A sealed system is what I think you mean? In this there should be 1. a pressure relief valve (this should discharge to a safe position, normally outside - could yours go into the cold water storage system? (it shouldn't, but...)) , 2. an expansion vessel, and 3. a pressure gauge somewhere, often all three are incorporated into the boiler itself. These are the industry terms, please Google them to check we are in understanding.
These three items replace the open-vented system components: feed-and-expansion cistern, cold feed, and open vent pipe. Other than that the systems are pretty similar, but I'm assuming we're talking about what your talking about, which may be a mistake.
Does the system have a combi (combination) boiler that provides hot water on demand, or is there a copper cylinder in the house?
Can we have a picture of the cylinder please, as I'm assuming that what you have is a traditional vented cylinder, but if it's an unvented cylinder, then it's a potential bomb if plumbed incorrectly.
Are you changing anything or just replacing pipes you have damaged in the process of lifting a floor?