bleeding air from a liquid system; high point?

We have a high efficiency furnace that heats water which is piped thru a concrete slab. This is a question about air in the liquid and bleeding of the system.
There is one accumulator in the system. It hangs down from the piping and I assume it has a bladder. This is what I don't understand. Why is it below the piping? If air gets into the system (which it has) it has nowhere to escape. It continues to race around the loop causing noise in the pump and flow regulation valves.
Shouldn't the system's accumulator be above the piping to allow air to move into it and out of the normal liquid flow? And if the accumulator is not designed to bleed, then shouldn't there be a high point reservoir, with a bleed valve on the top?
Thanks
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werwer wrote:

There should be a high point with a bleed valve, but I ASSume that the accumulator has a bladder in it and therefore it doesn't matter where/how you mount it.
nate
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Somewhere there is a vent valve of some sort. http://www.taco-hvac.com/en/products/Air%20Vents/products.html?current_category or perhaps http://www.watts.com/pro/_productsFull_tree.asp?catIdd&parCat%0&pidf0&ref=2 Series AS Air Separators are used in commercial and residential, hydronic heating applications to separate air from water in the piping system. They consist of a heavy, cast-iron body construction with a 1/2 in. female threaded connection for an expansion tank, and a 1/8 in. female threaded connection for a float vent. Series AS provides continuous purging and venting of air in the system when installed with Watts Model ET expansion tank and Model FV4 float-vent. Maximum Pressure: 125psi (8.6 bar).
If you still have air racing around the systems, something is wrong. Keep the feed valve to the boiler open also or it will just suck in more air during the cooling cycles as the water contracts.
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Yes, there is one of those. Ok, now I'll have a closer look. Thanks.
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The item you call an accumulator is commonly known as an expansion tank. As water in the system is heated it expands. The expansion tank allows the water to expand without increasing the system pressure. Mounting the expansion tank below piping is typical practice.
As another poster mentioned, an air removal device should be in the system. Air removal devices come in several different shapes and sizes. It can be a cast iron air scoop with a brass automatic one way valve on top or a brass device with a funny looking teepee top and a large body. A functioning air removal device will eliminate all the free air in the system.
Google around a bit and find some pictures of air removal devices for hydronic heating systems. Inspect your system to find the device or it's absence. If you don't have one then get one installed. If there is one in the system then it's apparently not working so find out why.
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