LMSC Thanks Rob. Is this generally the case for water in general or if this is zero water specific — because my question was zero water related.
How does this (hardness roughly doubling alkalinity) play for the service boiler please ?
I’m not sure what you mean by that really. Zero water is basically pretty close to pure, similar to RO. If you’re asking how long the jugs remain effective for I don’t know, but if you’re diluting to 40mg/l alkalinity all the time you’ll get consistent results you’ll just need to use more water from the jug as the filter wears out (the filter might not gradually stop working).
At service boiler temps it drops, say around 125c above approx 50mg/l hardness: 40mg/l alkalinity will scale.
If you pull water from a service boiler to test it, if scale is forming your test will always show the water as non-scaling afterwards. You can use a test to see if the water has more minerals compared to what you’ve been putting in, but you can’t determine when it started scaling and how much scale has formed without knowing how much water you’ve lost when steaming.