air in system plus soot, hot water circulation ok on top floors and not the bottom floor

i have a gas, hot water system that uses radiodors, circulator and expansion tank, i guess its a standard type of system. the boiler is in the basement, there are two additional floors above, each floor has two units, with a total of 6 units with six heating zones. i'm having a weird problem, i have heat going to the top floor two units, 1st floor two units are also ok, the problem is the two basement units, the heat is not circulating to the bottom two units. i checked the zone valves and the electrical power to them, this seems ok, i also get no circulation to these units when the zone valve is set to open, i know the zone valve is not stuck because i can open the system to bleed out the air. So i've also blead the system, each unit individually, i did this for the second time yesterday using a bucked which a friends suggested, although it took longer i could see the bubbles better, i did also notice the water being almost black with a lot tiny black particles, i guess this is soot from the boiler, is this normal? So it seems like i still have air in the system, what could be causing the air to enter the system, and why would the bottom two units not get circulation and the top floors are ok, thanks in advance.
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Black, stinky water in a boiler is exactly what we like to see.
Are you sure your circ is working?
On 5 Feb 2007 06:03:17 -0800, snipped-for-privacy@yahoo.com wrote:

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heat is good upstairs, and i can manually blead the water out of the basement zones, very weird.
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Heat upstairs and being able to bleed water doesn't mean the circ is working.
On 5 Feb 2007 09:18:24 -0800, snipped-for-privacy@yahoo.com wrote:

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The basement loops may be air-bound. This is a factor to consider when loops travel below boiler level. It's addressed with a appropriate way to bleed the whole loop at one time, usually when the system is installed, as opposed to individual bleeders that you have elsewhere.
Not something you want to tackle by written instruction only, for the very first time. You have six families depending on you to get it right the first time.
So I strongly advise you find yourself a highly recommended boiler service tech to A) fully diagnose/confirm what IS wrong and B) IF it's air-bound, watch him/her bleed the basement loops and let them know you would like to bleed this yourself next time so they can offer you any other instruction needed. And don't be too surprised if you'll need a new PRV (pressure reducing valve).
-zero
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