I have performed a couple of simple experiments to check if Hot Water has any effects on Pure Water, these are my findings.
Cold Water
I left a brass fitting in a bowl of Pure Water with a TDS reading of 000, within 6 hours the TDS reading had risen to 6 and then 12, 6 hours later, after 2 days the reading had settled at 22 and it was going up by the rate of 001 every day there-after for the last 4 days.
I then put this brass fitting in a bowl of Pure Water with a TDS reading of 000, I heated the water up to boiling point and checked the TDS reading, which had risen to 49 and still climbing.
I then boiled a bowl of Pure Water with a TDS reading of 000, I checked the water when it was uke warm and the TDS was 001, I then brought the water to the boil and the TDS had risen to 003.
I then boiled another bowl of water with a stainless steel fitting and the TDS had isen to 10 TDS.
My conclusions are that with hot water, if you have brass fittings then you water will rise to a level that will leave marks on the glass, and with stainless steel fitting the TDS level is acceptable.