Low pressure and flow complications

Houses that aren’t new-builds are a challenge when owners want to add modern features like power showers but suffer with low water pressure. The right pump could help.

The overall ‘water main’ consists of three separate sections; the water mains pipeline, the communication pipe and the supply pipe.

From the main pipeline, a communication pipe carries water to the boundary of a property, where a stop-tap then marks the end of the communication pipe and the water supplier’s responsibility.

The supply (service) pipe then runs from the external stop-tap to the property, where it enters from a depth not less than 750mm in order to protect from temperature extremes through a duct into the building. This supply (service) pipe between the external stop-tap and internal stop-tap is the property owner’s responsibility. This section of pipe can be the beginning of problems for homeowners where damage has occurred or where the pipe is simply undersized for intended use.

Water suppliers must provide water to the top floor of all properties (excluding those that are pumped, such as blocks of flats) and, as the water leaves the communication pipe, it must legally provide a minimum of 0.7 Bar pressure and 9 litres per minute (lpm) natural flow (OFWAT Standards) at this point. Now consider the supply (service) pipe, if the property in question is a terraced house where the external stop-tap is at the front door. The occupier should expect to see around these figures or greater as the water enters the property. However, if the property is set back from the road with a drive or has a substantial garden, we have a longer length of supply pipe that will suffer pressure and flow loss before it even gets to the internal stop-tap – this is the owner’s responsibility.

The majority of older properties were supplied with a 15mm supply (service) pipe, which puts the onus on the cold-water storage tank (cistern), rather than on the actual mains. When changing the system to an unvented hot water cylinder or combination boiler, the demand will be directly on the incoming mains water supply.


Challenging upgrades

Let’s consider upgrading to a combination boiler, new bathroom and new kitchen with around 1.5 Bar and 11 litres per minute when the plumber checks the flow rates mid-morning. They install a 28kW combination boiler, which could provide up to 11–12 lpm on the hot water. The trouble being, during times of high demand the flow drops to around 8-9 lpm, which isn’t sufficient for a mixer shower 4m above the point of entry.

The solution is a Salamander Homeboost fitted directly on the mains where it enters the property. It will step in and uplift the flow rate to 12 lpm – the Water Regulations maximum that can be drawn directly from the mains. A common misconception is that it is ‘illegal’ to pump directly from the mains: you can, provided the pump cannot deliver more than 12 lpm. Homeboost will monitor the flow of water and try to increase the flow up to 12 lpm but will drop into standby mode if the mains naturally increases.

The problem of low pressure and flow can be more difficult for unvented hot water cylinders and large combination boilers in properties of higher demand, like a home with an extension. A plumber could recommend an unvented hot water cylinder to meet demand when replacing the traditional cylinder and cold-water tank (cistern) gravity system.

The external stop-tap sits at the bottom of their drive, 7m from the internal stop-tap, with a supply pipe in 15mm copper pipe. They have in the region of 2 Bar and 15-20 lpm at this point and, with the unofficial recommended minimum pressure of 1.5 Bar and 20 lpm for unvented cylinders to function properly, they are already on the limit for performance.

When you take into account the pressure and flow losses through the supply pipe and the rise up to the cylinder, the system will suffer pressure and flow loss. This loss will be further increased when fittings are included, especially the combination valve before the unvented cylinder, which include the strainer, check valve, pressure reducing etc. It’s unlikely they would achieve sufficient working pressure for the system to perform as it should.


Finding solutions

Excavating and increasing the supply pipe size may help, although this could be disruptive. The other solution would be to install a break tank in the house and a set of pumps to increase the pressure and flow (Figure 1, above). A break tank is simply a cold-water storage cistern fed from the cold-water mains. These can be installed anywhere in the property but need to be of sufficient size to accommodate the highest period of usage. Normally, Salamander suggests 227l of usable water per bathroom and 136l of usable water per en-suite shower room. The tank can then supply a set of two negative head, single impeller pumps, which in turn will supply the cylinder and the cold water around the property. Essentially, this method stores the cold water and then replicates mains water at a higher pressure and flow. Taking an RP120SU into account, you should see a closed head (standing) pressure of 3.6 Bar, 3.4 Bar at a flow of 8 lpm and 3.15 Bar at 16 lpm on each pump.

A negative head pump would be used when supplying water to an unvented cylinder or combination boiler due to the high resistances and potentially low flow, especially when pushing through the likes of a flat plate DHW heat exchanger. A negative head pump works in a different way to that of a positive head pump. A positive head pump assists and improves a flow of water, while a negative pump creates a flow of water where one does not naturally occur. A positive head pump has a float and sensor, which together create the flow switch and as water naturally flows through the pump and out of an outlet (over 2 lpm in the case of Salamander) this lifts the float which activates a sensor, switching on the pump. Sometimes it can be difficult to achieve a positive head pump’s minimum flow requirements, such as restrictive outlets, where pipework or outlets run too close to the base of the cold water tank (cistern) such as ‘up and over pipework’ and loft conversions. Here a negative head pump should be used which, once an outlet is turned off, will run on for a very short period of time, charging up the section of pipe between the pump and the outlet and holding it under pressure. The next time the outlet is used, this releases the built up pressure, causing a pressure sensor to tell the pump to initiate, therefore not requiring a natural flow of water.

When the mains water is simply not sufficient, adding pumps can be an excellent solution.


About Salamander Pumps

Salamander Pumps is one of the UK’s leading manufacturers of shower and domestic water pumps. Innovation is used throughout the organisation to create higher quality products and offer outstanding customer service. www.salamanderpumps.co.uk


This article first appeared in the Jan/Feb 2019 issue of P&H Engineering, the magazine for members of the CIPHE. Find out how to join here.

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