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A combi will struggle if you have low pressure and have more than one person using a shower at a time.
Do all of your shower rooms use direct hot water or do they have electric showers, or shower pumps?
If your going for un-vented, then this is pressurised by mains water, so it can be located anywhere in the house.
You would normally locate it next to the highest demand of hot water to reduce the pipe run, ideally next to the showers, however if its cheaper to place it down stairs its not a big issue, just means you have to run the hot water tap for a little longer.
You shouldnt hear anything with the new unventted cylinders, you will hear water going through the pipes more than than any news from the cylinder, you may also hear the water circulating when its heating up but by in large you shouldnt hear it, if its behind a cupboard door.
I have a mega flow in a 4 bed house, and we leave the airing cupboard door and never hear anything. Please rate the service using the star rating at the top of this page.