Expansion tank is it big enough.

[Bogart]

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?
Thanks

 

[ShaunCorbs]

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

 

[king of pipes]

I think you need to employ the services of a heating engineer Bogart regards kop

 

[Bogart]

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.

 

[SimonG]

About 12 litres.

 

[Bogart]

Sorry lost me 12 litres?

 

[oz-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.

 

[Bogart]

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

 

[bacon_sandwich]

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

 

[snowhead]

Twin 18ltrs.