Because water will not flow uphill it is impossible to increase its pressure (head) without a booster set being used to assist in the increase.

The most common way to do this is by boosting the water with a pump. The pump being sized to produce the require pressure (head) and volume (flow).

In most areas domestic dwellings are feed from the local water company and they supplied adequate pressure to feed most properties. Unfortunately because of cost savings within these companies they have reduced the guaranteed water pressure and properties that are greater than 4 floors are finding that they have lose of water during peak times.

To overcome this problem a booster pump must be installed between the mains feed and the property. Because of water bylaws a break (air gap) must be designed in to the system so that a back flow cannot be created by the pump of system it is feeding. The most common way to do this is via a break tank that has a Type A air gap. The main water feeds in to this tank via a ball valve and the pump pumps the water from the tank and in to the property.

The next thing to consider is how the pumps are to operate. If a pump is run continuously without demand (closed valve) it will over heat and eventually burn out.

The original way of controlling the pump was by having a header tank in the loft of a high raised property. When the tank level dropped a float switch would turn the pump on until the level went up above the float switch. Most tall blocks of flats were served in this way and some blocks in London still work on this principle.

During the sixties and seventies hydrophumatic systems were designed to give automatic pressure control to large properties. These were large pressure vessels that were filled with compressed air and the water was pumped under pressure in to the tank. On the outlet of the tank was the system. As the system required water the pressure and the level in the tank would fall. The pump would start at a design pressure and run until the demand was met and the pressure and level in the tank had risen. The pressure tank would always require a large volume of air so that the draw down could be kept as long as possible. If the volume was lost over time a level switch in the tank would operate a compressor to bring the air volume up to the correct level. These systems are still used in some tower blocks and universities.

During the seventies and eighties a number of companies developed membrane (diaphragm} pressure vessels. This one item was the green light for cost affective booster sets. The booster units could be made as a package and the pressure vessel used as storage of pressure. The vessel having 2 primary functions, One to stop the set from hunting (pressure bounce) and two to hold the set off at low or no demand. The principle being the larger the vessel the longer the set will stay off. Most large buildings built in the last 20 years would have had a system like this installed.

The principle of operation is simple as long as the unit is sized and commissioned correctly. If we take the following as an example:

10 storey block of flats with a single rising main up the centre of the block. In the basement a break tank and booster set is installed. The top floor will require 3.5 bar so that it hot water system will work correctly. The pump should be sized to give a working range above the static head and the required pressure.

The static pressure being 10 floors x 3m head per floor = 3 bar pressure

The pressure required at the highest point being 3.5 bar.

Our pump will have to give a head greater than 6.5 bar.

The correct booster set should have a flow rate calculated and selected on a steep curve so that the vessel can exert a pressure of water and hold the unit off at low demand. If the pump had a very shallow curve the set will hunt and run continuously.

I would size a set to have a closed Valve of 9 bar. With a cut in pressure of 6.8 bar and a vessel pre charge pressure of 6.6 bar. We always set the vessel air charge .2 bar below the cut in of the final pump. (Standby pump on a two pump set).

Booster Pumps

Most booster sets of this type are fitted with run on timers. These are used to prolong the run of the pumps. Assisting in filling the pressure vessel and reducing the starts of the control gear. Most relays should not be switched on or off under load more than 20 times per hour.

The riser or the outlets to the systems should be fitted with pressure reducing valves. These being set below the riser pressure so that the systems have a constant pressure. Any PRV fitted in the system must be set below the minimum pressure the booster set produces. If the PRV is set above the minimum pressure the PRV will hunt and cause pressure problems in the system.

Over the last 10 years a new generation of booster sets have been developed. Pump sets fitted with inverter drives. The inverters adjusting the speed of the pump as the required flow increases and decreases. Because the speed can be varied the power consumption of the motor can be reduced. This in turn reduces the running cost of the pump. The pump of pump set can be design to give a constant discharge pressure with a variable output volume.

The two most common types of pump inverter are the:

1. Water cooled, pipe mounted. Works on the principle of water passing through a flow device and pressure switch. The design suits the domestic and light commercial market.
2. Motor mounted inverters that talk to each other and measure their performance via pressure transducers. These sets are designed for commercial applications and if set and serviced correctly will last for a number of years.

The correct selection of a booster pump is critical to achieve good energy saving. The design head has to be calculated correctly to give the set point (required pressure) of the inverter. The pump should be selected as far right as possible so the full capacity of the pump is used. The pump will work from left to right of the curve as the demand increases and decreases. If the pump is selected to the left of the curve it will run at high speed at low demand and will not achieve any energy saving.

We would offer 4 principle options to resolve water pressure problems.

1. Fixed speed domestic. Single booster sets with small pressure vessel and pressure switch. (Small houses and single water feeds to tanks).
2. Variable speed domestic. Single pump fitted with water cooled inverter to give constant pressure. (Small houses and systems with light loads that require a constant pressure).
3. Fixed speed booster set. Twin pump unit for larger properties. Fitted with pressure vessel and pumping against pressure reducing valves. (Larger domestic and commercial properties that require greater flows).
4. Variable speed booster set. Dual or triple pump units fitted with motor mounted inverters. Used on systems with large variable flows rates. (For large commercial properties that require constant pressures at a variable flow).

Loading Units are factors which can be applied toa  variety of appliances. They have been established by considering the frequency of use of individual appliances and the desired water flow rate.

Appliance	Loading Units
Hand basing	1.5 to 3 (depending on application)
WC Cistern	2
Washing machine	3
Dishwasher	3
Shower	3
Sink (13 mm tap)	3
Sink (19mm tap)	5
Bath (19mm tap)	10
Bath (25 mm tap)	22

By determining the number of appliances on a pipework system and summating the loading units. an equivalent flow in litres per second can be established from the follow conversion graph: