Amtrol Expansion Tank for Hydronic Heating System - Creating Air Bubbles?

Our house and heating system is 8 years old. Our second floor zone this winter has been getting air bubbles in it (for example, when the thermostat kicks on upstairs, you can hear a large rush of water and gurgling through the pipes in the wall) and this eventually gets worse and stops the flow of heat on the second floor. About once a week since December, we have to bleed the water and air bubbles for our 3 zones in the basement using a hose.

The system is a closed system and there are no air vents or release valves at the top floor. The water pressure for the rest of the house and plumbing is fine.

Can you give me an idea of what the problem could be - is it the expansion tank going bad? If it is not working right, will it create the air bubbles?

What pressure is the system set for when the boiler is at room temperature? I'm going to bet that you have the circulator pump on the return side pumping into the boiler and the expansion tank is connected to the boiler outlet piping. Is this a correct assumption? If yes, then you will likely need to either change the location of the pump or the point of connection of the expansion tank.

Amtrol Expansion Tank Question

The pressure is set at 21 psi. The Extrol expansion tank is the standard #30, 4.4 gallon volume and it is connected to the boiler outlet piping. We never had the problem for seven years and now it happens a lot. That is why we think the tank went bad and is creating the air bubbles in the system. The tank is warm on the top and cool at the bottom.

Sorry for not getting back sooner. A cold pressure of 21 is really high for most residential systems, did you perhaps misunderstand me and write what the pressure is when the system is hot? A quick and dirty method of figuring the required pressure when the system is at room temperature (around 70 degrees on the boiler thermometer) is to measure how high the highest heater is above the boiler, divide by 2 and add four. Most two-story homes with a basement do quite well with a cold pressure between 12 and 15 psi.

If your system has operated well for several years and no changes have been made then the location of the circulator pump in relation to the expansion tank is not the problem, even if it is not optimal. Tens of thousands of systems operate with no problem even with improperly installed pumps and expansion tanks.

Diaphragm expansion tanks can only be checked when they are isolated from the boiler system. Sometimes valves are installed to do this isolation but some local codes prohibit any valves in the expansion tank connection to the system. You cannot ascertain the condition of the interior bladder by feeling for a temperature difference or by knocking on the expansion tank side.

Your supposition that the bladder in your tank has failed and has released air into the system is certainly a possibility. Not having air vents at the heating units (baseboards or radiators) is in my opinion a design flaw but it also seems to be the norm these days.

Please watch the pressure and temperature of the boiler when it is firing and also when not firing. Note the "swings" of pressure and the temperature when the high and low pressure occurs and post back. If you can please take several pictures of the boiler and surrounding piping and also post the pictures.