noise from expansion tank

hello all. i have a single zone hot water heating system with an external coil. the circulator is continuously run for domestic hot water. it appears that air may be getting in the system. i had a "heating prof" in and he filled sysem and bled it from a valve located near the boiler. i live on the first floor and the heating pipes run from boiler, overhead, and then down to baseboard radiators. when he first filled and bled the radiators i heard nothing from radiators or boiler. the next day i heard some bubbling noise from the radiators when the mech zone valve opened. the noise became progressively worse each day. i have since bled the system again, and the radiators are much quieter. however, i now hear a gurgling/bubbling sound from the expansion tank each time the burner comes on. the burner comes on at 170/180 and goes off at 200 degrees. could the expansion tank be faulty and somehow letting air into sysem? i believe the expansion tank is one of the newer ones with a bladder. thanks for any help.email me at snipped-for-privacy@verizon.net just remove the 5 for spam purposes. or post message with any help. thanks again.
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You either have an air control system or an air elimination system. If you have a bladder type expansion tank you have an air elimination system. Running pipes overhead and then back down creates a place for air to collect that has separated out of the water. In an air elimination system the highest point in the piping is where you should have an air eliminator to prevent this kind of thing. It's a little tin can-looking thing about the size of a shot glass that has a float inside. It lets air out but not water.
If you have the non-bladder type of expansion tank you have a air control system. This means air is continually moving back and forth from the expansion tank to the circulating system. For this you need a boiler fitting to separate the air and return it to the tank, and a Automatic Tank Fitting on the tank. Also the piping must be pitched up to the tank.
MM
just remove the 5 for spam

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wrote: hello mm. it is an air elimination system. if there is such a device on the system is it possible it could be in the ceiling? because it is nowhere else to be found. i live in a condo complex of garden apts. everyone has the same system so i don't believe my problem should be on of configuration. although the configuration of all the systems may make them moe succeptible to problems. thanks for your help. jim

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The air eliminator may be near the boiler. In many systems, a device such as Taco Air Scoop separates the air bubbles from the water as the water is pumped through the system. The air eliminator screws into the top of the Air Scoop. This arrangement only works if the system has sufficient velocity to keep air entrained in the circulating water. If the flow is too low, the bubbles rise to the top of the system where they eventually trap the flow of water. Wiring the pump to run all the time and cycling only the burner may help. Adding an additional air eliminator to the top of the boiler may help.
I've seen air eliminators installed in attics and crawlspaces - not the preferred locations, but sometimes you have to. Good practice requires these devices to be accessible for service.
MM
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