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Technical Data Section 2

Chimney Sizing

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2/6Hoval Ltd. Northgate, Newark, Notts. NG24 1JN. Tel: (01636) 672711 Fax: (01636) 673532 e-mail: [email protected] web: www.hoval.co.uk

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Page

Contents Page 2

Basis Page 3General recommendation chimneys Page 3Flues Page 3Metal flues Page 3

Brick or concrete flues Page 3Ancillary equipment for chimneys and flues Page 3Rapid selection table (see also page 5) Page 3Outlet cones Page 3

Multi boiler installations Page 4

Maximum chimney heights Page 4Specific chimney calculations Page 4Calculation Page 4Procedure Page 4Chimney height in metres above boiler flue exit Page 4Maximum positive pressure Page 4All pressure jet models Page 4

Draught stabilisers Page 4

Gas fired pressurised combustion boilers operating with a fan dilution system Page 4

Rapid selection table Page 5Height for balanced draught at boiler flue outlet Page 5K Factors for flue fittings Page 5

Graph 1 Resistance of chimneys & flues at 205oC (smooth internal finishes e.g. metal) Page 6Graph 2 Flue gas velocity pressure at 205oC Page 6


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Hoval Ltd. Northgate, Newark, Notts. NG24 1JN. Tel: (01636) 672711 Fax: (01636) 673532 e-mail: [email protected] web: www.hoval.co.uk Rev 0Page 3

Most Hoval boilers have pressurisedcombustion chambers. All pressure

losses through burner and boiler are metby the burner fan, and normal efficientoperation does not depend on the chim-ney draught.

Where site conditions demand extendedflue systems, sizing data is needed. Asimple approach is outlined below.

All charts, graphs and tables are basedon the following:

Gross C.V.of oil 45.5 MJ/kgGross C.V. of natural gas 38.5 MJ/m3

Excess air 25%CO2 oil firing 12%CO2gas firing 9.5%Flue gas temperature 205oCFlue gas quantity:0.0671m3/s per 100 kW rating

All chimneys should be as slender asstrength considerations permit.Embellishments such as elaboratelydesigned copings at the chimney exitshould be avoided.

Considerable care is necessary wherechimneys for oil burning installations areto be used in conjunction with appli-ances burning other fuels or with incin-erators.Flanged joints in steel chimneys should

be tested to ensure they are gas-tightbefore they are covered with insulationor cladding.

Chimneys constructed of common redbuilding brick or concrete should alwaysbe lined internally. The lining may begunned solid insulating refractory, oracid-resisting brick depending upon re-quirements and temperature conditions.Metal liner inserts may also be usedwith this type of chimney.

The advice of specialist firms should besought before a decision is taken onthe most suitable type of lining.

To ensure the free flow of waste gases,

flues should be free from sharp turns orbends to the point of entry of the flue tothe chimney. Wherever possible the flueshould enter at an angle of 135o, ormore, to the vertical, terminating in aneasy bend in an upward direction. Toavoid turbulence and excessive heatloss, the chimney should be filled inbelow the lowest flue entry, though itmay be desirable to provide a small wellbelow the intersection to accommodatean inspection door.

Where more than one boiler or furnaceconnects to a chimney, a separate flueto the base of the stack is preferable toa common flue. Where a common flueis used, entries from individual boilersor furnaces should be inclined in the di-rection of gas travel and staggered. Incases where two or more flues arecoupled by a common breeching beforeentering the chimney, the breechingshould be designed to maintain a con-stant gas velocity by differential duct siz-ing.

Sudden expansions and contractions offlue should be avoided by the use of ta-pered transition pieces and all bendsshould be of the maximum practicableradius. The use of sharp bends or el-bows must be avoided, particularlywhere forced draught fans are used.

should be either cast-iron, mild steel or stainless steel. Such

chimneys should be well insulated toprevent condensation. Where flues en-ter brickwork or concrete, a mild steelsleeve ring should first be built in, witha clearance around the entering flue toallow for caulking with heat resistant ma-terial. The connection should not projectbeyond the internal face of the chim-ney.

should be lined with suitable insulating

bricks and should be adequately sup-ported to avoid movement due to ex-pansion, fracture or distortion.

Gas-tight cleaning doors of adequatesize should be provided at bends in theflue, at the end of horizontal flues andat the base of the chimney.

Pressure relief doors in addition to thaton the boiler smoke box may be re-quired in certain circumstances suchas long horizontal sections of flue. Fullguidance on this may be obtained fromBuilding Inspectors and the Fire Ser-vice.

This relates the boiler rating to the chim-ney size, and gives heights for the buoy-ancy of a chimney to overcome the re-sistance of the three flue systems de-scribed, to give a neutral condition atthe boiler outlet. A higher chimney wouldgive a negative pressure - a lower onewould give a positive pressure. Whereexcess fan pressure is available, lowerchimneys can be accepted, providingthat the loss of buoyancy does not ex-ceed the excess pressure available. Foreach metre reduction in chimney height,the approximate loss of buoyancy is 4N/m2.

Also shown in the table is the gas ve-locity at the base of the chimney.

With insulated chimneys, loss oftemperature will be between 0.5oC and1oC per metre so entry velocity is ap-

proximately equal to discharge veloc-ity.

If a higher efflux velocity is needed areduction cone can be fitted. Velocityand area are directly proportional.So the area of discharge at the coneexit =

Area of chimney x velocity in chimneyRequired velocity

To overcome the resistance of the cone,extra chimney height is needed - un-less this small extra resistance can bedealt with by the burner fan.



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Where two or more boilers connect to one

chimney or to a simple header, the rapidselection table opposite can be used byadding together the boiler ratings. In suchcases an efflux velocity of 5m/s should ap-ply with one boiler firing.The flue header should have a cross sec-tion area equal to the sum of the boileroutlet areas.

On very high chimneys consider whether(a) cooling of gases is not too great and

whether(b) natural draught variations will signifi-cantly affect combustion and justify modu-lating dampers.

The object in chimney design calculationsis to make sure the resistance of the fluesystem does not exceed the power avail-able.Discounting spare power from the burnerfan, it follows that:

Therefore:Chimney height in metres =

The resistance arises from five compo-nents:

1) Burner head.2) Boiler.3) Flue run to chimney.4) Chimney.5) Discharge from chimney outlet.

1) and 2) are amply covered by the burnerfan.3) For this resistance, add together theresistance of bends, junctions and fluepipe.4) For chimney resistance see graph 1.5) Discharge. Use velocity pressure fromgraph 2 multiplied by appropriate K. factor

as listed opposite.

The procedure is to calculate the resis-tance of the flue system connections,considering only the most distant boiler,and add this to the discharge resistance.With the sum total of the system resis-tance established, divide this by the nettchimney buoyancy per metre to obtain

the required chimney height in metres.

Maximum positive pressure at the boiler

outlet allowable with standard burners.

Resistance in N/m2

Buoyancy per metre in N/m2

By using nett buoyancy, the chimney re-sistance is already taken into account andmust not, therefore, be added to the total

of resistances.If the resultant chimney is too high, con-sider the following:- Improve layout of the flue system

(easy bend, swept junctions etc.)- Increasing area of flue connections.- Increasing chimney area.- Crediting the sum of resistances

with the excess burner fan poweravailable (see maximum positivepressures list).

Providing the total of the fan pressure plusthe chimney buoyancy exceeds the totalresistances, the system design will worksatisfactorily.However, a high positive pressure in theboiler flue requires extra care to ensurethat the complete chimney is gas tight withgood joint sealing.

Total sum of flue resistances Nett buoyancy per metre

1. Prepare diagram of flue system.Mark gas quantities in m3/sSectional area in m2

Velocity in m/sSelect velocities in chimneybetween 4.5 and 12 m/s.On multi-boiler installations avelocity of 6m/s is suggested.(Common flue area = sum ofboiler flue outlet areas or more).

2. Add up the resistances of the fluerun components and includedischarge resistance.

3. Add on the flue pipe resistance,which equals the pipe length xresistance /m (from graph 1).

4. Establish nett chimney buoyancy

from 5N/m2

per metre.(the flue gas buoyancy at 230oC)

If the chimney design requires a higherpositive pressure, consult Hoval Techni-cal Department which may be able to rec-ommend a burner with higher fan pres-sure.

On high efficiency forced draught burners,the presence of a draught stabiliser of theswinging flap type is undesirable for thefollowing reasons:1. It admits cold air which may result

in the gases reaching dew pointbefore they leave the chimney.

2. The gas volume is increased andchimney capacity thereby lowered.

3. On occasions positive pressureconditions may arise at the base ofthe stack.

Generally, draught stabilisers are not nec-essary for efficient boiler operation andshould not be fitted, except when a fan di-lution system is to be used and where thedraught from a tall chimney is greater thanthe boiler resistance leading to negative

pressure in the combustion chamber.

The dilution fan should be sized to the for-mula given on page 6 of data section 1.Air for dilution is drawn independently intothe dilution header from outside the boilerhouse.The diluted exhaust gases should be dis-charged from the same side of the boilerhouse as the air inlet to ensure wind pres-sures are balanced.To avoid excess suction from the dilutionfan acting on the boiler flue gas outlet, adraught stabiliser should be fitted in thevertical flue from the boiler to the dilutionheader.Boiler house ventilation and combustionair requirements should be as listed inTables 1a & 1b Pages 3 & 4 of Data sec-

tion 1.

=Chimney ChimneyBuoyancy Resistance

20kW - 150kW = zero175kW - 300kW = 25N/m2

325kW - 600kW = 30N/m2

625kW - 1450kW = 60N/m2

1500kW - 6000kW = 90N/m2


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Hoval Ltd. Northgate, Newark, Notts. NG24 1JN. Tel: (01636) 672711 Fax: (01636) 673532 e-mail: [email protected] web: www.hoval.co.uk Rev 0Page 5

1.Figures suitable for smooth boresteel flue pipes and chimneys whichare well insulated over full length.

2.Chimney buoyancy taken as 5 N/m2

per metre.

Resistance of fitting = K x Velocity Pressure (See Graph 2 overleaf).

The factors given are in common use and equal those advocated by various Design authorities. However,each factor can only relate to a specific configuration and some attempt should be made by the designerto sense the true situation and adjust the K figure accordingly.

Model Number &nominal output (kW)

Chimney / Flue i/d(mm)

Chimney height above boiler flue outlet (m) Flue gas velocity(m/s)

Type A Type B Type C

125

150

225

300

375

300

400

450

500

600

600

700

750

950

1000

1200

1300

1450

1400

1500

175

200

200

200

200

225

225

250

250

250

275

300

250

250

250

250

400

450

400

450

3.0

2.5

5.0

10.0

28.0

5.0

9.5

4.5

8.5

17.0

8.5

9.5

3.5

5.0

7.0

10.5

5.0

4.0

10.5

6.5

5.0

4.2

8.5

17.0

45.5

7.5

15.0

7.5

13.5

26.5

13.5

14.0

6.0

7.5

10.0

15.0

7.0

6.0

14.5

9.0

6.5

12.0

11.5

23.0

-

10.0

21.0

10.0

18.5

36.0

18.5

19.0

8.5

10.0

13.5

21.0

10.0

8.0

20.5

13.0

3.56

3.46

5.19

7.36

9.21

5.47

7.3

6.98

7.39

8.87

7.33

7.18

5.52

6.99

7.53

9.03

7.50

6.78

8.07

6.83

Type A90o BendEasy Sweep

Type B135oBend45o Junction4 metres connecting pipe

Type CAs B with the addition of2 x 90o Bends - Easy sweepDoes not allow for extra horizontalruns.

90obend

K = 1.2

90obend(easy)

K = 0.4

135obend(easy)

K = 0.3

branch(at 45o)

K = 0.5

discharge

K = 1.0

taperdischarge

K = 1.1

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1

1

1

1

1

1

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chimney entry

at 90oK=1.2at 60oK=0.7at 45oK=0.5

Note, if base ofchimney is leftopen doubleabove values.



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1. Calculations based on flue gastemperature of 205oC

2. Resistance = K x Velocity pressure

3. P = 175 x V2

273+tgWhere:

V = Flue gas velocitytg = Flue gas temperature0

2

4

6

8

10

12

14

16

0 10 20 30 40 50 60 70 80

Graph 2

FlueGasVelocityPressurem/s

Velocity Pressure in N/m2

Chimney Inside Diameter (mm)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

100 200 300 400 500 600 700 800

4m/s

5m/s

6m/s

7m/s

3m/s

8m/s

9m/s

10m/s

12m/s

ChimneyPres

sureDropN/m

PerMetre

2

Graph 1 for Steel

P at tg = V1.742 x 64 x 273+230 D1.266 273+tg

Where: V = Flue gas velocity (m/s) D = Flue diameter (mm) tg = Flue gas temperature (oC)

( )( )

500 800400300200100 700600

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

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