Mains water supplies and how to improve.

Both Thermal Stores (including Heat Bank Thermal Stores) and Unvented Cylinders are limited in their performance by the mains water supply to a property.

Even is a hot water cylinder can provide 30 litres per minute of hot water, if the cold mains into the property is only 10 litres per minute, then you will only get 10 litres per minute out to taps.

Considering that a bath tap uses around 18 litres per minute, and a shower around 10 litres per minute, it soon becomes clear that to get 2 to 3 outlets running simultaneously requires a fairly decent mains supply.

There are 2 properties of the mains that should be looked at, and your mains water supplier is legally obliged to help you get readings:

Flow Rate. This is simply the maximum amount of water that can flow into the property, typically measured in litres per minute, and is the key to mains fed water systems. The simplest way to measure flow rate is to time how long it takes to fill a bucket to known volume.  For example, if it takes you 30 seconds to fill a 10 litre bucket, the flow rate is 20 litres per minute. The flow rate will tell you how many water outlets can run at the same time.  It is important to try and get a reading of the total, so it is no good just doing a test using a kitchen tap. Good to get all taps going at once and add up the totals. Better still to isolate the supply into the property externally (using road valve) and take the flow reading from the actual mains supply pipe within the property - although this does involve breaking into the pipework as close as possible to where the mains enters the property. 

Pressure. This is read using a pressure gauge, typically connected to a tap.  Pressure is an indication of how high in the air the water can reach. A 1 bar mains can push water up 10 metres, 3 bar up 30 metres, and so on.  Pressure is a measure of the force behind the water. However, you can have a very high pressure supply, but find that you only get a small amount of water coming into the property because the pipes that carry the water are small, or very long, or because the mains gate-valve is closed most of the way.  Pressure is in fact 'lost' in forcing water through things. Therefore, where you have a long supply pipe of small bore (15mm for example) pipe, it takes a lot more force to get water through the pipe and pressure is lost - the higher the flow the higher the pressure losses. 

Fist thing to always check is there are not half closed valves, or length of contorted small bore pipes within the property.  Occasionally properties have a very good supply in, but us is pipework within the property that causes the pressure losses. Also some water meters and water softeners are the problem. 

The local water pressure is set by your water supplier. Legally they only have to guarantee 1 bar pressure (and 9 litres/minute flow), which is very low for most mains fed systems, and there is little you can do to help the situation. The ONLY way to improve water supply pressures is to install a cold water storage tank and a pump - it is illegal to pump the mains directly.  The mains fills the tank via a ball float valve, and the pump forces this water to outlets at a higher pressure.  

Given a set local water pressure, the flow rate into the property is dependant on the supply pipe sizes and lengths.  This can be improved by replacing the supply pipe with a larger bore pipe. This can often be a very costly procedure however and may involve digging up driveways etc. Where there is a good local pressure and pipes are known to be small, this is the best solution, cost permitting. 

A third option, and the cheapest, is to fit an accumulator.  This is a steel vessel that stores mains water at the pressure it is supplied. Air within the steel vessel (trapped in a rubber diaphragm) is compressed by the mains water.  When a tap is opened the water can rush out of the vessel to the tap - faster than the mains can get into the property.  It is best imagined as like blowing up a balloon slowly and then letting the air out quickly. Accumulators only have one connection (like a balloon) and tee directly into the mains supply pipe, within the property.  They cost around 200 to 300 and are very easy to install.  The down-side is that they run out of water - once the vessel has empties you are back to the original mains supply flow rate. They also take up space;  a vessel large enough to fill a bath would be 60cm diameter and nearly a metre tall.