Boiler off...knock in radiator?

pascack

I had issue with boiler leaking water last winter which turned out to be ancient expansion tank. Prior to that, I had some knocking.  Not much.
Today, I’m sitting in our house. It’s summer so at times house is hot, though I do have room air conditioners on first floor, used occasionally.
The radiator knocked!  The boiler is off!
Anyone have any idea why it would knock now?  Should I worry?
Thank you. 

dopey27177

Do you have a coil in the boiler to make domestic Hot water?

If you do lower the aquastat to 180, you may have phantom water circulation in the heating system, this sometimes is caused by the lack of a check valve in the return line for each zone. or a flow control valve before the your zone valve.

Jake

pascack

No. I have a separate hot water heater. I assume the knock was boiler related because I was sitting next to our large radiator when I heard the bang. It’s been very hot here and I do not have central air conditioning. Don’t know if that makes a difference. The system never had knocks at all till the expansion tank needed draining. That improved, but still had a knock here or there when boiler working. 
Thanks for your thoughtful reply. 

dopey27177

When you say expansion tank do you mean a tank at the ceiling or a compression tank with a bladder.

If is a tank with a bladder make sure the pressure in the tank is set to 15 PSI when the system is cold.

If you have a tank at the ceiling, when you are draining it you must make sure that the system has no pressure in it. Then drain the tank.

When refilling the system you need to make sure the pressure in it is 4 pounds greater than the static .

simple formula the height from the floor to the top o the highest convector or radiator.

Assume the height is 20 feet, multiply 20 X .434. That equals 8.640 psig. Add 4 psi and that gives you 12. 640 psi. So your system pressure is 15 PSIG. By adding the extra pressure it will prevent air bubles from forming and turning to steam at that location.

Enclosed is a catalog of fittings to use on an overhead expansion tank. If you have this overhead tank it is suggested that you install a gauge glass so you can monitor the water level in the tank.

see enclosures.