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I just flushed out my baseboard heating system and refilled it with new water. I am wondering, should the cold water supply to the system stay open or should it be a closed loop until flushed again at another date?

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Closed.

Two reasons - main one, you need to know if it's leaking. A properly functioning system should not need "make up water" on a regular basis.

The other, related reason - if the system is leaking and constantly taking in new water, it's also taking in new dissolved oxygen in that water - and 99.9% of hot water heating systems are built with a mixture of copper and iron parts (or all iron parts) that will rust terribly if continuously supplied with oxygen. A non-leaking system rusts out what oxygen is in the water, and then stops rusting. Every time you add new water it rusts more; so the less you add new water, the less the interior of the system rusts.

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Not to mention the build up of scale from new calcium in the make-up water, if it's hard... –  User58220 Dec 29 '13 at 22:26
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With the fill valve open, and there is not a backflow preventer installed (more common than you think) you could contaminate your drinking water if the incoming water pressure were to drop. –  mikes Dec 30 '13 at 2:19
    
I've already answered this but if you have a backflow preventer along with a 12-15 PSI regulator installed, would it make sense to leave it on in that case? –  masedesign Jan 7 at 21:42
    
No. Read the answer again. The answer does not assume that you are missing either of those devices. –  Ecnerwal Jan 7 at 21:51

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