I had to drain my heating system yesterday, as I had to replace the expansion tank (diaphragm ruptured).

After replacing the tank,I was able to get heat circulating in the first two floors, but not the third.

I have one circulator, with three Taco zone valves (with manual valve opening levers).

I do NOT have shutoff valves , but have purge valves. No bleeder valves in any of the three circuits.

The layout of the three return lines is (top to bottom):

Purge valve, with zone valve below. All three zone valves connect to a manifold which connects to a 1 inch pipe. Water supply enters this pipe. The circulator is below this. Below the circulator is the boiler inlet.

Obviously I did something wrong with the third floor. I believe I have air trapped. Is there an order to fill the system? I've found procedures to fill systems when there are shutoff valves installed, but I don't have any.

The Outside temperature is 40F today. It was below freezing yesterday. Could I have a frozen pipe?