by san-ho-zay » Sun Jul 15, 2012 10:31 am
Boiling water reduces temporary hardness (bicarbonates, a.k.a. KH). The result is the "scale" on your kettle. It doesn't affect permanent hardness (calcium and megnesium, a.k.a. general hardness or GH). If you have hard water the effect of warming some water as part as of a water change is not likely to be significant. Boiling also drives off dissolved air, which would be significant if you only used boiled water but is not going to be an issue if you mix with cold water, which is saturated with dissolved air.
In other words, using a small proportion of water from the kettle to air cold water should be OK.
I can't think why your boiler would introduce nitrates. A quick search reveals that corrosion inhibitors often contain high nitrite levels but they would only be used in a closed circuit like radiators. Are you able to persuade the boiler to give you water that's heated on demand only? If so, you could test that, which would point to your Economy 7 storage tank. Did you test your hot water for nitrite also?
Richard