Introduction

The use of roof tanks to ensure adequate water pressure in buildings, and especially tall buildings, is very common. The alternative to roof tanks is the use of pressurised systems, where a number of booster pumps provide the necessary pressure.

Roof tank solutions were originally created more than a century ago, as buildings grew taller and taller. The required water pressure for both fire-fighting and domestic use increased and mains water was insufficient to supply a whole building. Moreover, reliable and efficient pumps for pressurised systems were not available. The immediate solution was to use standard pumps to lift the water to the tank. From the tank, gravity ensured a natural downwards flow and sufficient pressure. Despite improved and energy-efficient pressure booster technology, many buildings still have roof tanks.

Water at the ready

Roof tanks allow the users to have both water pressure and water supply in situations where electrical power supply is intermittent. Roof tanks vary greatly in size, but common to them all is that they feature “water at the ready”, storing water for domestic purposes and fire-fighting. The simple construction basically entails a tank, inlet and discharge piping, a float switch, and a pump. When the water level in the tank drops below a certain level, the float switch engages the pump, refilling the tank.

Traditions abound

The establishment and usage of roof tanks is often deeply rooted in local design traditions. Several cities and geographical areas around the world still employ roof tanks, and will continue to do so for years. In Nairobi many roof tanks dot the skyline, forming an integral part of the city’s water supply system. In the rest of East Africa, roof tanks are very common as well.

In Europe, roof tanks are employed much less, where instead pressurised systems are primarily chosen. Numerous types of pressurised system configurations are available, each having its own pros and cons. Common to the different types of pressurised system, is less of a demand for space and lower life cycle cost. However from a functional point of view, roof tanks of today work adequately in many aspects. The technology is mature, and operation is stable. The user receives the water pressure required.

On the negative side, roof tanks involve elements that are not always desired. Examples include higher capital costs due to the tank set-up and greater structural requirements, high operating costs, a lack of pressure control, and difficulty in maintaining the roof tank itself.

Hygienic aspects

In addition to serving as a storage device and creating pressure, roof-top tanks unfortunately can also serve as breeding grounds for bacteria constituting a major health risk. The exceptionally resistant bacteria legionella often appears as an unwelcome guest in water systems. In order to survive, the habitat for legionella and other micro-organisms arises in the biofilm created in the water system. Biofilm is created inside pipes and water tanks, serving as a protective barrier and breeding ground for the bacteria. Regular cleaning and maintenance of water tanks in many countries is required by law, so the additional costs, including disinfection, should also be taken into consideration.

Roof tank system

Roof tanks ensure both water pressure and water supply in case of power failure and saves space as there is only one set of pumps with one discharge at the roof tank. In some jurisdictions, it is a legal requirement to have a roof tank storage especially in homes partly as a buffer for water outages. 

This solution, however, requires pressure reduction valves on each floor in order to avoid undesired high static pressures at the tap, which creates unacceptable noise while tapping. An additional pump booster set may be required to serve the upper four to six floors in order to create sufficient pressure. The static pressure there being too low due to the insufficient geometric height to the roof tank.

The roof tank system in this case has the below disadvantages:

• Water is pumped past where it’s required

• Insufficient pressure on the uppermost floors

• Excessive pressure on the lowest floors

• Pressure reduction valves have to be fitted

• Need for higher pressure grade of pipe work

• Space requirement for tank

• Risk of microbiological growth in roof tank

Zone-divided system. 

In this set-up, the supply system is split into several zones with a set of pumps supplying between 8 floors and a maximum of 12 floors each. This ensures adequate water pressure on all floors without using pressure reduction. The minimum pressure on the upper floor in each zone is kept at 1.5 – 2 bar. The maximum pressure on the lowest floor in each zone does not exceed 4 – 4.5 bar if pipework is sized correctly.

In addition, no space is required for booster pumps in the upper levels (all being located in the basement). The system is less vulnerable to pump failures and no pressure reduction is required. 

                  

                                      

Fig. A roof tank system with small booster for upper floors (left) vs. a three zone system on a 24floor building

 

The pressurized systems comes with the following disadvantages;

• More riser pipes are required for the building

• High pressure grade pipes and pumps are required

• Sensitive to power black-outs

 (The power black-out issue is resolved in buildings with back-up power supply)

 

How to make a comparison of a pressurized system vs roof tanks

To make a realistic comparison, various costs need to be considered. These include initial cost of booster sets, initial piping costs, pressure reduction valves and tanks, maintenance costs, energy costs and lost revenue costs. These are explained below:

 

Initial costs for booster sets Cib

This includes a booster set or pumps and all the equipment and accessories needed to operate the booster sets:

• Pumps

• Frequency converters (not required in roof tank system)

• Control panels

• Pressure sensors

• Diaphragm tanks

 

Initial costs for tanks and pipes, Cip

In tall buildings, capital costs for piping, valves and tanks often exceed the costs for boosters many times over. 

The calculation of costs includes:

• Vertical riser pipes including pipe insulation, pipe bearings and mounting

• Costs of tanks-In case of electrical breakdowns tank volumes are sized so they will be able to supply water for up to 12 hrs. (Not required in pressurized systems).

• Pressure Reduction Valves (PRVs) including mounting.

PRVs are included in the calculation where the pipe layout

Imposes static pressure at the taps to exceed approximately 5 bar. (Not required in zoned pressurized systems)

Note: Costs for horizontal water distribution pipes over

suspended ceilings, pipes in office floors and water fixtures are not included. Costs for these are the same regardless which boosting configuration is chosen.

 

Maintenance costs, Cm

Maintenance costs over a 20-year period are estimated as below:

•Maintenance of booster sets is estimated to constitute 50% of the booster’s initial purchase price.

• Pipes and PRVs: 5% of the initial investment

• Roof and break tanks: 20% of the tank’s initial cost

Both roof top tanks and break tanks must be emptied and cleansed every year according to local regulations. Booster systems that operate with roof tanks are in this sense disadvantaged in comparison with pressurised systems.

Energy costs, Ce

For a roof tank system, it is easy to calculate energy demand as the number of hours to fill daily demand can be estimated by dividing daily flow requirement by pump capacity. To perform an energy calculation for a pressurized system a load profile is needed. The consumption profile shows the changes that occur in flow during a typical 24-hour period. 

In an office building, as well as in most commercial buildings, water consumption varies greatly depending on the time of day. In the morning, the largest flow occurs with the start of service activities such as cleaning, coffee making, cooking, and washing. The demand fluctuates for the rest of the day, but does not reach the high morning level. As the building hosts only office space, there is virtually no consumption in late evening and overnight. The load profile is based on the duration curve.

In typical office block application, the load profile can be estimated as;

100% flow demand- 1 hour/ day

60% flow demand- 4 hours/day

40% flow demand-10 hours/day

0% flow demand- 9 hours/day

The energy calculations are performed at three different duty points which are regarded as representative for the consumption profile.

• Duty point 1 is liable for only one hour per day at the

peak flow 

• Duty point 2 is liable for four hours per day 

• Duty point 3 is liable 10 hours per day 

• The remaining nine hours are estimated as having no

Consumption.

 

Lost revenue costs, Cr

As real estate becomes more valuable, the amount of

saleable area gets more and more important. In many

instances it is profitable to extend the height of a building.

Another and more effective way to increase the saleable

area is to reduce “wasted” space for building services such as the space taken up by more pump systems used in a zoned system. This cost needs to be taken into account in comparing the systems.

 

Total or Life Cycle Cost comparison.

Bringing all the above factors together for comparison yields the below formula: 

 

LCC = Cib + Cip + Cm + Ce + Cr

 

Initial costs for booster sets Cib

Initial costs for tanks and pipes, Cip

Maintenance costs, Cm

Energy costs, Ce

Lost revenue costs, Cr

 

CONCLUSION

Pressurised and zoned divided systems are superior to the roof-top tank solutions – both when it comes to initial investment, maintenance and energy efficient operation. Creating flow in the water system consumes power and so does creating pressure even when there is little or no flow.

Therefore, booster configurations with several booster sets and low pressure levels are preferable as the power consumption will reduce significantly as the provided pressure reduces. The pressure in zoned systems is low hence no PRVs are necessary.

The roof-top tank system power consumption is the highest of all systems as all of the water is pumped past consumers at a high pressure and then allowed to gravitate back to where it is supposed to be used. Again, pressure reduction valves have to be applied in order to remove surplus pressure. Traditionally, there has been great focus on initial cost both when choosing booster sets and when settling for a system configuration of boosting systems. However, calculations with all significant factors considered shows that doing so is unwise. 

It is a fact that zone divided systems call for increased investment in booster sets, but a total or life cycle cost analysis shows that investment in boosters is of minor importance in the longer term. Focus should be given to the entire boosting configuration as energy consumption is the most important element to consider: energy consumption turns out to account for more than all the remaining costs added together.

 

REFERENCES

Reprinted with excepts from Grundfos white papers on same topic as well as  below references:

1.  International Plumbing Code – 2012 Ed.

2.  Drinking water standard DIN 1989-1:2001-10

3. Water Boosting commercial buildings- Engineering Manual, Anders Nielsen (2016)

4. State of Green, Alfred Heller, Jens Norgaard et.al. (2015).

 

For more information and actual example contact us or visit: 

http://www.grundfos.com/grundfos-for-engineers.html

 

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