Your guide - unvented hot water system

With more homeowners installing an unvented hot water system in their homes (you've probably heard of a Megaflo system), we decided to understand exactly what an unvented hot water system is and the benefits of having one.


What is an unvented hot water system?

Let's first understand the traditional hot water system which remains the most popular in the UK, mainly due to convention and cost. The traditional heat only system is where a cold water expansion tank is kept in the loft space and it's the height of the cold water tank above the hot water cylinder that produces the pressure that delivers the hot water to the taps - essentially relying on gravity to drive the pressure. This works well for taps on lower floors of your home however the pressure is often not enough for the shower or the bath. This gets even worse when you are running two showers simultaneously.

An unvented hot water system, such as a Megaflo, is essentially a hot water tank which is pressurised to the mains water pressure, meaning that you get a strong flow of hot water no matter where you are in your home. The cold water is fed into the hot water tank directly from the mains, with no requirement for a cold water tank. It's the mains pressure which forces the hot water out of the top and then delivered to the taps.


What are the benefits of installing an unvented hot water system?

If you are struggling with pressure in your shower then an unvented hot water system will solve this without having to install a noisy pump. It's also an ideal system if you have a busy family home and want to be able to run various taps at once. It will support two or more bathrooms and you will feel very little change in pressure when you have various taps running - unlike conventional systems where you can often be left with a trickle of water! The unvented hot water system is recommended for large and busy homes so that you really notice the benefit.

Unvented hot water systems are also energy efficient as the tank is insulated with polyurethane foam, meaning that once the water is heated it stays warm for longer, so there is less of a need to re-heat the system.


What kind of boiler system do you need?

In a similar way to all hot water systems, any unvented hot water system can be heated by gas, electrical immersion units or solar power.


How much does an unvented hot water system cost?

An unvented hot water system is of course more expensive than a standard hot water tank, however the benefits are considered to be worth the investment. Ensure that you choose a good quality system which will last well. The most popular system is a Megaflo but leading brands such as Worcester Bosch, Viessmann and Gledhill are also reputable.

An unvented hot water system is normally more expensive to install than a standard hot water tank as the high pressure means that you must get a boiler specialist who is qualified under G3 building regulations to install it.

Expect to pay in the region of £1,000 - £1,500 for the supply and installation.


What about servicing?

Like your boiler, you should have the unvented hot water system serviced annually. All unvented hot water systems have a pressurised vessel to allow the increase in volume of water as it heats up. Some systems, like the Megaflo, operate this internally within the tank itself through an air-bubble which acts as the expansion vessel. Over time, this air-bubble can be absorbed into the water therefore an annual service is important to recharge this air-bubble.


Is there anything else to consider?

As the unvented hot water system operates at high pressure, it's important that your pipes are able to withstand the pressure in order to allow the system to function well.

Published: July 27, 2018

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