Understanding Your Unvented Hot Water System

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Many of the modern properties in Milton Keynes are fitted with Unvented Hot Water Systems.


How to tell if you have an Unvented Hot Water System

Unvented tundish plumbing
Unvented Hot Water System – Tundish

You can tell if you have an unvented hot water system if you have a large (usually white) water cylinder in your property and if you have a tundish (see picture) this indicates that you have an Unvented Hot Water system.

If you have a hot water cylinder but no tundish then it’s likely that you have a vented system which is more common on older properties and is fed by a cold water storage tank in the loft.


How your Unvented Hot Water System Works in 3 Simple Steps.

  1. In an Unvented system your hot water is fed directly from the cold mains water supply into the tank.
  2. Once inside the tank the hot water is heated up to temperature either by your central heating system or by an electric immersion heater or a combination of both.
  3. When you turn a hot tap on cold water pushes in through the bottom of the tank and forces hot water out of the top. ( A process called stratification means that the hottest water will always rise up to the top of the tank and the coolest water will fall to the bottom.)


Unvented Hot Water System Safety

  • An unvented hot water system stores a large volume of hot water at extremely high temperatures. Hot water at high temperatures and high pressures creates a risk of major hot water leaks or, in extreme cases, explosions.
  • Due to the potential safety risks, your unvented hot water tank comes with a number of safety devices which need checking annually by a G3 Hot Water Qualified Plumber.
    hotwater cylinder explosion
    Hotwater cylinder explosion


    The Safety Devices

  • The first safety device on your cylinder is a pressure reducing valve. This limits the water pressure coming into the system in case of surges in the mains water supply.
  • The heaters on your cylinder should also be fitted so that they automatically cut out once a certain temperature is reached.
  • The second safety device is an expansion vessel this is a pocket of air that absorbs excess pressure. It may be inside your cyclinder or you may have a big metal ball external to the cylinder. This is the second line of defence against excess pressure and needs checking and refilling on a regular basis.
  • The next line of defence against excess pressure is the pressure relief valve (PRV) on the incoming water supply. These usually have a red cap and will discharge water into your tundish if the pressure in the pipework gets to high.
  • The last line of defence is the temperature and pressure relief valve (T&P) this also has a red cap and discharges water into the tundish if the temperature or pressure inside the cylinder gets too high.

unvented hot water



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