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Discuss Expansion tank is it big enough. in the Central Heating Forum area at

  1. Bogart

    Bogart Member

    Doing a bit of a revamp of heating system some rads out new ones in . Took old galvanised expansion tank out of loft. Now I reckon this thing had a capacity of a good 100 litres. Looking at replacements they seem to be 18 litre and that is it. Now I have some 22 rads and what with a run to and from the boiler I recon there is roughly about 300 litres in the system. Using an online calculator it states that my 300 litres say at 5degC will be 317 litres at 75degC. Will not be this as obviously water might leave the boiler at 75degC but the return might be only around 45deg C so average is 60degC hence an averaged rise of 55degC for the sake of calculations. This give a heated volume now of 313.5 litres ie expansion of 13.5 litres.All this is leading to is an 18 litre expansion tank sufficient for my volume of water? If not is there an alternative to not using a cold water cistern sized thing which is way oversized?
  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    You sure it was an f and e tank and not supplying your cylinder (for your hot water) ?

    Also by the looks of your post your out of your depth as that calc is for a sealed system

    My best advice is get a gas safe engy in
    • Agree Agree x 1
  3. king of pipes

    king of pipes Trusted Plumber GSR Top Contributor!!

    I think you need to employ the services of a heating engineer Bogart regards kop
    • Agree Agree x 1
  4. Bogart

    Bogart Member

    Definitely expansion tank the water tank is next to where the expansion tank was. Not sure how you think my figures are for a sealed system.
  5. SimonG

    SimonG Trusted Plumber Top Contributor!!

    About 12 litres.
  6. Bogart

    Bogart Member

    Sorry lost me 12 litres?
  7. oz-plumber

    oz-plumber Plumber

    If what you are saying is correct - just go an purchase a 24 litre expansion tank.
    Could be an overkill - but it will work.
    I did some oldschool calcs - 18 should be fine but if it were me, I would put in a 24.
  8. Bogart

    Bogart Member

    I would if I could find one. Here we seem to go from 18 litres straight to 114 litres ie 25 gallons
  9. bacon_sandwich

    bacon_sandwich GSR

    Are we not at apples and pears here. Assuming the expansion tank in the loft is for a vented gravity system and the tanks he is looking at are pressure vessels for an unvented sealed system ? although i dont have any trouble sizing vessels in small increments here
  10. snowhead

    snowhead Well-Known Member

    Twin 18ltrs.
  11. Matchless.plumb

    Matchless.plumb Trusted Plumber Top Contributor!!

    I'd recommend an Plumbing and heating engineer.
  12. Chuck

    Chuck Active Member

    That seems rather a lot. Between two and three times what I'd expect for a domestic system. DId you measure it by counting buckets when you drained it?

    How big was the old expansion tank? If you look carefully inside it you can probably see the high-water and low-water levels represented by rings of crud and corrosion.
  13. Bogart

    Bogart Member

    No tanks am looking at are for gravity fed system.
  14. Bogart

    Bogart Member

    Did actually think of that still a possibility.
  15. Harvest Fields

    Harvest Fields Trusted Plumber GSR Top Contributor!!

    So are you trying to convert the system or trying to keep it as a gravity fed?
  16. Bogart

    Bogart Member

    I think I am not too far of the volume. Worked it out by radaiator size and then adding volume in pipework. The boiler is some 15m plus away from the house and feed and return are in 32mm plus there is a fair bit of 28mm and 22mm knocking about soon adds up. Old expansion tank was I reckon a good 100 litres minimum has been at the tip for weeks so no chance of looking at levels.
  17. Bogart

    Bogart Member

    Keeping as gravity fed.
  18. Chuck

    Chuck Active Member

    Fair enough. As a rule of thumb, you want 10 cm depth of water in the FE tank when the system is cold and then space for an additional 5% of the system water to expand into. So, by this method you would need a bit more than 18 litre.

    Polytank, for example, do 55 litre rectangular tanks in three shapes:


    I'd consider a single tank as a nicer solution than two 18 litres linked into a pair. The footprint will be smaller and easier to install and insulate.

    The fact that the previous tank was 100 litre suggests that the previous installer decided (or discovered!) that a standard 18 litre (4-gallon) FE wasn't big enough.

    [Afterthought: Check before buying that Polytank approves use of their rectangular tank as a CH FE application. I'm 99% sure they're suitable, but I can't find it explicitly specified on the website.]
    Last edited: Aug 8, 2017
  19. Bogart

    Bogart Member

    Strangely enough I had seen their tanks before online and at the back of my mind thought might be the solution. They have today confirmed suitable, suppose is obvious as they supply the B and Q feed and expansion tank.
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