Most codes require an auto bleeder to allow air to escape from the system. This should be a device installed at a high point in the hot water loop. These bleeders are known to fail periodically and may need to be replaced. I would also be questioning why you need to bleed air so often. This is an indication of a possible leak. It is a closed loop system, so once the air is purged from the system, there should be no problems. If there is a leak air will continue to be introduced into the system via the fresh make up water valve, or from the leak itself once the water has drained enough. If your taking in water, your also leaking water somewhere. Another problem you could end up with is, by introducing alot of fresh water the system could become contaminated if your boiler water is treated. Mixing treated water with fresh can create too much bacteria growth and cause system falure. A quick way to make an initial guess is to take a small sample (a small coffee cup) and smell it. If it has a bad odor, it is probably contaminated.
Joe makes a good point on the expansion tank as well. If you give it a couple taps on the side it should sound empty, or have a ring to it. If it sounds full, it's saturated and needs to be replaced.
Let's hope it's as simple as changing the bleeder or expansion tank. That should be fairly inexpensive. I would not recommend using the plumber you spoke with because if he would have simply bled the system and left it, he would not be providing complete service.
Hope this is helpful.
