Greenhouse Heat Loss Calculations

J. Raymond Kessler, Jr.

 

       The selection of the number of heaters and the capacity of the heaters for a greenhouse depends on estimating the expected heat loss from the structure. Heat exchange between a greenhouse interior and exterior is the sum of all sources of heat and the rate of heat loss. Sources of heat in a greenhouse include solar, heater, lighting, electric motor heat, and heat from specialized equipment such as carbon dioxide generators. Sources of heat loss from a greenhouse include conduction and convection of heat through the greenhouse glazing, heat loss by air exchange between the inside and outside air (usually infiltration through cracks in the structure), heat loss by evaporating water, and heat loss by thermal radiation.

 

Detached, Gable-roof Greenhouse

Example: Glass-covered, all metal frame greenhouse, 30 feet wide and 100 feet long. Curtain wall constructed of 4-inch concrete block, 2 feet tall. Glass wall above the curtain is 6 feet tall. The average wind velocity expected is 15 mph. A 60°F temperature difference is expected between the low outside (0°) and desired inside temperature (60°F). Actual wind speed and low outside temperature for your area can be obtained from the U.S. Weather Bureau.

 

1.    Set up the table below.

2.    In table 1, obtain the appropriate heat-loss value for both gables. For a 30-foot width, it is 18 mBTU/hr. Enter in the table below.

3.    In table 1, find the appropriate heat-loss value for both roofs. For a 30-foot width and 100 foot length, it is 266 mBTU/hr. Enter in the table below.

4.    Calculate the length of the glazed side wall perimeter, which is 100+30+100+30=260 feet. Find the heat-loss for the glazed side wall, 6 feet tall and 260 feet perimeter in table 2. Because there is no entry for 260 feet, look up the value for 200 feet (95 mBTU/hr) and 60 feet (28 mBTU/hr) and add them together, it is 123 mBTU/hr. Enter in the table below.

5.    Find the heat-loss for the 4-inch concrete block curtain wall, 2 feet tall and 260 feet perimeter in table 2. Because there is no entry for 260 feet, look up the value for 200 feet (32 mBTU/hr) and 60 feet (9 mBTU/hr) and add them together, it is 41 mBTU/hr. Enter in the table below.

6.    Determine the climate factor (K) for your area. In the example, in table 3 at a 15 mph wind velocity and 60°F temperature difference, K=0.84. Enter in the table below.

7.    Determine the glazing construction factor (C) for the type of greenhouse. In the example, in table 4 for a glass-covered, all metal frame greenhouse, C=1.08. Enter in the table below.

8.    Determine the non-transparent curtain-wall construction factor (CW) for the type of greenhouse. In the example, in table 5 for a 4-inch concrete block curtain wall, CW=0.64. Enter in the table below.

9.    Determine the heat loss for each portion of the greenhouse by multiplying the standard heat loss by the climate factor (K) and the construction factor (C or CW).

10.  Add the 4 heat loss values together to arrive at the total heat loss per hour from the greenhouse. The heating system must deliver at least 391, 272 BTU/hr to maintain a minimum 60°F at an outside temperature of 0°F and wind velocity of 15 mph.

 

Greenhouse

Component

Standard heat loss (mBTU/hr)

Climate factor

(K)

Construction factor

( C or CW)

Heat loss (mBTH/hr)

Gable

18

×

0.84

×

1.08

 =

16.330

Roof

266

×

0.84

×

1.08

 =

241.315

Wall (glazed)

123

×

0.84

×

1.08

 =

111.586

Curtain wall

41

×

0.84

×

0.64

 =

22.042

 

 

 

 

Total heat loss

 =

391.273

 

Detached, Quonset-roof Greenhouse

       Determination of heat loss from a Quonset-style greenhouse requires some modifications because of the difference in shape. Quonset-style greenhouses may be covered with polyethylene film plastic, fiberglass reinforced panels or polycarbonate. A curtain wall is rarely used. Two components are considered in the heat loss calculations: 1) the two ends collectively, and 2) the roof covering that extends for the length of the greenhouse ground-to-ground.

 

Example: Metal-frame, Quonset greenhouse that is 30 feet wide and 100 feet long covered with two layers of polyethylene, each with a covering width of 40 feet. A temperature difference of 60°F and an average wind velocity of 15 mph are expected.

 

1.    Set up the table below.

2.    In table 6, obtain the appropriate heat-loss value for both ends. For a 40-foot covering width, it is 40 mBTU/hr. Enter in the table below.

3.    In table 6, obtain the appropriate heat-loss value for the ground-to-ground roof covering. For a 40-foot covering and 100 foot length, it is 316 mBTU/hr. Enter in the table below.

4.    Determine the climate factor (K) for your area. In the example, in table 3 at a 15 mph wind velocity and 60°F temperature difference, K=0.84. Enter in the table below.

5.    Determine the glazing construction factor (C) for the type of greenhouse. In the example, in table 4 for a double layer, metal frame greenhouse, C=0.70. Enter in the table below.

6.    Determine the heat loss for each portion of the greenhouse by multiplying the standard heat loss by the climate factor (K) and the construction factor (C).

7.    Add the 2 heat loss values together to arrive at the total heat loss per hour from the greenhouse. The heating system must deliver at least 209,328 BTU/hr to maintain a minimum 60°F at an outside temperature of 0°F and wind velocity of 15 mph.

 

Greenhouse

Component

Standard heat loss (mBTU/hr)

Climate factor

(K)

Construction factor

( C)

Heat loss (mBTH/hr)

Both ends

40

×

0.84

×

0.70

 =

23.520

Covering

316

×

0.84

×

0.70

 =

185.808

 

 

 

 

Total heat loss

 =

209.328

 

Gutter-Connected Greenhouse

       Heat loss calculations for gutter-connected greenhouses are simple continuations of the procedures for detached gable- or Quonset-roof greenhouses. Three components are considered in the heat loss calculations: 1) the ends or gables, 2) the roof or covering, and 3) the walls. The wall height is the distance from the ground to the gutter, and the wall length is the perimeter of the greenhouse. The heat-loss calculated for one roof or covering is multiplied by the number of bays. Then multiply the heat-loss calculated for one end or gable by the number of bays.

 

Example: Gutter-connected, Quonset-style greenhouse with 4 bays each measuring 20 feet wide, 5 feet tall, and 100 feet long. The walls are 12 feet high, and the covering width is 24 feet. The greenhouse frame is metal, and the entire greenhouse is covered with double-layer polyethylene. A temperature difference of 65°F and a wind velocity of 20 mph are expected.

 

1.    Set up the table below.

2.    Calculate the length of the side wall perimeter, which is 100+(20×4)+100+(20×4)=360 feet. Find the heat-loss for the side wall, 12 feet tall and 360 feet perimeter in table 2. Because there is no entry for 360 feet, look up the value for 300 feet (288 mBTU/hr) and 60 feet (58 mBTU/hr) and add them together, it is 346 mBTU/hr. Enter in the table below.

3.    Determine the heat-loss from the 4 combined roofs. In table 6, obtain the appropriate heat-loss value for the roof covering. For a 24-foot covering and 100 foot length, it is 190 mBTU/hr. Multiply this by 4, it is 760 mBTU/hr. Enter in the table below.

4.    Determine the heat-loss from the 8 combined ends. In table 6, obtain the appropriate heat-loss value for both ends of a bay. For a 24-foot covering width, it is 15 mBTU/hr. Multiply this by 4, it is 60 mBTU/hr. Enter in the table below.

5.    Determine the climate factor (K) for your area. In the example, in table 3 at a 20 mph wind velocity and 65°F temperature difference, K=0.96. Enter in the table below.

6.    Determine the glazing construction factor (C) for the type of greenhouse. In the example, in table 4 for a double layer, metal frame greenhouse, C=0.70. Enter in the table below.

 

Greenhouse

Component

Standard heat loss (mBTU/hr)

Climate factor

(K)

Construction factor

( C)

Heat loss (mBTH/hr)

Walls

346

×

0.96

×

0.70

 =

232.512

Roof

760

×

0.96

×

0.70

 =

510.720

Ends

60

×

0.96

×

0.70

 =

40.32

 

 

 

 

Total heat loss

 =

783.552

 


Table 1. Standard heat-loss values for gables and roofs of a gable-roof greenhouse





Greenhouse

Length

in feet

Greenhouse width in feet

16

18

20

22

24

26

28

30

32

34

36

38

40

50

60

Gable loss (both) in mBTU/hr

5

6

8

10

11

13

15

18

20

23

26

29

32

50

72

Roof loss (both) in mBTU/hr

5

7

8

9

10

11

12

12

13

14

15

16

17

18

22

26

10

14

16

18

19

21

23

25

27

28

30

32

34

35

45

54

20

28

32

35

39

42

46

50

53

57

60

64

67

71

88

106

30

42

48

53

58

64

69

74

80

85

90

96

101

106

133

160

40

57

64

71

78

85

92

99

106

113

120

127

135

142

177

212

50

71

80

89

97

106

115

124

133

142

151

159

168

177

222

266

60

85

96

106

117

127

138

149

159

170

181

191

202

212

265

318

70

99

112

124

136

149

161

173

186

198

211

223

235

248

310

372

80

113

127

142

156

170

184

198

212

227

241

255

269

283

354

424

90

127

143

159

175

191

207

223

239

255

271

287

303

319

398

478

100

142

159

177

195

212

230

248

266

283

301

319

336

354

443

532

200

283

319

354

390

425

460

496

531

567

602

637

673

708

885

1,062

300

425

478

531

584

637

690

743

797

850

903

956

1,009

1,062

1,328

1,594

400

566

637

708

779

850

920

991

1,062

1,133

1,204

1,274

1,345

1,416

1,770

2,124

500

708

797

885

974

1,062

1,150

1,239

1,328

1,417

1,505

1,593

1,682

1,770

2,213

2,666

 

Table 2. Standard heat-loss values for greenhouse walls


Wall length in feet

Wall height in feet

2

4

6

8

10

12

14

Wall loss in mBTU/hr

5

1

2

2

3

4

5

6

10

2

3

5

6

8

10

11

20

3

6

9

13

16

19

22

30

5

9

14

19

24

29

34

40

6

13

19

26

32

38

45

50

8

16

24

32

40

48

56

60

9

19

28

38

47

58

67

70

11

22

33

44

55

67

78

80

13

25

38

51

63

77

90

90

14

28

43

58

71

86

101

100

16

32

47

64

79

96

112

200

32

63

95

128

158

192

224

300

47

95

142

192

237

288

336

400

63

127

190

256

316

384

448

500

79

158

237

320

395

480

560

 

 

 

Table 3. Climate factors (K) for various average wind velocity and temperature conditions

Inside to outside temperature difference (°F)

Wind velocity in mph

15

20

25

30

35

30

.41

.43

.46

.48

.50

35

.48

.50

.53

.55

.57

40

.55

.57

.60

.62

.64

45

.62

.65

.67

.70

.72

50

.69

.72

.74

.77

.80

55

.77

.80

.83

.86

.89

60

.84

.88

.91

.94

.98

65

.92

.96

.99

1.03

1.07

70

1.00

1.04

1.08

1.12

1.16

75

1.08

1.12

1.17

1.21

1.25

80

1.16

1.21

1.26

1.30

1.35

85

1.25

1.30

1.35

1.40

1.45

90

1.33

1.38

1.44

1.49

1.54

 


Table 4. Greenhouse construction factors (C) for common types of greenhouses

Type of greenhouse

C

All metal frame (tight glass house—20- or 24 inch glass width)

1.08

Wood and steel (tight glass house—16- or 20 inch glass width—metal gutters, vents, headers, etc.)

1.05

Wood house (glass with wood bars, gutters, vents, etc.—up to and including 20-inch glass width

 

 

Good tight house

1.00

 

Fairly tight house

1.13

 

Loose house

1.25

FRP-covered wood house

.95

FRP-covered metal house

1.00

Double glass house with 1-inch air space

.70

Plastic covered metal house (single layer)

1.00

Plastic covered metal house (double layer)

.70

 

 

 

 

Table 5. Curtain-wall construction factor (CW) for various types of non-transparent materials

Type of material

CW

Glass

1.13

Asbestos-cement

1.15

Concrete, 4-inch

.78

Concrete, 8-inch

.48

Concrete block, 4-inch

.64

Concrete block, 4-inch

.51

 


Table 6. Standard heat-loss values for Quonset-style greenhouses for the combined ends and the entire covering along the length of the greenhouse





Greenhouse

Length

in feet

Covering width in feet

18

20

22

24

26

28

30

32

34

36

38

40

End loss (both) in mBTU/hr

8

10

12

15

17

20

23

26

29

33

36

40

Roof covering loss in mBTU/hr

5

7

8

9

9

10

11

12

13

13

14

15

16

10

14

16

17

19

21

22

24

25

27

28

30

32

20

28

32

35

39

41

44

47

51

54

57

60

63

30

43

47

52

57

62

66

71

76

81

85

90

95

40

57

63

70

76

82

89

95

101

103

114

120

127

50

71

79

87

95

103

111

119

127

134

142

150

158

60

85

95

104

114

123

133

142

152

161

171

180

190

70

100

111

122

133

144

155

166

177

188

199

211

222

80

114

127

139

152

164

177

190

202

215

228

240

253

90

128

142

157

171

185

199

214

228

242

256

271

285

100

142

158

174

190

206

221

237

253

269

285

301

316

200

285

316

348

380

411

443

475

506

538

570

601

633

300

427

475

522

569

617

664

712

759

807

854

902

949

400

570

633

696

759

822

886

949

1,012

1,075

1,139

1,202

1,265

500

712

791

870

949

1,028

1,107

1,187

1,265

1,345

1,424

1,503

1,582