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Water heaters - unvented megaflows and cylinders

As with central heating systems there are quite a few variations available that may suit a new installation or indeed you may already find them in your own home. Perhaps one of the main options when it comes to choosing a cylinder is whether the cylinder is of an unvented or vented type. For further information on unvented water cylinders please visit the Megaflow page and for vented cylinders please visit the water cylinders page.


megaflow unvented cylinder

Megaflow are a renowned brand for unvented hot water cylinders, we always like to recommend Megaflow's because of their reliability and quality. We are qualified to work on and install unvented Megaflow cylinders - please note not all Gas Safe engineers are qualified to work on this type of system.



water cylinder
Water Cylinders

Open vented or traditional cylinders are still widely used mostly in older systems. Most vented cylinders are fed by a cold-water storage cistern usually located in the attic, whereas unvented cylinders are mains water fed.



Direct or Indirect?


Both unvented and vented cylinders can be either what is called 'direct' or 'indirect'. This can be referring to two important aspects, how the mains water is fed into the system, either directly into the pressure vessel contained within a cylinder or indirectly into to the cold water storage tank. Or how the water is heated within the hot water cylinder, directly using an electric immersion heater or indirectly using a heat exchanger supplied via a boiler. Many households use a combination of the two, indirect for normal heating and only use their immersion heater as back-up. There are a number of differences between each method and a number of key benefits and disadvantages as outlined below.

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unvented cylinder

Water delivery - if you use a combination boiler or unvented cylinder such as a Megaflow the mains water entry is primarily considered to be direct. This is because the water is fed directly to the taps, showers, and other appliances directly and at mains pressure.


Advantages - Unvented cylinders provide hot water at mains pressure throughout, this also means you do not need a cold water storage tank and associated pipework. As you do not rely on gravity, you are considerably less restricted in where you site your cylinder.


Disadvantages - You can not use a unvented cylinder with power showers and some mixers. Also because an unvented cylinder requires specialist installation they can be more expensive than conventional cylinders.


directly heated cylinderWater heating - in a directly heated system an immersion heater will be used to heat all of the water contained within the cylinder. As the immersion heater is ran from electric there is no reliance on your boiler to provide the heat.


Advantages / Disadvantages -An immersion element is convenient and relatively easier to install and operate. However the biggest disadvantage is cost, electricity is relatively far more expensive to heat your domestic hot water supply than through a heat exchanger.

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conventional cylinder

Water delivery - if you have a traditional gravity fed or partially pumped system (not sealed) then the mains water entry is classified as indirect. This is because the mains water is fed to the cold water storage tank, and from here on to the appliances. Thus all appliances requiring hot water are fed indirectly via the cold water storage tank.


Advantages - Perhaps the main two benefits of a vented cylinder is the fact they are much less complicated which means they are easier to install, lower in price and easier to maintain.


Disadvantages - In contrast to an unvented cylinder a vented cylinder must still retain a cold water storage tank, not so good if you want to save space. You must also rely on a gravity fed system for your hot water pressure which can be substantially less than mains water pressure.



indirectly heated cylinderWater heating - in a indirect heating system your water will be heated indirectly by a primary heat exchanger within the vessel being supplied by your heating boiler.


Advantages / Disadvantages - Using a heat exchanger within the vessel is far more economical option. Hot water from the boiler flows through a series of coils within the hot water cylinder, which in turn heats the water. However if you solely rely on your heat exchanger and you have a problem with your boiler, you could be left without heating whilst you make the repairs on the boiler.

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Combination vented cylinders


combination vented cylinderA combination water storage cylinder has its own cold-water header tank fitted on top of the hot water cylinder. Essentially a combination cylinder combines the hot water vessel and cold water storage tank into one unit. This means you can do away with your storage tank in your loft and get everything built into one relatively compact unit.


Advantages / Disadvantages - A combination water storage cylinder is ideal for a small flat or where space is at a premium, it is also cheaper to install than alternative storage systems and also maintain. However the main disadvantage in using a combination cylinder is water pressure, as the cylinder still relies a cold water storage tank, albeit a built in one, it still relies on gravity. Given that the cold water storage tank only sits above the cylinder, pressure levels can suffer a great deal. To try and combat this somewhat, the combination cylinder must be located above the highest outlet.

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Other water heating systems


mult-point water heater

Instantaneous Multi-Point Water Heater

These are room-sealed appliances (requires no ventilation) that can give continuous hot water to various number of taps at once. They are totally independent of any central heating system and so can be fitted in homes without central heating. They can also be used to give instant hot water with any other type of heating. When a hot tap is turned on, the burner fires up heating the water and delivering it at about 8-10 litres/min (dependent on incoming water supply) for as long as the tap remains open.  Although several taps, showers etc, can be supplied, realistically water can only adequately flow from one draw off point at a time. 



sink water heater

Sink water heaters

These are a small, flueless appliance giving single point continuous hot water to supply a single tap or spout. Sink water heaters are ideal for cloakrooms or small offices where a single heat source is required. There are two options to where they can be sited, over a sink (as pictured) or more concealed under the sink, both options are convenient for hand washing, but not much more.


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What we can do for you


Choosing a system that not only suits your needs and matches your own criteria whilst also being efficient and cost friendly is no mean feat. We try to offer as much information as possible so you can base your decisions safe in the knowledge that your choice will be the right one. Why not let our technical surveyor advise you and offer practical suggestions based on years of experience.


We always want you to feel confident in your decision and confident in us to do the job to your specification. Based on you requirements our surveyor can advice what can and can't be done, what is and isn't practical and what will be more economical in terms of your money and the environment. So if your cylinder is need of repair or you are thinking of upgrading your old cylinder or installing a new cylinder please give us a call.


Please contact us to discuss your requirements:corgi and oftec approved


TG Services

1 Wintershill Farm

Wintershill Estate

Durley, Southampton

SO32 2AH

Tel: 02380 000701



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