"regardless of which unvented cylinder you choose, be sure the overall system is suitably designed..."
The design and installation of any Megaflo or Unvented hot water system is of paramount importance. Correctly sized pipes ensure optimum performance and efficiency in delivering high pressure hot and cold water to all outlets - get this wrong, and the whole system falls apart!
All unvented installations are subject to the legal requirements of Building regulation G3 and should only be installed by an “approved installer”. This requirement ensures the safe installation of a Megaflo or Unvented Cylinder, but does not regulate the quality, design or installation of the associated pipe work, fittings and components.
Megaflo Unvented specify and design systems in-order to maximise the performance of an unvented hot water system. A poorly installed Megaflo system will under-perform a traditional vented system and will fail to offer any real benefits - don't get this wrong!
With an unvented hot water system, it is essential that the cold water supply has adequate pressure and flow rate to meet the maximum flow demand for both hot and cold water outlets.
Pressure: the force of water delivered.
Flow rate: the volume of water carried in the pipework.
It is generally recommended that, as a minimum, the incoming mains is at least 25mm with pressure of 1.5 bar and 20 l/m flow rate.
Firstly, it is important to understand that with any mains fed system (i.e. without stored water), the performance is directly related to the incoming flow and pressure.
The higher the mains water pressure the faster the water will flow, however the size and length of pipe work is equally important. As water flows, pressure is lost through the pipes and fittings. Even with a high mains water pressure, the resistance in a length of small bore pipe will reduce the available pressure, resulting in reduced flow rates.
In existing properties, the water supply can be gauged by opening a tap connected directly to the mains supply. Typically, these are the outside tap or kitchen tap. As you open up more than one tap, the ability of each tap to maintain the flow rate is an indication of the water available. Your local water supplier should be able to assist in testing pressures and flow rates properly. We always test the flow and pressure when we carry out a survey.
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.
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.
First thing to always check is that all valves are fully opened, including the main stop tap. Even with a good supply into a property, if the internal pipe work is undersized, damaged or restricted in some way, this could have a significant affect on the flow rates. Also some water meters and water softeners are a problem.
The local water pressure is set by the water undertaker, and 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 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
If the mains supply pipe work is unable to provide the peak water requirements then there are various options available:
See Accumulators and Pump Sets
That said, most properties have more than adequate water supplies, and mains fed hot water systems are an attractive and viable option.
As the Megaflo or Unvented system is pressurised, it can be sited virtually anywhere; it does not rely on a header tank to provide a head of pressure.
Common locations are within an airing cupboard, a downstairs cupboard, utility room, loft, garage or basement. However, consideration should be given to the requirement for a safety discharge pipe to run from the cylinder to either outside or to a soil and vent pipe.