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How To Calculate Heat Loss For A Room:

The unit of heat is the British thermal unit (Btu). It is the amount of heat that can raise the temperature of one pound of water one degree Fahrenheit.

Heat is lost from a room through any area exposed to a lower temperature, and is also lost by heating the incoming air due to infiltration and ventilation. Different materials and different types of construction vary the rate of heat loss…

The usual heat losses are through walls, windows and doors, and in heating cold air due to infiltration and ventilations. The ceiling is another source of heat loss unless there are heated rooms above. Heat loss through the floor may be omitted when there is a heated basement located under it.

Since most houses have all the rooms heated, it is necessary to include only outside walls in the total wall area.

Heat Loss Per Cubic Foot Per Hour

Btu heat loss to incoming air by infiltration for various window construction and walls exposed

Weather Strip or Storm Sash

Table 1

Exposure

Yes

An outside wall of room

0.018

0.009

Two walls

0.027

0.014

Three walls

0.036

0.018

Entrance halls

0.036

0.018

Sun rooms

0.054

0.027

Heat Loss Per Square Foot Per Hour

BTU Heat Loss for Various Windows Btu per degree F

Windows And Doors

Table 2

Single Glass 1.13

With Storm Windows

0.75

Double glass with sealed space between

0.65

Doors with or without glass

Same as windows

Glass Block, 3-5/8” thick

. 49

Wall Construction

No Insulation

½” Batting

3-5/8” Batting

6” Batting

Framed-Siding or shingles, building paper, sheathing, plaster or wall board

0.26

0.20

0.08

0.045

(2x6 Studs)

Framed-Brick veneer, building paper, sheathing, plaster or wall board

0.28

0.21

0.08

8” Brick-1” furring, lath and plaster

0.32

0.22

8” Hollow tile or cinder block, 1” stucco finish

0.37

4” brick and 8” cinder block

0.34

12” concrete, plain

0.57

Ceiling Construction

Ceiling- ½” plaster board flooring on joists

0.25

0.10

0.07

0.05

Ceiling-lath and plastered unfloored

0.7

0.26

0.10

0.05

Example:

A single glass window is 3’-0” by 4’-0”, the outside temperature is 0F and the inside temperature is 70F…

How much heat is lost through the window?

Heat loss equals constant (glass rating in this case) x temp difference (F) x area (sq ft. LxW).

Heat Loss = 1.13 (single glass constant) x 70 (temp difference) x 12 (sq ft).

Heat Loss = 949.2 Btu

To take this one step further, Hot water heating (radiant heat) has a radiation factor between 150 to 240 Btu per hour per square radiator foot (Every manufacturer of water heaters and radiators will specify the Btu for their product). For convenience and for easy math, I will use 200 Btu per square foot of heat. As an aside, a solar heat collector does not produce high temperatures, so a solar system usually uses 150 Btu per square ft of heat.

So, using the example above, to determine the size of hot water radiation to offset the heat loss…

This formula is used: HL/R=radiator size.

HL= Heat Loss (949.2)

R = Radiation factor, (for easy math 200)

949.2 / 200 = 4.75 sq ft of radiator size to offset the heat loss.

Example for Radiator sizing for a room:

Heat loss for a dining room in a one-floor house. The room is 11’2” X 11’2”X 8’0”. There are two outside walls, each 11’2” X 8’0” and three windows, each 1’6”X 4’6”. The windows are double glass with sealed space between. The walls are frame with wood siding and 35/8”bats. The ceiling has 6” batt insulation. The temperature difference is 70 degrees.

Calculation

BTU Heat Loss

Dining Room

Room Dimensions 11’2” X 11’2” X 8’0”

(I don’t know about you, but , at this stage, armed with a calculator, I find converting feet to inches preferable, as I still haven’t figured out what 2” is converted to so my calculator can understand it so)…

Room Dimension is 134 x 134 x 96 = 1723776 divide that by 1276 (12 x 12 x 12) this will convert the inches to cubic feet.

Volume Cubic feet

997.5 X 0.014 ( outside wall with weather strip) X 70 (temp difference)

equals 977.5 rounding up…

978

Total Wall Area

(134 x 96) x 2 (Two walls) / 144 (12 x 12 this will give you square ft)

(178.66 For math purposes) 179

Less window and door areas

22.08 (L X W per window) X 0.65 (double glass w/sealed space) X 70 ( temperature difference

(1004.64) 1005

Net Wall area

(179 – 22.08 = 156.92 again round up) 157 x 0.08 (framed with 3-5/8 batting) x 70 = 879.2

879.2 880

Square feet of ceiling

125 (L X W 134 X 134 /144 ) x 0.05 (6” batting) x 70

= 437.5 438

Total …. 978 + 1005 + 880 + 438 = 3301 Btu heat loss

Square feet of radiation needed: 3301/200 = 16.5 or 17.

I hope this was informative for you…. For an added bonus, since we were speaking of hot water heaters above, I thought it would be nice to add the following.

Electric delivers 1000 Btu @ .08

1 gallon of water equals 8.33 pounds

Gas delivers 100,000 Btu @ 3.50

Oil delivers 140,000 Btu @ 4.00

To determine how much a water heater will cost, convert gallons to pounds X temp rise (for this discussion will be 40 degrees.) X cost (i.e. electric for this demonstration is 8 cents per 1000 Btu which is 1Kwh)

So a fifty-gallon water heater would be:

50 (gallons) X 8.33 (weight of water) = 416.5 pounds

416.5 (pounds) X 40 (water rise) = 16660 Btu s needed.

Electric 16660/1000 (Btu) = 16.66 X .08 (cost of electricity will vary in your area) = $1.33

Gas 16660/100,000 (gas Btu) = .1666 X .70 (cost of gas… you know the rest) = $.1166 or $.12

Oil 16660/140000 (oil Btu) = .119 X 1.00 (cost of oil… again you know the rest) = $.119 or $.12 .

For 100 gallons: 33320 Btu s needed

Electric: $2.67

Gas: $1.15

Oil: $.96

To be honest, a fifty gallon water heater would be overkill for both the gas and oil fired system, for a family of five… Think 40 gallon for gas, and 30 gallon for oil.

Finally:

To determine the volume of a half-sphere the formula is: V = 0.2618d (diameter of sphere, think dome) cubed (diameter multiplied together three times).

So, the volume of a forty foot dome (give or take) is 0.2618 x 64000 (40 x 40 x 40) = 16755.2 cubic feet.

Jack