I have a dual-energy system set up like in figure 5/page 11 of this manual When the system switches to gas (at -12C outside temp), I'm getting a banging sound in the gas boiler at the end of the cycle. I think what is happening is that the water is "stuck" in the exchanger and boiling; it takes a few seconds for the aquastat to sense that the water is at temperature and stop the boiler.
Here is what I have observed:
- If the circulating pump is running, there is no banging at the end of the cycle. But, as you can see from the diagram, the circulating pump and thermostat are connected to the electric boiler. When the thermostat calls for heat when the system is on gas mode, it simply starts the circulating pump. The gas boiler doesn't know anything about this. The gas boiler only starts when the water temperature falls below the temperature set on the aquastat.
- The aquastat does appear to be working. If I roll it back to 120 degrees from 180 degrees, there is a long wait until it turns on. From what I can see, it is a simple aquastat — just a dial with the temp in F.
Edit
- Perhaps the aquastat is not working properly. If I set it at 120F and the burner fires without the circulating pump turned on, it will start banging. I think this is steam being created. It seems that the aquastat should turn the burner off as soon as senses 120F. ON the other hand, the burner does turn off after some banging… I'm not sure if this is the aquastat turning off or something else.
Here is the manual for the gas boiler
I'm not sure how to troubleshoot this further (air in the system? check valve?)
Edit 2
d.george's answer led me to a solution. There is no flow-switch installed and there probably should be. The gas boiler instructions clearly state that water should be circulating when the burner is firing. So I stuck a relay in between the thermostat and the circulating pump connection, putting the contact side in the gas boiler thermostat loop (shown in red below). 
Best Answer
Unless you are getting your electricity at a price that is cheaper than the natural gas used in the boiler, I see no need for the electric boiler. I am not familiar with this make of boiler so my writing is on how I feel the units should perform. In the instruction manual they mentioned an optional control that can modulate the gas input from 50% to 100% of the full input that would or could make the boiler more efficient and save on operating cost. Since I am not physically there, to check the control wiring of the 2 units, I must assume that when the gas boiler is firing the electric boiler is shut down, if they are wired that way. That being said there should be a device that keeps the pump running long enough to rid the boiler of excess heat. I am assuming that this unit is a coil boiler and those boilers usually have a flow switch in the boiler's piping to insure water flow before the boiler is allowed to fire and a delay relay to insure that the excess heat is removed before the pump shuts down. You did not mention if this a new installation with a problem that you are observing or if is an older installation with a new problem. More information may be needed to properly give a proper answer. Hope this helps