Hot Water Storage and inadequate mains pressure

8th Aug 2017


The one thing that can compromise the flow rate and pressure advantage of a mains pressure storage water system such as an unvented cylinder is an inadequate mains water supply.

Stuart Elsy, Managing Director, OSO Hotwater gives an explanation of the varying technologies and products available to boost water performance and advice for installers and specifiers on correct diagnosis of water supply problems and the optimum products to select.

Cylinder manufacturers’ technical helplines receive incoming calls from installers complaining that the water pressure in a given property is inadequate as the client is unable to run 2 or 3 showers simultaneously. In at least 90% of cases this diagnosis is incorrect. There is usually nothing wrong with the water pressure, the problem is lack of water flow rate.

When a plumber measures water pressure prior to an installation of a mains pressure system, as they normally should, they typically measure the pressure with all taps closed and no water flowing. This is the static water pressure. If they continue the test but open a cold water tap, they are then measuring dynamic water pressure. Dynamic pressure is what makes a system work correctly. Dynamic is always lower than static, is a function of available flow rate and reduces further with every extra outlet opened. Ideally an installer should open 3-4 cold water outlets simultaneously, measure the flow rate of each whilst they are all running together and add them up. This will give a strong indication of the maximum water flow rate available to the property.

As a general rule of thumb, 25 litres per minute is satisfactory for a property with 2 showers. More is needed if there are extra showers. Our own experience at OSO is that 30 – 35 lpm can be considered average and this will satisfy all but the largest of properties with 4 showers plus.

Water pressure varies dramatically around the country and to a lesser degree, during different times of the day. Some localities are well known for poor pressure, however it is relatively rare for static water pressure to fall below 2 bar. A water pressure of between 3 and 6 bar could be considered average, and pressures in some areas can reach 8 to 10 bar, especially at night.

An unvented system will work perfectly well at 2 bar, indeed some manufacturers’ factory set pressure reducing valves will restrict the pressure to this point or even lower without compromising performance.

If static pressure is OK, as it usually is, the typical solution to a poor flow rate is to add an accumulator to a system. Accumulators are large expansion vessels, typically up to 500 litres, that are fitted immediately after the stop cock. They store cold water under mains pressure and drive this water into a plumbing system when the demand from hot and cold outlets exceeds the available mains water flow.

An accumulator system should be supplied with a dedicated “upstream kit”, including a double check valve, line strainer and pressure reducing valve. These are required to protect the integrity of the rubber bladder holding the water store and are needed in addition to the standard inlet control kit supplied with all approved unvented cylinders. It should be remembered that the concept of an accumulator driven unvented cylinder is a patented idea. Prospective purchasers must source the products from one of the 2 suppliers who are license holders of patent 2349908 and who will ensure that the correct selection of equipment is supplied to suit the installation.

The performance difference that a correctly specified accumulator system can achieve is incredible. Multiple outlets can be supplied at full flow. In most cases only one accumulator is required, but some commercial installations such as hotels or sports centres can benefit from multiple accumulators to ensure optimum performance.

Most plumbers know that installing a booster pump on the water mains contravenes the water regulations if the pump is attempting to draw more than 12 lts/min, however if static mains pressure is inadequate, typically less than 2 bar, there are certain WRAS approved products known as Charger Pumps that will trickle charge an accumulator by drawing water from the mains in an otherwise dormant period to increase the static pressure in the accumulator and therefore the entire hot and cold water system in the property. This will ensure a good performance regardless of the static pressure.

In a very large plumbing system, the optimum solution to a low static pressure problem can be a break tank and booster system. Once again, water regulations approved products are available.  These products store cold water in a ball valve controlled break tank and boost the water into an unvented cylinder via a flow switch controlled pumping set to achieve optimum performance.