I have a vaillant gas condensing open vent boiler which I am plumbing in at present. In the short term I plan on running my underfloor heating off it just to dry the floors out prior to fitting timber floor. Thats all it will do for the next 3-6 months. The water volume in the ufh is approx 50 litres. (Long term I will have a heatrae sadia system fit cylinder installled which comes with an expansion vessel, safety blow off and auto bypass etc. My question to anyone who can advise me is; Do I need to buy and fit an expansion vessel just for the ufh short term (3-6months). i will obviously fit a safety blow off and an auto bypass valve. Thanks.