Curious Radiant Heating Problem

Working on a previously healthy 4 port manifold radiant system. The thermostatic part of the system seems to be working... the pump (Grundfos UP 15-42F) & boiler (shows 140) come on & the motoroized actuators respond correctly.
The mystery is that only 2 zones (closest to the boiler input end of the manifold) get flow (both input & return tubes heat up)... even when manual depressing the actuator valve, there is no discernable heat in either of the other 2 tubes. The manifold & actuators were new 6 months ago and it is reported that everything worked fine, until recently.
What could have happened that would cause these 2 zones to not flow????
Any thoughts greatly appreciated.
jr
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It was 95 degrees today. Heat is not needed for another three months. Don't worry about it.
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System is not balance properly or zone valves do not work or perhaps you have air the system and you not getting full circulation "which is most likely" DIDO

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What happens when you close all of the zones except one of the zones w/o flow?
Did you try purging the zones that aren't flowing?
On 26 Aug 2006 19:25:46 -0700, snipped-for-privacy@yahoo.com wrote:

--
All things come to him who waits... As
long as he works like hell while he waits.
  Click to see the full signature.
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Thanks for all the responses... the culprit here was a huge amount of calcium buildup and corrision... the two bad loop's manifold valves were completely eaten apart, that is, the shaft was no long connected to the closing seal... so, up or down, nothing was happening as far as opening or closing the circuit. Should have checked that earlier, but this was a new manifold installed just 8 months ago.
That solved, the question remains as to why so very much corrosion in such a short period of time...????
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Dissimilar metals. Easily corrected with proper water treatment, and sacrificial anodes.
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You need to call these folks:
http://www.gewater.com/index.jsp
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