Pressurised Cylinders vs Thermal Stores vs Vented Cylinders


You may be considering installing new, or changing an existing system to provide mains pressure hot water at your taps. If you are planning a solar installation, it makes sense to do the two jobs at the same time. Both types of mains pressure system work perfectly well with solar water heating. Often customers contact us having committed themselves to spending up to £2000 on a mains pressure hot water cylinder - before you do this, please look at the other options. We can probably supply you with a higher quality, less expensive tank, and if you consider a thermal store, you may be surprised to find that this is a better option and at a lowe price.

General Any system providing a mains pressure supply is only as good as the supply entering the property. A product may boast for example - 35 litres per minute flow rate at 3 bar pressure - but if the supply to your property is inadequate and can only muster say 15 litres at 1 bar, this is all you will achieve. Figures quoted against products are indicative of their potential based on an adequate supply.

How does it work? In general, a mains pressure system comprises a means of heating water taken directly from the incoming water main prior to delivery to your tap. There are several ways to do this and many different manufacturers interpretations on product design.

The Choices There are two main types of domestic Mains Pressure Hot Water Systems.
Unvented Mains Pressure Systems
Vented Mains Pressure Systems
The term vented refers to whether the system is partially vented to atmosphere or completely pressurised and therefore unvented.

Unvented Mains Pressure Systems

Navitron Unvented Cylinders are made from Copper, rather than stainless steel. This material, although more expensive, has superior heat-transfer properties, and is therefore ideal for maximising heat transfer from solar collectors.In an unvented system, incoming cold mains water typically enters a pressure vessel where it is heated either Directly by means of electrical heater/s, or Indirectly by means of a primary heat exchanger within the vessel being supplied by your central heating boiler. The hot water stored within the vessel is forced out by the incoming cold water when you open a tap hence - Mains Pressure Hot Water.

Vented Mains Pressure Systems

All Navitron Thermal Stores are supplied with a mixer valve and pressure vessel included as standard, to comply with all the latest water regulations. In a vented system, heat is imparted to the incoming mains water by means of a secondary heat exchanger within the vessel. Water in the vessel is used rather like a battery for heat - a THERMAL STORE. The incoming mains water passes through it's heat exchanger within the vessel drawing heat from the store. The Thermal Store temperature is maintained in the usual way - either Directly with electrical heaters, or Indirectly by your boiler. Because the water within the Thermal Store does not need to be under pressure (as with an unvented system) it is vented to atmosphere. The body of water within the Thermal Store is used purely as a medium for storing and imparting heat to the secondary exchanger. Considerations Unvented and vented systems both provide hot water at mains pressure. They achieve this by slightly different methods. There are however certain considerations when choosing the system that suits you best.

Unvented Mains Pressure Systems

In an unvented system you will be storing a large volume of hot water under pressure. For reasons of safety such systems must be installed by qualified technicians with relevant experience, CITB training and G3 certification.
As a precaution, pressure-relief pipework and valves must be installed to protect against unsafe pressure build-up within the vessel which could result in explosion.
Your Local Authority (Building Control Dept) will need to be advised of your intention to install an unvented system.
For reasons of safety, your system will require annual maintenance to ensure safety equipment is functioning correctly (BS2870)
An unvented system must be commissioned and certified by the installer.
You should not fit uncontrolled heat sources such as stoves to a mains pressure tank

Vented Mains Pressure Systems

A vented system does not store a large volume of hot water under pressure. The Thermal Store, whilst containing hot water, remains at atmospheric pressure. There are no hazardous issues with a vented system. Pressure relief valves and pipework are not therefore required. Vented systems do not require Building Control approval. Because of the simplicity and safety of such systems, installers do not need certification or specialist training. Vented systems do not need certification. There are no special annual maintenance requirements.

Potential Drawbacks of Mains Pressure Hot Water Systems Under water bye-laws you may not add a pump directly to your mains fed system to increase pressure or flow rate. If the supply in your house is poor there will be no simple way to improve it. Additional performance usually entails bringing in a new, larger supply pipe to the property. Even then, you should ensure the new improved supply will be adequate, as some areas suffer from frustratingly low pressure. Always remember - your system can never be better than the supply to it. Mains pressure systems are at the mercy of fluctuations in supply pressure. This may not be an issue in the majority of cases. Mains pressure systems can give 'power shower' performance, providing supply pressure is good. If you like a vigorous showering experience and your supply pressure is poor, remember - you cannot install a shower booster pump. If you simply must have a power shower then consider a gravity fed system. You can do pretty much anything you want with a gravity feed.

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