How to calculate air-con BTU

[TKL]

Hi ,

Anyone know how to calculate total BTU require for a room space? Pls help with formula. Thanks

Teo

[Dave Harnish]

Hi Teo,

Here's the 'ballpark' chart I've used for window units for many years:

Hope that's of some help.

God bless,

Dave Harnish Dave's Repair Service New Albany, PA

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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!)

John 3:3

[Joseph Meehan]

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.

[Travis Jordan]

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.

[Ermalina]

Check out the links/downloads on this page:

[I've downloaded and used, a few years ago, the "Load Calculation Software" from Lennox."]

[nicksanspam]

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 w>Here's the 'ballpark' chart I've used for window units for many years:

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

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