HEATING SYSTEM DESIGN.doc

8/10/2019 HEATING SYSTEM DESIGN.doc

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Title HEATING SYSTEM DESIGNDescription A basic description of heating system installationsAuthor Mrennan

!rganisation "esearch Machines plc#

HEATING SYSTEM DESIGN

There are many diferent wet system designs; they can begrouped into two main categories, either;

- Open Vented Systems- Sealed Systems

There are also many diferent types o system pipe design to beconsidered such as;

- Two-pipe- Single-pipe- Gravity- Microbore

Open Vent Systems

This is the most common type o system that has been installed overrecent years within the Industry. It is called Open due to the act that thewater circulated through the system radiators and cylinder calori!er isopen to atmosphere using an open saety vent pipe. The e"pansion andcontraction o this water due to heating and cooling is accommodated bya eed and e"pansion cistern, and water is supplied to the system by theuse o a #eed and $"pansion pipe %cold eed&.
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Open Safety Vent pipe

The 'ent pipe is a saety device installed to the (entral )eating *ystem. Itenables any steam or high temperature water to be removed rom theheat e"changer in the event o boiler controls ailure. The 'ent pipe isgenerally installed to the main )eating +ow, or directly to the boiler. Thepipe should rise continuously to discharge above the #eed and $"pansioncistern. There should be no valves installed, either manual or automatic inthe route o the vent pipe.ith the introduction o more powerul pumps, and the increase inresistance in central heating circuits, due to high resistance boilers andradiators, the correct positioning o the vent pipe is essential.

It is important to avoid pressures below atmospheric which may draw inair through the open vent pipe, or small lea-s within the system, such asthrough valve pac-ing which may be watertight but not airtight.It is also important to avoid a rise in water level in the vent pipe such thatwater is orced over into the eed and e"pansion cistern thus aerating thewater in the system. The ingress o air will eventually cause the corrosionprocess to accelerate, leading to sludge being ormed and subseuentbloc-ages and restrictions within the system or corrosion o heat emitters.

Feed and Expansion Pipe (Cod Feed !

This describes the dual unctions o this pipe are to eed water down intothe system to permit water on heating up, to e"pand into the cisternabove. 'ery oten this pipe is reerred to as the cold eed pipe, even insome /ritish *tandards. This is a poor description as its main unction is tohandle the e"pansion o heated water within a system. Its main tas- is todeal, thereore, with the results o heat being applied to water. It also hasto assist the saety vent pipe in the event o overheats within the heate"changer. In such situations it is vitally important or the eed ande"pansion pipe to -eep the boiler ull o water. (ommonly a 0oc-shield

gate valve is installed, generally ound at the connection to the heatingsystem on the return pipewor- within the boilerhouse plantroom.


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Feed and Expansion Ciste"n

This cistern is connected to the mains water supply through a balloperated +oat valve and replenishes a system with water ater installationor remedial wor- being carried out. 1nother important unction o thecistern is to accept the e"panded heated water within the system. It is oneo the most neglected and yet one o the most important pieces oeuipment in the system. The common components ound with the cisternare the Open *aety 'ent 2ipe, /all operated +oat valve, #eed and$"pansion pipe, and an Over+ow arning pipe.

Seaed Systems

The *ealed system is not open to the atmosphere and incorporates asealed e"pansion vessel which replaces the eed and e"pansion cistern othe open vent system. $"pansion and contraction o the system water isaccommodated within this vessel. 1 pressure relie valve is installed as asaety eature similar to a open saety vent pipe.


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The system provides a number o bene!ts;

- The system is sealed to atmosphere, therefore corrosion isreduced within the installation

- System can run at a higher temperature than an opensystem

- No feed and epansion cistern is re!uired

In the open system the eed and vent arrangement protects the system inthe event o boiler ailure. The sealed system is protected by an overheatthermostat on the boiler and a separate pressure relie valve. Only boilersspeci!cally intended or use with a sealed system should be installed,these will contain an appropriate high temperature cut3out as standard

Expansion Vesse

The e"pansion vessel should be !tted on the suction side o the pump.The point at which it is !tted becomes the neutral point o the system.


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The e"pansion vessel is divided into two compartments and separated bya +e"ible diaphragm. The sealed side is charged with nitrogen gas or air.The open side is connected to the system. 1s the water is heated ite"pands and compresses the nitrogenair until the e"pansion isaccommodated within the e"pansion vessel.The diagrams above indicate the e"pansion vessel in a cold state %1&when the temperature increases %/& and !nally when at system operatingtemperature %(&. 1t this point the pressure in the system has risen due tothe e"pansion o the system water, compressing the gasair until ulle"pansion has ta-en place, at this point the diaphragm will become

virtually static, with increased pressure, but eual on both sides o thediaphragm.

P"ess#"e $eief Va%e

1 saety pressure relie valve must be !tted either on the appliance or onthe +ow pipe. The saety valve must open when the system pressureincreases above the set system pressure and discharge in a sae manner.There should be a discharge pipe to low level in the boilerhouse plantroom, and on combination boilers the discharge will be to a saeposition outside the property. The discharge pipe should always terminatein a downward direction to avoid ris-s to people in the area.2ressure relie valves are oten used to drain the system or areas in theplantroom, this is a practice not recommended, as the seating o the valvecan become contaminated with any debris rom within the system.

P"ess#"e Ga#&e


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1 pressure gauge recording the pressure o the system is reuired toenable the system to be charged rom cold to its design pressure. Thisgauge is also useul when establishing whether the system is losingpressure, thereore indicating a water lea-.

Ci"'#atin& P#mp

In modern systems, the pump provides the motive orce behind thecirculation o water within the heating system. The position o the pump iscrucial to whether the system operates under negative or positivepressure. In open systems the pump is installed on the +ow pipe, eitherater the open vent pipe and cold eed or where the open vent and coldeed are connected to the boiler, the pump would be installed on the +owpipe rom the boiler. This allows the system to operate under positive

pressure. The pump head is also a most important actor in the design oan open central heating system. 2umps are ad4ustable having generallythe option o difering speed settings and controls which enables theinstaller to ad4ust and vary the delivery o the head o water within thesystem. The pump will reuire ad4usting to provide su5cient pressure and+ow rate to the inde" radiator to achieve the reuired temperaturediferential across the +ow and return connections o the radiator. This isachieved by the altering the pump speed. hen a system is beingcommissioned the pump speed is set to give the correct pressure %head&and +ow rate o water. 6omestic 2umps usually have several speeds%oten 7 speeds& and can easily be set by a controller attached to the

motor. 'ariable speed drives are used or larger 73phase electric motordriven pumps.


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Index Ci"'#it and aan'in& t)e System

The resistance within the +ow pipe to each heat emitter will varydepending on the length and route o the pipe design. The radiator withthe greatest resistance will normally be the urthest rom the boiler,although on some modern installations circuits o less length have moreresistance due to a greater number o pipe runs and emitters. The radiatorwhen established id thereore -nown as the inde" radiator. The radiatorshould be balanced at the loc-shield valve on the return connection o theheat emitter to achieve an appro"imate 889( drop in temperaturebetween the +ow and return connections. 1ll other radiators on the circuitwill have less resistance than the inde" radiator. This means thatadditional resistance has to be created by ad4usting the loc-shield valveson the emitters to gain the reuired temperature drop between theconnections. I no attempt is made to balance the system, poorperormance can result as the heated water will only circulate through thecircuit with the lowest resistance. This may lead to the emitters at the endo the circuit not reaching the desired temperature.


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The Inde" (ircuitneeds to be identi!ed so that the pumpcan be si:ed.

"ample #

The system shown below is divided into two sub3circuits 1 /.

1 pipe si:ing calculation would determine which o the two sub3circuitshad the most resistance and thereore which was the Inde" (ircuit.

The reason for $nding the %nde &ircuitis to si'e the pump(

The pressure developed by the pump should be capable o overcomingthe resistance in the Inde" (ircuit.

I the pump pressure can overcome the resistance in the Inde" (ircuit,

then it can overcome the resistance in other circuits o lesser resistance.I it was ound that the Inde" (ircuitwas (ircuit %/& in the above diagramthen we would include the +ow o water through radiators .

I we e"amine (ircuit %/& then the Inde" (ircuit+ows past ?adiator


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Inde" (ircuit.

Si*in& of Expansion Vesse

*i:ing o the $"pansion vessel essential, the vessel must be large enoughto accommodate all the e"pansion o the water.

The ollowing inormation is reuired to enable the correct si:e o vessel tobe calculated;

- The volume of water within the system( Manufacturerssupply data includes water capacities of components such asboilers and heat emitters(

- %nitial pressure of the system, calculated from the staticheight, which is the vertical distance from the epansionvessel to the highest point of the system(

- The boiler )ow temperature

Pump

Sub-Circuit B is

Index CircuitSub-Circuit A

BOILER

No.1

No.2

No.3 No.4

No.5

Heat Emitters

Radiator No.5 Included in

Inde !ircuit


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capacities of expansion vessel

Typical volumes o water contained within systems are as ollows;

&ast iron heat echangers #* litres

+ow water content heat echangers ( litres

Small bore pipewor. # litre/.w of system output

Microbore system 0 litres

Steel radiators 1 litres/.w of system output2ot water cylinder/calori$er 3 litres

The volume ound rom the previous table shall be multiplied by theappropriate actor rom the table below;

Maimum boiler )ow temperature 4actor

56& or greater #(*

+ess than 56& but not less than 116& *(5

+ess than 116& *(1


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Fiin& t)e Seaed System

2rovision must be made or !llingpressurising the system. Theconnection or the !lling a sealed system shall include a stop valve anddouble chec- valve assembly to prevent water rom passing bac- intothe mains water supply.

1 typical method would incorporate;

- 7 temporary hose connection to the mains water

- 7 stop valve should be installed to the service main outlet

- 7 double chec. valve and stop valve should be installedto the system connection of the temporary hose

Where an approved flling loop is used, which incorporates allnecessary valves; it must be in accordance with manuacturersinstructions.


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P"ess#"isation +nit

1nother method commonly used to pressure a sealed heating system iswith the use o a 2ressurisation unit, which is sealed to atmospherethereore the term sealed. 1 pressure sensor is !tted into the system

as indicated in the diagram. I the pressure alls below a set pressure, apump is operated which eeds mains water into the system pipewor-.hen the boiler increases in temperature, the pressure will alsoincrease due to the e"pansion o water. This increased water volume isaccommodated within the e"pansion vessel. In the event o a boilercontrol ault or the pressure increase being too great, the interloc-between the pressurisation unit and the boilers will switch the boilersof until the ault is recti!ed. $"cess pressure is released through apressure relie valve. (ontinuous !lling o the system by the pumpwould indicate a lea- in the system.

1s well as -eeping the system water levels maintained .The

2ressurisation unit can also pressurise the water in the system. Thebene!t o this is that the boiler +ow temperatures can be increasedabove 8@@9( without it boiling. This means the same volume o waterwill carry more energy to the heat emitters and smaller diameterpipewor- may be used which are easier to accommodate within piperuns.


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T,o-Pipe System

The preerred method o installing the heating circuit is to use two pipes,one pipe being the +ow rom the heat source %boiler&, and the other beingthe return to the heat source. The main advantage is that appro"imatelythe same +ow temperature is available to each radiator, thus allowing thecorrect si:e radiator to be selected in the initial design o the system.

1 total heating system in a property, may be made up o a number odiferent designs o two3pipe systems serving various numbers o heatemitters. $ach emitter being supplied by the )eating +ow and return.


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Sin&e Pipe System

The single pipe design o water supply to heat emitters is by ar thesimplest installed, using one pipe to supply all radiators on the circuit withheated water. 2ipe si:es and heat emitter si:es are generally larger toovercome resistance and meet output demands o the system. 1disadvantage o this type o design is that the same +ow and return

temperatures at the heat emitters cannot be maintained due to thecooling o the heated water as it circulates through the system.

The reduction in temperature at the +ow and return connections to theheat emitters through a single pipe system is not desired, with reerenceto the diagrams attached. The !rst radiator %1& will receive water at the

boiler output mean temperature at the +ow connection and atercirculation through the radiator, the water will lose temperature due to


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some o the heat being e"tracted or the reuired output o the radiator.The water is then circulated through the return connection o the radiatorat a lower temperature bac- to the single pipe which has bypassed theradiator to ultimately mi" at a lower temperature than the boiler outputmean temperature. This water is then circulated urther to radiator %/&

where the process continues.

G"a%ity Ci"'#ation

1 Aravity central heating system depends on the diference in density ohot and cold water. (old water is heavier than hot water and, thereore,the cold water will drop and the hot water will rise. These types o systemswere initially the only types used, until through the years they weregradually modernised by the urther installation o controls andcomponents.


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This type o system comprises o a heat source %boiler& installed to asystem o pipewor-, which in turn circulates the heated water through thesystem to the radiators and calori!ers.hen the boiler is raised to temperature this allows the heavier cold water

to all through the return pipe and return to the boiler to be reheated, thusin turn displacing hot water within the boiler into the system andrepeating the process.

F#y P#mped Heatin& and Domesti' Hot .ate" System

Bodern practice is to use a positive orm o circulation, or both )eating

and )ot water, this removes the reliance on gravity as the main orm ocirculation along with all its inherent disadvantages.2umped circulation, to both heating and hot water, allows or smallerdiameter pipewor- and a more +e"ible approach to the routes o pipesand in general a cost efective and e5cient installation.The use o the pump, to serve both the main )eating circuit and the )otwater circuit %primary&, allows the independent control o both domestichot water and heating. The other advantage being speed o recovery andthe control o temperature within the system.


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Instaation Fa'to"s

The installation o a ully pumped system, is dependant on a number oimportant actors;

- 8ump position

- 8ump head

- Open vent and &old feed position

- &orrect pipe si'e

- 8ipe layout

- The pressure within a system, both negative and positivebeing correctly understood

- The correct installation of controls

- 9etermining the :inde radiator; on the circuit

- Balancing of the system


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oie"s

hen the total heat reuirements o the system have been calculated, the

installer will need to select a boiler to meet both the system and clientreuirements.

1 number o considerations must be made beore the selection o theboiler such as;

- +ocation

-


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and heating. It is normal to select a boiler o a larger output in the evento the system being e"tended in the uture. Cltimately the boiler will beeither Open3+ued or ?oom3sealed.

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