Hi,
To work out how long your resin will last, the calculation is simple.
Litres of resin in demin tank x 22,727 divided by the TDS of the water going in to demin tank = no of litres of water which the tank will handle before the resin needs changing.
So, to give a worked example, if your demin tank has 10 litres of resin, and the TDS of the water from your Merlin is 18, you'll need to change the resin after you've produced about 10 x 22,727/18 = 12630 litres of water. This is the problem with the Merlin. If your tap water TDS is very low, and you don't use much water, it's great - if not, resin and waste water costs can be astronomical. It's a great new machine, and we sell it in the right set of circumstances, but each case should be considered on its merits.
Yours, Andrew