Quote:
Originally Posted by SailinAway
I feel quite sure it's water. It makes a gurgling sound. I won't let the upstairs fall below freezing.
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It is water. But when the system is fully charged - no air in it - it can't gurgle.
Air in the system allows it to emit a gurgle... and the separator when the circulator is active allows for the air to be removed from the system protecting the boiler.
If it has a real strong gurgle, almost sounding like water flowing, there may be a problem with the fill valve. It replaces the volume of air removed by the separator with additional water keeping the system charged.