Any guidance on whether a flow rate of 20litres/minute is enough flow for an unvented cylinder/thermal store to run a shower with another outlet, say kitchen sink or flush a toilet being used at the same time?
An unvented cylinder will work with low or high flow rates.
At 20 litres per min you will definitely notice the modulation of the shower by another appliance being used simultaneously. How much that would concern you is a personal thing.
If you can't live with it, install a GAH DualStream.
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