 4:59 am
TKL wrote: