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#51995 - 05/17/05 09:34 AM Electric Heat - A.C. calculations
Yoopersup Offline

Registered: 03/04/03
Posts: 826
Loc: Michigan
Where can I find a quick reference Chart for square footage to find Watts needed to Heat and cool a small building 16x20x 8 ft ceilings in Northern area. well insulated

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#51996 - 05/17/05 10:53 AM Re: Electric Heat - A.C. calculations
gfretwell Offline


Registered: 07/20/04
Posts: 9023
Loc: Estero,Fl,usa
The right way is a manual J calculation but you can get a quick and dirty with this
Obviously if the inside to outside temp delta is huge you will need to adjust the number.

Home Type or Characteristics ..... Heat loss (Btu/hr/ft2)

1) No insulation in walls, ceilings, or floors; no storm windows; windows and doors fit loosely .... 90 to 110

2) R-11 insulation in walls and ceilings; no insulation in floors over crawl spaces; no storm windows; doors and windows fit fairly tight. ..... 50 to 70

3) R-19 insulation in walls, R-30 in ceilings, and R-11 in floors; tight-fitting storm windows or double pane windows. ..... 29 to 35

4) "Superinsulated" house with R-24 wall insulation, R-40 in ceilings, and R-19 in floor; tight-fitting storm windows or double pane windows; vapor barrier sealed carefully during construction. ..... 21 to 25

5) Earth-sheltered house with little exposure; well insulated. ..... 10 to 13

For example, if a home's energy-saving features are best described by #2, and the home has a heated space of 1,500 square feet (139.35 m2), then 1,500 ┬┤ 50 and 1,500 ┬┤ 70 is the heating load range. Roughly 75,000 to 105,000 Btu/hour
Greg Fretwell

#51997 - 05/17/05 02:10 PM Re: Electric Heat - A.C. calculations
jkochan Offline

Registered: 01/19/05
Posts: 61
Loc: Phoenix, AZ USA
Here's a link that might help:
Click Here

[This message has been edited by Roger (edited 05-18-2005).]

#51998 - 05/17/05 10:25 PM Re: Electric Heat - A.C. calculations
frank Offline

Registered: 05/29/01
Posts: 361
Loc: windsor ontario canada
I remember the old days when i was told to stand across the street on the oposite curb hold my hand out a foot from my body and raise my thumb.I your thumb blocks out the entire house it's a 1 ton,two thumbs is a two ton ect LOL.I was in that trade for 10 years and rarely seen load calculations done properly.It's very important that the a/c unit be slightly undersized rather than oversized as too large a unit will short cycle reducing life expetancy.Most electronic thermostats will force the a/c to run a minimum of 10 minutes and lock it out for 5 if someone fools with it for this reason.Most newer units with mechanicaly siezed compressors are oversized with cheap thermostats.

#51999 - 05/18/05 05:07 AM Re: Electric Heat - A.C. calculations
Alan Belson Offline

Registered: 03/23/05
Posts: 1801
Loc: Mayenne N. France
Wood work but can't!

#52000 - 05/18/05 05:58 AM Re: Electric Heat - A.C. calculations
Roger Offline

Registered: 05/18/02
Posts: 1779
Loc: N.C.
Alan, it was the long URL in jkochan's post.

BTW, jkochan you didn't do anything wrong.


#52001 - 05/18/05 12:47 PM Re: Electric Heat - A.C. calculations
Alan Belson Offline

Registered: 03/23/05
Posts: 1801
Loc: Mayenne N. France
There's only one way, that's to do some math,
which is easy enough.
Take 3800 btu as roughly = 1kW hour.

16' x 20' x 8' building, pitched roof, no ceiling- IE insulation in rafters....
Assume this is a timber structure with say glass wool or styrofoam infill, well sealed, with an insulated floor. Let floor be solid, with 2" styrofoam and walls + roof be say 6" glass wool or equivalent with vapor barriers-It's very important the insulation won't get wet.

Losses calculated as Watt losses, (do not confuse with BTU 'R' figuring, I assume you want electric heating?)
let roof = 6.0W per deg. F differential
let walls =10.0W "
let floor = 7.0W "
Note this floor is 2" of styrofoam equivalent + concrete as a solid floor,
OR to be as good as the walls if a basement is sited below, ie 6" of glass wool between the joists.
Calculation of solid floor losses is very complex and the above value is a good general figure based on UK Govt. tables.
Openings say 13% of the floor area, allowing 10% for a window (and a way to get in!)
Air changes, let's use one per hour.
Air-losses will be 20W per deg.F differential

Total losses = 43W per deg F differential

Let the comfort level be 70F
Winter, North, let min temp be 2F* (30 FROST)
= a differential of 68F
*This value, for your area, neeeds to be assessed.
Need 43 x 68F differential = 2.9kW HEAT.
Allow 20% additional safety factor,
say 3.5kW = 13334 btu.

Aircon: This is more difficult, as there are variables to consider, such as area of glass, aspect with regard to sun, materials, color, and intended use and traffic into the building increasing the air changes.
But lets say Summer temperature = 100F*
Let airchanges be 2 per hour= 40W/F diff.
= 30x 63 =1890W (7180 btu) to pump heat out.
This is NOT the compressor load as a heat pump is more than 100% "efficient".
There were recent posts on converting values to tons. I have no experience of air-con, (being british) so others will, I'm sure, advise.
I usually work in Metric, but was schooled in btu, sq ft, pounds etc. We had to give all that up and learn it all from scratch when we joined Europe.
Hope this helps,
Wood work but can't!


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