Mini Expansion Vessel

[Bollod]

First post since intro!
Worcester 28cdi in property when we bought. Worcester engineer when servicing says should have a mini expansion vessel in cold supply because a water meter is fitted. Boiler has worked with very few problems over the years so why is mini expansion vessel needed now? Always had a water meter and it is approx. 30 meters from boiler position.
Retired plumber friend says not needed because that length supply pipe gives plenty of expansion. (I understand the idea of hot water pushing back into cold supply)
However, since a liquid is not compressible how would it allow expansion.

Don't want to spend money unnecessarily so am asking for guidance

 

[townfanjon]

Yes Worcester want them , it will help protect the plastic block within the boiler , if the boilers had brass blocks they wouldnt really need them .

 

[Bollod]

Yes Worcester want them , it will help protect the plastic block within the boiler , if the boilers had brass blocks they wouldnt really need them .

Thanks for that, looks like I must bite the bullet!
Seems to me that with boiler manufacturers giving longer guarantees you have to jump through more hoops so not to give them any wriggle room to avoid paying out!

 

[Stanios]

Liquid is compressible your pressure gauge on the boiler should tell you that!

 

[Bollod]

Liquid is compressible your pressure gauge on the boiler should tell you that!

As a new member of the forum I am sorry but have to disagree. The pressure shown on the gauge is pressure exerted on the water in the sealed system, it does not actually compress the water.
If liquid were compressible hydraulics on a digger or the brakes on a car could not work!

 

[Stanios]

As a new member of the forum I am sorry but have to disagree. The pressure shown on the gauge is pressure exerted on the water in the sealed system, it does not actually compress the water.
If liquid were compressible hydraulics on a digger or the brakes on a car could not work!

Its fine to disagree!
I pressure test sections of pipes and radiators (no expansion vessel there) and can pump it up to 10 bar if I like, every stroke of my pump sends additional quantity of water into an already full sealed radiator compressing the watter. So from 1 bar to 10 bar I will deliver another 200ml of water. I can then cap a radiator that had 1 bar of pressure and another that had 10 bar and you will find the 10 bar radiator has more water.

 

[Bollod]

Fair point but would suggest that the rad had expanded slightly under the pressure to take the extra volume.
And my point about hydraulics still stands.

 

[AlexGas]

Liquid is compressible...

The answer is NO, one could compress water though, but only by a tiny smidge. It could be a long thread...

 

[Stanios]

The answer is NO, one could compress water though, but only by a tiny smidge. It could be a long thread...

You can compress any liquid apparently https://www.google.co.uk/search?q=c...1&ved=0ahUKEwiV1YSGvsbOAhUlP5oKHS58B58QvwUICg

 

[AlexGas]

You can compress any liquid apparently https://www.google.co.uk/search?q=c...1&ved=0ahUKEwiV1YSGvsbOAhUlP5oKHS58B58QvwUICg

The answer is yes, You can compress water, or almost any material. However, it requires a great deal of pressure to accomplish a little compression. For that reason, liquids and solids are sometimes referred to as being incompressible.

 

[AlexGas]

In order to compress water, you would need so much pressure. Even at the deepest regions of our oceans, the density of water is only a little higher than that of the water at the surface, and the compression is approximately around 1.8%
I never knew that... Thanks Google!

 

[Best]

If water wasn't compressible then a sealed heating system or unvented unit would have no pressure inside it and if you isolated the supply to it water would not blast out if you opened a valve. Obviously it does, because the water will have pressure. The expansion vessels do add pressure in one sense, but without their air charge the water will still be compressed

 

[AlexGas]

If water wasn't compressible then a sealed heating system or unvented unit would have no pressure inside it and if you isolated the supply to it water would not blast out if you opened a valve. Obviously it does, because the water will have pressure. The expansion vessels do add pressure in one sense, but without their air charge the water will still be compressed

It's compressible by next to nothing!!!!!
The volume of a quantity of liquid is fixed by its temperature and pressure. Liquids generally expand when heated, and contract when cooled. Water between 0 °C and 4 °C is a notable exception. Liquids have little compressibility. Water, for example, will compress by only 46.4 parts per million for every unit increase in atmospheric pressure (bar).[11] At around 4000 bar (58,000 psi) of pressure, at room temperature, water only experiences an 11% decrease in volume.[12] In the study of fluid dynamics, liquids are often treated as incompressible, especially when studying incompressible flow. This incompressible nature makes a liquid suitable for transmitting hydraulic power, because very little of the energy is lost in the form of compression.[12] However, the very slight compressibility does lead to other phenomena. The banging of pipes, called water hammer, occurs when a valve is suddenly closed, creating a huge pressure-spike at the valve that travels backward through the system at just under the speed of sound. Another phenomenon caused by liquid's incompressibility is cavitation. Because liquids have little elasticity they can literally be pulled apart in areas of high turbulence or a dramatic change in direction, such as the trailing edge of a boat propeller or a sharp corner in a pipe. A liquid in an area of low pressure (vacuum) vaporizes and forms bubbles, which then collapse as they enter high pressure areas. This causes liquid to fill the cavities left by the bubbles with tremendous localized force, eroding any adjacent solid surface.

 

[AlexGas]

That's only 46.4 parts per million for every unit increase in atmospheric pressure (bar)!!!!!!!!!!
Next to nothing!!!


It's here https://en.m.wikipedia.org/wiki/Liquid

 

[Bollod]

I thank AlexGas for for his very complete explanation! (And Google )

 

[Stanios]

To be honest is this was such an issue for combi boilers they would fit them internal as standard

 

[townfanjon]

To be honest is this was such an issue for combi boilers they would fit them internal as standard

One of the reasons I fit Viessmann

 

[Bollod]

To be honest is this was such an issue for combi boilers they would fit them internal as standard

There is no issue if no non return valve in cold supply. I've now read two installation instructions on line that do specify a MEV if that is the case.

 

[huddster]

As previously mentioned the mini expansion vessel will help reduce stress on the plastic manifold. If the boiler has pre heat on, massive amounts of pressure will build up (especially when not in use during the night) and weaken the manifold which will eventually pin hole. Ive only come across it once but apparently a very common problem on Worcester.