There are a couple of old threads on DVS if you want to search, ie.
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We have a Brivas system which is gas central heating and the vents act like a DVS system. I can't recommend it more, it's brought the humidity right down and the place is drier (we used to get mould quite bad because we're in a valley) and it's lovely and toasty in the winter. It cost about 7k - get several quotes as the difference between quotes was huge. But I'm sorry I can't give you advice on what a standard DVS system is like.
They installed it in a day. The cost of heating our place is minimal, we keep the temp at 19-20 degrees year round.