hot water danfoss.pdf

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Domestic Hot Water Circulation System - Background information Data sheet a ter Circ mestic Hot Wa System Domestic H Circulation System Dome Hot Water Circulation System Do m Domestic Hot Water Circulation Sys lation System Domestic Hot Water Circulat W ater Circulation System Domestic Hot Water Cir estic Hot Water Circulation System Domestic Hot W a System Domestic Hot Water Circulation System Dom Circulation System Domestic Hot Water Circulation c Hot Water Circulation System Domestic Hot Water m Domestic Hot Water Circulation System Domestic H ation System Domestic Hot Water Circulation System Water Circulation System Domestic Hot Water Circu mestic Hot Water Circulation System Domestic Hot W m Domestic Hot Water Circulation System Dom n System Domestic Hot Water Circulation ulation System Domestic Hot Water Circulation System Domesti Water Circulation System Hot Water Circu mestic Hot m Do ESMC ESMB ABN CCR TVM-W MTCV

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Page 1: hot water danfoss.pdf

Domestic Hot Water Circulation System- Background information

Data sheet

a

ter Circ

mestic Hot Wa

System Domestic H

Circulatio

n System Dome

Hot Water C

irculatio

n System Do

m Domestic Hot Water C

irculatio

n Sys

lation System Domestic Hot W

ater Circ

ulat

Water Circ

ulation System Domestic Hot W

ater Cir

estic Hot Water C

irculatio

n System Domestic Hot Wa

System Domestic Hot Water C

irculatio

n System Dom

Circulatio

n System Domestic Hot Water C

irculatio

n

c Hot Water C

irculatio

n System Domestic Hot Water

m Domestic Hot Water C

irculatio

n System Domestic H

ation System Domestic Hot W

ater Circ

ulation System

Water Circ

ulation System Domestic Hot W

ater Circ

u

mestic Hot Water C

irculatio

n System Domestic Hot W

m Domestic Hot Water C

irculatio

n System Dom

n System Domestic Hot Water C

irculatio

n

ulation System Domestic Hot W

ater

Circulatio

n System Domesti

Water Circ

ulation System

Hot Water C

ircu

mestic Hot

m Do

ESMC

ESMB

ABN

CCR

TVM-W

MTCV

Page 2: hot water danfoss.pdf

2 VD.57.X1.02 © Danfoss 04/02 SIBC

Design principle

Recommended scheme of hot water installation with circulation

Example of MTCV / basic version / placement in domestic hot water system

A) Indirect heating connection with parallel instantaneous system for domestic hot water production- independent CCR system

B) Indirect heating connection with parallel instantaneous system for domestic hot water production- dependent CCR system

Data sheet Domestic Hot Water Circulation System

B

A

cold water

hot water

cold water

Page 3: hot water danfoss.pdf

SIBC VD.57.X1.02 © Danfoss 04/02 3

Background Domestic hot water systems have developedover the last few years. These developmentshave been stimulated by new factors andtrends like:

• Increasing cost of domestic hot waterproduction

• Increasing cost of water• Required high level of reliability of the

hot water supply• Quality and hygiene of water i.e.

legionnaire contamination

Danfoss has developed a universal andcomprehensive solution for the regulation ofhot water systems, providing the followingadvantages for users:

A. Traditional sizing methods common inthe seventies and eighties, calculated therequired circulation flow in whole hot watersystem i.e. from heat loss through thepipes through the nominal temperaturedrop from the heat station to the lasttapping point and nominal waterconsumption.

The disadvantage of this sizing method isthat circulation flow in an individual riser isproportional to nominal water consumptionfrom all tapping points of the riser. Thesame flow will be calculated in the nearestand farthest riser from the heat exchanger(in case of risers with the same number oftapping points, e.g. in multi-family houses).It is obvious that heat losses are biggest atthe farthest riser and the water will be at itslowest temperature there.

The traditional sizing method does notcompensate for this fact and as a resultthe water temperature will be different atdifferent points in the installation. Fig. 2and 3 illustrate sizing results for a typicalinstallation in a 10 storey building with 12hot water risers. The temperaturedistribution graph (fig. 2) illustrates thattemperature varies from 52 °C to 47 °C inthe farthest riser.

B. The sizing method currently being usedcompensates for heat losses through thepipes. Heat losses are calculated, takinginto consideration thermal insulation of thepipes and the difference between theambient temperature and watertemperature. Due to heat losses, thetemperature drop is assumed to be in therange of between 5 to 10 K, depending onthe hot water nominal temperature.

This method allows for an equal watertemperature in all the risers and at thesame time a different circulation flow inindividual risers, (fig. 2 and 3).

Below are basic assumptions and formulas

Thermal BalanceSizing methods

Data sheet Domestic Hot Water Circulation System

used in the calculations:

Water flow in installation is in relation totemperature drop, where:

hww tc

Qm

∆= ∑ &

& [kg/s] or hww tc

QV

∆= ∑

ρ

&& [l/s] (1)

∑Q& - heat losses in installation [kW]ρ - water density [kg/m3]cw - specific heat of water, [kJ / (kg K)]∆thw - temperature drop of hot water

(assumed to be between5 and 10 K)

Heat losses are calculated from the formula:

qlTlKQ hwhwj =∆=& (2)

q - unit heat loss through hot water pipes[W/m],

Kj - heat transfer coefficient for pipes andthermal insulation [W/(mK)],

lhw - pipe length [m]∆t - difference between ambient

temperature and hot watertemperature [K]

Calculation of the circulation flow in theindividual risers is made on the basis ofincoming and outgoing flow to the specificnode.

• Reduction of the domestic hot waterproduction costs.

• Reduction of water consumption - no needto wait for running water to achieve theright temperature at the tapping point

• Thermal balancing - equal temperature ofhot water at all tapping points

• Possibility of thermal disinfection of the hotwater system against Legionnaire. Thiscan be done automatically by protectingthe installation against calciumsedimentation and corrosion

• Reducing scalding risk during thedisinfection process

• Possibility of monitoring and controlling thehot water temperature during thedisinfection process.

Calculation scheme of the hot waterinstallation node

ppp tQV ∆,, &&poc VVV &&& +=

oo

ot

QV

∆,,&

&

Page 4: hot water danfoss.pdf

4 VD.57.X1.02 © Danfoss 04/02 SIBC

Thermal BalanceSizing methods

Data sheet Domestic Hot Water Circulation System

Substituting (3b) and (3c) to (3a) and dividingthis new formula by (3b), the followingformula appears:

po

o

c

o

QQ

Q

V

V&&

&

&

&

+= thus:

po

oco

QQ

QVV &&

&&&

+×= [l/s] (4)

where:

oV& - nominal water flow in the riser (branch flow) [l/s],

oQ& - nominal heat loss in the riser [W],

cV& - nominal total flow in the hot watersystem (incoming flow from waterheater) [l/s],

pV& - nominal flow in the circulation header(passing flow) [l/s],

pQ& - nominal heat loss in the circulation header (in remaining part) [W].

Passing flow in the circulation header iscalculated from the formula:

ocp VVV &&& −= [l/s] (5)

When calculating flow values in the individualrisers, it is assumed that the temperaturedrop in the first riser (from the heat station tothe last tap on the riser) is the same as in theremaining part of the hot water system. Flowin the whole system is a sum of flow in itsparts, thus:

The flow in the other branches is thencalculated using formulas (4) and (5).The calculated flow in branches andrecommended water velocity determine thesizing of the pipes.The recommended water velocity vma , variesfrom 0,2 ¸ 0, 5 m/s, and must not exceed 1 m/s.It is not recommended to use steel pipessmaller than DN 15 due to risk of scaling.

Having sized the pipe diameters, hydrauliclosses and consequently flow can becalculated in all branches of the system.Hydraulic losses are a sum of linear and locallosses, (it can be assumed that local lossesare between 20 - 40% of linear losses,depending on the system configuration) andlosses on the valves and other equipmentmounted in the system.

The following formula is used for calculatingthe pressure losses (hydraulic losses):

wRp ppRlp ∆+∆+×∑×÷=∆ )()4,12,1( [Pa] (6)where:

pp∆ - total pressure losses in hot water andcirculation system to size thecirculation pump, [Pa],

4,12,1 ÷ - 20 - 40% reserve for local pressurelosses in hot water system withcirculation

l - lengths of the branch, [m];R - unit linear pressure loss, [Pa/m],

Rp∆ - local pressure loss on valve(controller, equipment), [Pa],

wp∆ - local pressure loss on water heater(boiler, heat exchanger), [Pa].

Thermal BalanceBalancing methods

When the pressure losses have beencalculated, the hot water system must bebalanced. There are 2 basic methods ofbalancing:

1. STATIC METHOD - Hydraulic balancingShould excessive pressure be reduced ina certain node of the system, we can sizethe throttling (regulating) devices. Thistype of balancing does not secure anequal temperature in all points of thesystem due to:

• Changing hydraulic (pressure) losses while thescaling process in the pipes occurs (higherfriction of the pipes walls)

• A change from nominal consumption of hotwater (influence on flow and watertemperature)

• A different than assumed result in thecalculation of ambient temperature (differentfrom the calculated heat losses)

• Different conditions than assumed incalculations for parts of the installation locatedclose to external walls, ventilation shaft, etc.This is especially the case in larger systems.

• The dynamic nature of hot water consumption,when nominal conditions are rare.

2. DYNAMIC METHOD - Thermal balancingThe method is based on the usage ofthermostatic balancing valves (circulationvalves), which secures a constanttemperature in all the circulation risers ofthe system, irrespective of changingoperating conditions.Regulation is made simply by setting therequired temperature on the valve.Complicated hydraulic calculations tobalance pressures in all nodes of thesystem are no longer needed. Thethermostatic balancing valve automaticallyallows minimum circulation flow in order tomaintain the set temperature.

In the example mentioned below, thecalculation results are illustrated for the 2sizing methods (authors: PhD W. Szaflik,MSc A. Malysa, Szczecin TechnicalUniversity, Poland).

A - sizing method of determining circulationflow proportional to sum of nominal flowof all tapping points on the riser

pw

pp tc

QV

∆××=

ρ

&&

ow

oo tc

QV

∆××=

ρ

&&

poc VVV &&& +=

∆to = ∆tp

(3a)

(3b)

(3c)

(3d)

Page 5: hot water danfoss.pdf

SIBC VD.57.X1.02 © Danfoss 04/02 5

1

31 2 4 5 86 7 9 10 1211

72 3 4 5 6 8 9 1110 12

Thermal Balance(continuous)

Data sheet Domestic Hot Water Circulation System

Fig. 1 Scheme of hot water system with 12 risers and circulation, in 10-storey building.

Fig.2 Circulation flow in 12 insulated risers of hot water system.

B - sizing method of determining circulationflow proportional to heat losses in eachpart of hot water system andcirculation

Example:• 10 storey building, 12 hot water risers with

circulation• nominal inlet temperature of hot water

Tcwu = 55 °C,

• nominal temperature drop = 5 K• heat transfer coefficient for uninsulated

steel pipes K = 12 W/(m2K),• pipes diameters are stated in table fig. 1• nominal ambient temperature:

- To = + 5 °C for cellars- T1 = + 25 °C for installation and

ventilation shafts

Fig.3 Temperature in last tapping points of 12 risers of hot water system.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

1 2 3 4 5 6 7 8 9 10 11 12Number of risers

Flo

w [l

/s]

Static method Dynamic method

45.00

46.00

47.00

48.00

49.00

50.00

51.00

52.00

53.00

1 2 3 4 5 6 7 8 9 10 11 12

Number of risers

Tem

pera

ture

[ C

]

Static method Dynamic method

o2 3 4 5 6 7 8 9 10 11 121

number

hot waterdimension (mm)

circulationdimension (mm)

lenght (m)

12 risers

from sub-station

riser horizontal pipe

1.II

1.IX

1.X

Page 6: hot water danfoss.pdf

6 VD.57.X1.02 © Danfoss 04/02 SIBC

Data sheet Domestic Hot Water Circulation System

Legionella pneumophila -a threat to health

During the last few years there has been agrowing interest from the public media, andfocus from sanitary and epidemic institutionson the hygienic quality of water, especiallysecondary bacteriological contamination ofwater. The reason for this is the discovery ofthe legionella bacteria, which grows in:

• Domestic hot water systems• Cooling towers• Evaporative condensers• Large air-conditioning systems• Hot whirlpools• Respiratory equipment• Small portable humidifiers

Below follows an outline of the main findingsand conclusions from the research that hasbeen done in relation to legionella.

The first information about legionella datesback to 1968. Regular investigation ofbacterium, however, was not started until asrecently as 1977 after illness caused twenty-nine deaths among American Legionmembers who were attending a convention ina Philadelphia hotel in 1976.

Legionella covers app. 50 classified species,of which as many as 18 can cause seriousdisease. These are types like: L.pneumophila, L. bozemani, L. micdadei andothers.

Legionnaire is an infection caused by thebacterium Legionella pneumphila (app.90%).

The disease has two distinct forms:• Legionnaires’ disease, the more severe of

the infections, that includes pneumonia,and

• Pontiac fever, which is a milder illness.

Due to the fact that the infection is transmittedwhen inhaling aerosolized, contaminated waterinto the lungs, the presence of bacteria inwater systems creates a risk wherever thereare aerosol-producing devices.

The perfect conditions for transmission of theinfection exist in water tanks and waterinstallations in dwelling buildings, commercialbuildings and public buildings, like: hotels,hospitals, sanatoriums, etc.Typical examples are showers, whirlpoolspas, humidifiers and decorative fountainswhere water-air aerosol is the ideal„transportation" of the bacteria into the lungs.

What are the usual symptoms oflegionallosis?Patients with Legionnaires' disease usuallyhave fever, chills and cough, which may bedry or may produce sputum. Some patientsalso have muscle aches, headache,tiredness, loss of appetite, and occasionallydiarrhoea. Laboratory tests may show adecreased functioning of the kidneys. ChestX-rays often show pneumonia. As it is difficultto distinguish Legionnaires' disease fromother types of pneumonia by symptoms alone,other diagnostic tests are required.

Who gets Legionnaires' disease?People of any age can get Legionnaires'disease, but the illness most often affectsmiddle aged and older people, particularlythose who smoke cigarettes or have chronicdiseases. People whose immune system issuppressed by diseases as cancer, kidneyfailure, which requires dialysis, diabetes orAIDS are also at increased risk. People whotake drugs that suppress the immune systemare also at higher risk.

In the Europe alone, over 5.000 to 10.000cases of legionnaires’ disease are reportedeach year. About 5% of known cases oflegionnaires’ disease have been fatal.

What is the treatment for Legionnaires'disease?Erythromycin is the antibiotic currentlyrecommended for treating people withLegionnaires' disease. In severe cases asecond drug, rifampin, may be used inaddition. Pontiac fever requires no specifictreatment.

What are the best conditions for thebacterium to breed?Legionella bacteria are most commonly foundin natural and man-made aquaticenvironments.The bacterium breeds easily at temperaturesbetween 22 - 43 °C and the optimaltemperature for their growth is36 +/- 1°C. The time of regeneration of cells atthis temperature is 6-8 hours.

Bacteria breeding in biofilms and microbescan still occur at a temperature of 67 °C. Thegrowth of bacteria is possible within pH 5,5 -9,2 and the optimal pH is 6,8 - 7,0. Scale,sediment, biofilms and presence of amoebaare also favourable conditions for themultiplying of bacteria.

Bacteria are eliminated at a temperatureabove 46 °C. The graph below illustrates theempiric dependency (dependency ofBrundrett), of bacteria on their ability tomultiply at different temperatures. The axis ofordinates in a logarithmic scale (vertical),represent concentration of bacteria in waterand the axis of abscissa (horizontal) - time.

Dependence between bacteria concentrationand time is linear at a constant temperature(in chosen logarithmic scale). We can thusread directly from the graph that at 50 °C,2.3 hours are needed to reduce bacteriaconcentration from 1000 units to 100 units.The pasteurization process is much faster at ahigh temperature. The graph illustrates that inorder to achieve legionella-free water at atemperature of 50 °C, a pasteurization time of7 hours is needed, while at a temperature of54 °C only 1.3 hour is needed. At atemperature of 48 °C, the pasteurizationprocess must continue for as long as 30hours.

Page 7: hot water danfoss.pdf

SIBC VD.57.X1.02 © Danfoss 04/02 7

It is also interesting to study the dependencyof bacteria concentration in a changing watertemperature (author: PhD J. Wollerstrand,Lund Institute of Technology, Sweden). Thedependency is shown on the lined curve. Atfirst there are 100 units of bacteria colonies inthe water. The colonies are amplifying in atemperature up to 40 °C, after that they startto decrease at a temperature of 50 °C, afterwhich, on further temperature increase, theystart to grow again.

The conclusion is that a periodical increase oftemperature (without control overpasteurization time) will reduce theconcentration of bacteria, but will noteliminate them completely. A similar

Data sheet Domestic Hot Water Circulation System

Legionella pneumophila -a threat to health

1

10

100

1000

10000

0 1 2 3 4 5 6 7 8 9 10

time (hours)

bact

eria

gro

wth

(nu

mbe

r/m

l H2O

)

30 - 40 oC

46 oC

50C58 C 54C

48 oC40 oC

50 oC

40 oC54 oC

phenomenon can occur in a hot watersystem, or parts of it, which are rarely used.

It is important to mention that the bacteriagrow not in the water itself, but in the biofilms,which always cover surfaces which are incontact with the water.

A recently discovered form of bacteria growthare colonies inside amoeba and othermicrobes that are living in water and are aspecific “umbrella" for bacteria. In this casethe required pasteurization temperature maybe up to 70 °C. In such a high temperature,the scalding risk is very high and has to beconsidered as well as the risk of pipe scalingand corrosion.

What is being done to preventlegionellosis?We can use several methods,which havebeen the subject of research, to controllegionella in portable water systems. Theyinclude the chemical methods:

• ChlorinationIn this method doses of Cl2/l are regularlyadded to water heaters and waterinstallations and the concentration of Cl2 iskept at a level of 2-3 mg/l.The use of chlorine and its compounds iseffective, but requires constant monitoringdue to the risk of creating cancerouschemical compounds. Furthermore,chlorination increases corrosion of thepipes.

• OzoningThe use of ozone O3 - a strong oxidizer,results in the reduction of bacteriaconcentration in the water by 99 % in only5 min. Unfortunately, the side effects ofusing ozone, limits its application in hotwater systems. Also the fact that oxidizerscan undergo a chemical reaction withmaterials in the water will reduce theireffectiveness.

The above-described chemical methodshave the following common disadvantages:- Practical difficulties in dosing ofchemical additives and constant control oftheir concentration in water- Negative influence on water quality- Increased corrosion

The physical methods developed toeliminate bacteria in water systems are:

• Ultraviolet LightThe devices used emit ultravioletradiation UV of 254 nm and at anintensity of 2.04 mW-s/cm2, which resultsin the reduction of bacteria by 90 %. Theeffectiveness of this method is, however,influenced by the turbidity and the colourof the water and temperature. Scale andsediment in water will significantlydecrease the effectiveness of a.m.method.

• Thermal disinfectionThe recommended physical method ofbacteria pasteurization is thermaldisinfection. The thermal disinfectiontakes place when heating up water tothe "disinfection temperature" andmaintaining this temperature for thespecific "disinfection time".

Fig. 4 Legionella growing process depending on temperature

Page 8: hot water danfoss.pdf

8 VD.57.X1.02 © Danfoss 04/02 SIBC

Data sheet Domestic Hot Water Circulation System

Legionella pneumophila -a threat to health(continuous)

Thermal disinfection must be regularlyrepeated, depending on the measures ofthe legionella contamination and theinstallation type. Thermal disinfectionmethodology including ”disinfectionparameters" is subject to specificregulations in individual countries.

Comments:Legionella pneumophila in hot water systemscan pose a severe threat to health.Currently known pasteurization methods cancontribute to a significant reduction of thisrisk. Thermal disinfection is the mosteffective, easiest and at the same timecheapest method. The full thermal disinfectionsystem by Danfoss ( MTCV + TVM-W + CCR)will provide the possibility to lower the risk oflegionnaire and at the same time help protectthe installation against excessive scale (CCR)and the user from scalding (TVM-W).

Given that the research in the area oflegionella as accounted for above is accurate,Danfoss offers a solution that can control a

temporary increase in temperature in order tofacilitate a disinfection process.

All this can be achieved at a low cost (MTCV).The following guidelines must be observedwhen designing, installing and maintainingwater installations:

• Keep the temperature of the drinking watersupply below 20 °C and in the hot watersystem at a temperature of 55 - 60 °C.

• Insulate hot and cold water pipe tomaintain the recommended temperature.

• Avoid ”dead ends" in the installation whendesigning the circulation.

• Keep the system clean and free ofsediment, deposits of solids, scale andcorrosion.

• Regularly perform thermal disinfection at atemperature of up to 70 °C.

The above-mentioned directions willcontribute to lowering the risk of infectioncaused by Legionella pneumophila.

(Corrosion and fur) limeproblems

Installations for domestic hot water is subjectto two independent processes:

• Corrosion• Precipitation of sediments

The operating durability of domestic hot waterinstallations are affected by these processes'speed of evolvement and the concentrationratio of chemicals dissolved in water, theexistence of colloids and suspensions, thewater flow speed and its temperature.However, the most important role is played bythe chemical content and temperature.

CorrosionThe phenomenon occurs mainly ininstallations made of zinc-plated steel pipes,which commonly were used in apartmenthousing in the 70s and 80s. Replacing thismaterial with others (copper, plastic)effectively decreases the risk of corrosionoccurring. However, we must remember thealready existing installations, and the fact thatsteel is still accessed and used in newinstallations for domestic hot water (e.g.distributing horizontal pipe).

Installations are supplied by water, which isrequired to fulfil sanitary norms. Thetreatment of the water is carried out withconsideration to these norms. However,ingredients included in water, even if they areneutral or even favourable from a health pointof view, can provide water with strongcorrosion features.

The first factor, which the speed of corrosiondepends on, is the ratio of protectiveingredients (as acid carbonates, hydroxides,ions of calcium) and corrosive ingredients (aschlorine, chlorides, nitrates, oxygen andcarbon dioxide) included in the water.

The second factor is the water temperature,which has influence on the change ofcomposition and structure of arising productsfrom the zinc corrosion and thus on theprotective features of these products.Whereas in cold water products of corrosioncreate a tight and protective coat which stickswell to the basis, they become grainy andloosely connected with metal at a temperature40 - 80 °C. The maximum for zinc corrosionsoccurs at a temperature of 65 °C - the speedof corrosion in this environment is e.g. 10times higher than at a temperature of 55 °C!

An additional unfavourable factor istemperature fluctuations. This phenomenoncauses a pole reversal of the zinc-ironarrangement, which significantly acceleratescorrosion. As a result of the perforation of thezinc coating (increase of temperature above65 °C) a pole reversal of the zinc-ironarrangement and a sudden steel corrosionoccurs through the simultaneous restrainingof the zinc corrosion.

Precipitation of sedimentsAs a result of the physical - chemical changesand under influence of the heating of thewater sediments occur, which are the reasonfor faulty functions in the domestic hot waterinstallation ("overgrowing" of pipelines,increased roughness of the surface of thepipelines) and has influence on hydraulics ofthe system.

Damaging chemicals included in water causethe evolvement of sediments (commonlycalled scale sediments ). It can be e.g.calcium, magnesium, iron and manganesebicarbonates. However, the main ingredient offur in domestic hot water is calciumbicarbonate in form of crystals of calcite or/and aragonite.

Page 9: hot water danfoss.pdf

SIBC VD.57.X1.02 © Danfoss 04/02 9

Calcium creates so-called carbonatehardness. It occurs in water as well as solublecalcium bicarbonate (Ca(HCO3)2) in balancewith hard soluble calcium carbonate (CaCO3).The balance is described as follows:CaCO3 + H2O + CO2 « Ca(HCO3)2

The condition of maintaining well solublebicarbonate in the water is the existence ofcarbon dioxide in the system. This minimalcontent of carbon dioxide is balance dioxide.Unfortunately, during heating up the removalof CO2 that causes the removal of the balanceinto direction of hardly soluble calciumcarbonate is a consequence.In similar ways magnesium, manganese andiron bicarbonates are created (it should benoticed that Mg(HCO3)2 is characterised bymuch higher solubility, which decreasesduring increase of the temperature,twater >75°C).

(Corrosion and fur) limeproblems(continuous)

Data sheet Domestic Hot Water Circulation System

Fig. 5 The rate of thermal decomposition of calcium bicarbonate in the first minute of heating.

Many factors influence the rate of carbonatesediments arising. The most important are:

• Water temperature• The initial concentration of bicarbonates in

water• Period and way of heating

The high hardness of the water and the hightemperature increase the rate ofdecomposition of calcium bicarbonate, whichmay be the reason for the acceleratedprocess of precipitation of calcium sediments.During analysing the above-mentionedprocess you may state that the increase ofthe temperature from 45 °C to 60 °C causes afour-time increase of the decomposition rateof Ca(HCO3)2 - the main element of fur. Thisfact explains the frequent occurrence ofproblems with "overgrowing" in domestic hotwater installations.

Thus inspection of an adequate water qualityand temperature guarantees that theinstallation works properly. For the user of theinstallations supplied with very hard water thisindicates the strict obligation of maintainingproper temperatures. Otherwise theinstallation may become totally "overgrown"even after a few years' operation. In caseinstallations are supplied with soft water, evenat a temperature of 65°C, it does not presenta problem (for concentrations below 10 °n -almost nine-times decreasing ofdecomposition of bicarbonates).

In order to understand the process of furarising it should be noticed that thermaldecomposition is not identical - in a directway - to this arising. After decompositionprecipitation follows which depends onseveral factors both accelerating andinhibiting. You may number among them:• The material which installation is made of

(brass, copper have strongly anti-chemosorptive features which inhibitscrystallisation i.e. precipitating ofsediments;

• The way the water is heated;

• The heat load of the heating surface ofdomestic hot water heaters;

• The flow speed of the water.

Conditions for the arising of bicarbonatesediment:

Based on the quoted data you may generallystate that: higher temperature, longer heatingperiod and higher concentration ofbicarbonate means faster arising of sediment.From the presented table you can read thatthe critical temperature for the initiation of furarising depends on carbonate hardness(carbonate hardness is caused by the contentof calcium and magnesium bicarbonate).

00,1

0,20,3

0,40,5

0,60,7

0,80,9

35 40 45 50 55 60 65 70 75

Temperature (˚C)hardness of water 15 ˚n hardness of water 16 ˚n hardness of water 17 ˚n

mva

l/min

Carbonate Period of beginning ofhardness of sedimentation (hours)water (°n) 30 °C 40 °C 50 °C 60 °C 70 °C

7 120 48 2410 120 24 515 120 24 2 immediately16 120 24 117 120 24 1

Page 10: hot water danfoss.pdf

10 VD.57.X1.02 © Danfoss 04/02 SIBC

Thus the temperature is already critical whenTkryt= 50 °C at a hardness higher than 15 °n,where the initiation of sedimentation arisingfollows after one hour. For the sametemperature Tkryt = 50 °C and at a hardnesslower then 7 °n , the initiation ofsedimentation arising follows after "only" 24hours.

SummaryThe problem connected with fur precipitationand corrosion in domestic hot waterinstallations may be limited to a minimumunder the following conditions:

(Corrosion and fur) limeproblems(continuous)

Data sheet Domestic Hot Water Circulation System

• Knowledge of quality parameters of theinstallation water (obedience to qualitystandards)

• Maintenance of a proper constanttemperature in domestic hot waterinstallations and circulation installations

• Limiting the temperature fluctuations to anessential minimum (controlled overheats ofinstallation in function of time andtemperature)

• Assurance of proper flow speeds andselection of proper plumbing materials.

Scalding problems Besides comfort hot water serves a vital rolein maintaining a good health. To serve thisrole the hot water must be produced at anappropriate temperature.

What does an appropriate temperature of hotwater mean?

An appropriate temperature is a temperature,which will:

• Reduce the risk of a Legionella infection toa minimum

• Reduce the risk of scalding and cutaneousburns to a minimum

• Reduce problems of precipitation to aminimum

This document is intended to give a clearguidance, but does not supersede anycurrent legislation or standard.

The owner of a residential building has theduty of care and must ensure that the tenantscan use the building and its facilities safely.On one hand the risk of Legionellacontamination requires the maintenance of ahigh temperature (around 55 - 60 °C,sometimes even more!). On the other handthis high temperature can cause seriousinjuries.

Thousands of children, elderly and disabledpersons are severely scalded, burned anddisfigured because of dangerous and unsafedomestic hot water systems. Almost 30% ofall burn injuries are related to hot water. Themajority of these injuries involve elderly andchildren under the age of five. Studies ofthermal injuries presented on the belowgraph, show the relative importance of timeand surface temperature in the causation ofcutaneous burns (authors: Dr. Moritz AR, Dr.Henriques FC - American Journal ofPathology). According to this data severe,full-thickness scalding that causesirreversible second and third-degree burnscan occur in 1 to 30 seconds at atemperature between 54 - 70 °C. At 63 °C,first-degree burns can occur in less than 2seconds, but with a water temperature of49 °C it would take much longer -approximately 5 minutes.

Since infants, young children and elderly maynot be ably to respond quickly to a situationinvolving contact with hot water - a constantsafe water temperature is essential forpreventing scalds by tap water.

When reducing the temperature in order toprevent scalding you reach a temperature,which is ideal for Legionella bacteria to growin the water system. A temperature highenough to kill the bacteria will scald users ofthe hot water and increase the risk ofprecipitation lime deposit. Only effectivesystems can be used to minimize the risks forscalding, Legionella and lime deposits.However, in many cases a high hot watertemperature reduces either one of these risksand increases the potential risk from theother.

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SIBC VD.57.X1.02 © Danfoss 04/02 11

Data sheet Domestic Hot Water Circulation System

Scalding problems(continuous)

Comments The Danfoss solution based on electroniccontrols of valves installed in thecirculation system (MTCV and CCR) andindividual for each flat - thermostaticmixing valve for water (TVM-W) reducesall of the above-mentioned risks andprovides the possibility to achieveindividual regulation in a wide range.A TVM-W - thermostatic mixing valve -tempers the water temperature in such away that the temperature will not exceedthe level at which the thermostat is set.The MTCV and the CCR allow themaintenance of an appropriate circulationtemperature in the systems simultaneouslywith recognizing thermal disinfection inminimum time and reducing the scaldingand precipitation of deposit problem -providing maximum guarantee of thermaldisinfection.

30

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0,1 1,0 10,0 100,0 1 000,0 10 000,0

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Fig. 6

Local legislation in many countries e.g.:CBRF - Community Based ResidentialFacilities, ASSE - American Society ofsanitary Engineering, CPSC - ConsumerProduct Safety Commission requires asafe pre-set temperature. The Danfosssolution can be adapted to this andthereby provides maximum satisfaction forboth the owner of the building and the end-users.

References • Brundrett, G.W.: Legionella and Building Services, Butterworth-Heinemann Ltd. 1992.”• Chudzicki J. Bakterie Legionella w instalacjach sanitarnych, Politechnika, Warszawska, 1997• Evans, C.C.: Clinical Aspects of Legionnaires Disease. Health Estate Journal, June 1993• Makin, T. Legionnaires Disease Infection Control. Health Estate Journal, June 1993• Seminar. Bakteriefrit varmt brugsvand - hvordan ? Odense Congress Center, Februar 2001• Walker, J.T. & C.V. Keevi: The influence of plumbing tube materials, water chemistry and

temperature on bio fouling of plumbing circuits with particular reference to colonisation ofLegionella pneumophila ICA Project 437 C. International Copper Association Ltd., New York,1994

• Wollerstrand J., Lund Institute of Technology, Sweden, Cyrkulacja Cieplej wody Użytkowej wświetle nowych wymagań temperaturowych i sanitarnych, Międzyzdroje 1999,

• Yu, V.L.: Resolving and Controversy on Environmental Cultures for Legionella - a modestproposal, Disinfection Control and Hospital Epidemiology, 1998, vol. 19, No. 12, pp. 893-897.

Please note that Danfoss does not assume responsibility for the research that has been performed with regards tolegionella. Danfoss offers a product to fulfil the requirements of the research results.

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Danfoss A/SSales DepartmentBuilding Controls DivisionHydronic BalancingDK-6340 NordborgDenmarkTel.: +45 7488 2222Fax: + 45 7449 0394

http://www.hydronicbalancing.com

Danfoss Trata d.o.o.Jožeta Jame 16, P.O.B. 48201210 Ljubljana - ŠentvidSloveniaTel.: +386 1/58 20 200Fax: + 386 1/51 99 824

http://www.danfoss-trata.si

Data sheet Domestic Hot Water Circulation System