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Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions.Temperature is a monotonic function of the average molecular kinetic energy of a substance. When a substance is heated, molecules begin to vibrate and move more, usually creating more distance between themselves. Substances which contract with increasing temperature are unusual, and only occur within limited temperature ranges (see examples below). The relative expansion (also called strain) divided by the change in temperature is called the material's coefficient of linear thermal expansion and generally varies with temperature. As energy in particles increases, they start moving faster and faster weakening the intermolecular forces between them, therefore expanding the substance.

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  1. P

    URGENT Do I need a expansion vessel fitted to my system

    I have just had a water meter fitted and since been told I need a small expansion vessel fitted to my domestic hot water system as the meter will have a NRV in. I have also been told no I don't as my boiler is over 10meters away from the meter and any expansion will be taken up within the...
  2. R

    Why do I have 2 Expansion Vessel

    I have Gledhill 300L unvented water cylinder storage system with Vaillant Boiler. We are using it for UFH and radiators. My plumber has fitted 2 expansion vessels 1) Zilmet - 18 Litre Red Heating Expansion Vessel & Sealed System 2) MULTIFUNCTIONAL TANKS WITH REPLACEABLE MEMBRANE Why has he...
  3. M

    Expansion vessels and charges

    What causes an expansion vessel to loss it's charge and at what rate would you expect it too? Thanks
  4. M

    Expansion vessel hose

    Do many merchants sell 1 meter long 3/4 male to 3/4 female. The 3/4 male screws in to the composite valve and the other end the expansion vessel. I presume came supplied with the composite valve. The flexi is leaking.
  5. J

    Advice combi boiler plumber says can't fix expansion vessel/access it?

    Hi there, I was wondering if anyone could advise if I should get a different plumber for a 2nd opinion. Basically, my combi boiler is in my attic. It is overpressurised at the moment and the overflow pipe always drips. I had a plumber round, he seemed good and honest and knew the neighbour so...
  6. R

    Hot Water Tank Expansion Vessel Question

    Am I correct in saying that the capacity of an expansion vessel as written on the appliance data label is the capacity of the water compartment alone and excludes the air compartment capacity? Thanks
  7. C

    Expansion vessel for potable water for a combi boiler

    Hi,a customer of mine has a combi boiler and they don't have an expansion vessel for the potable hot water supply. I know there's an expansion vessel for the closed-loop central heating circuit but I doubt the combi boiler has one for the potable water. I was thinking to put one in the airing...
  8. N

    Ferolli expansion vessel

    I need a new expansion vessel, problem is I've got a optima 901 and the flue goes up about a foot and then into the back wall and the vessel is at the back, it's one of the round ones, I've got to get a engineer into do it, I'm assuming that they take the flue out and lift it from the back...
  9. C

    Expansion Vessel Pressure

    Hi, Should an expansion vessel pressure match the pressurisation unit? For example, if the pressurisation unit is set at 1.2 bar should the expansion vessel be set at 1.2? Many thanks
  10. K

    No water sound when turning filler tap by expansion vessel.

    Have an Ariston unvented hot water tank in airing cupboard with the red and white expansion vessels and a filler tap. As advised years ago on these forums when I hear that sound that the central heating water needs topping up I usually wait until cold so the gauge goes to zero and top up to 1...
  11. M

    Sizing expansion vessels

    Has anyone got a good rule of thumb for sizing an expansion vessel for a central heating system. A combi has a 8 litre vessel. And will deal with the expansion of say 8-10 rads. Could one use this as a ball park figure and multiply up from there Obviously pipe work to take in to account 22mm...
  12. H

    Pressure loss in Heating System post fitting new Expansion Tank

    Hi, hope someone can help me with a heating pressure loss problem. My valiant boiler has been loosing pressure over the last few months, resulting in me having to top up the water every week or so. Managed to get a boiler engineer to visit who diagnosed a faulty expansion tank. (Water evident at...
  13. M

    New expansion vessel on water system - is pressure too high?

    Hello all, I'm currently in Ireland and in a small cottage where the plumbing system has me scratching my head a bit. When the place was done up the builders decided that the tank in the loft was no longer needed and put a tank out back in a small extension where ALL (well, except for the cold...
  14. S

    Vaillant Ecotec Plus 832 Expansion Vessel

    Hi Guys, Had a quick question regarding expansion vessel on a Vaillant Ecotec Plus 832. Was servicing the boiler and upon attaching hand pump to vessle after having drained the boiler, returns and flows isolated and drain off on boiler open, the vessle showed 1 bar charge. Was wondering why...
  15. W

    Maxi Pod expansion tank

    I have a Maxi Pod installed and the overflow of the expansion tank keeps overflowing. i think it maybe because my expansion tank is to small. It is only a 7 gallon tank and I believe Maxi Pod recommend a 20 gallon expansion tank. Question: why do Maxi Pod recommend such a large tank?
  16. M

    Unvented cylinder problems - over pressure or faulty expansion tank?

    Hi, New to this forum and also complete novice who knows pretty much nothing about plumbing. However I have done quite some google searching trying to find some answers. I live in a 1st floor flat which uses an electric duel element direct unvented water heater cylinder (150 litres). Recently I...
  17. D

    Leaky Gledhill expansion relief valve

    Hi I have a leaky Gledhill expansion relief valve block a little like this one with the same spec 3bar on the black input and 4.5 bar on the red expansion relief value. It protects a 250L Gledhill unvented tank. All the input pipes have isolation valves so pretty easy to exchange components...
  18. S

    Hi all our boiler has just had a new expansion vessel fitted but pressure is still going to 2.9 bars when using central heating . Is this normal? Than

    Hi All our boiler has just had a new expansion vessel fitted bit the pressure is still rising to 2.9 bars . Is this normal ? Many thanks
  19. C

    What expansion vessel should i order for Boiler

    Hi, Currently have a Potterton Promax Combi 28 HE A boiler and looking to replace expansion vessel due to issues with it. Looking to make sure i get the right part for it Found this part online but it says it will Fit a Promax Combi 28 HE Plus A,Is the HE plus A and my boiler HE A the same...
  20. M

    System boiler kw sizing and expansion vessel

    Hi, After a bit of a saga with my plumber, I need to chose boiler size as soon as possible, the plumber is not going to do any heat loss calculations or do anything to work out accurately so I am on my own. Appreciate this is not what should be doing but after some level of finger in the air...