What could be preventing the heat boiler from circulating water upstairs to the radiator, frozen pipes

boilerheating

I recently bought a Cape Stye house and it is equipped with a Weil McLain Series 1 Boiler. The boiler is up and running fine except it is only heating the first floor; the base board radiators upstairs doesn't get hot, I opened the bleeding valve on each radiator to see if air/water is circulating, and they both bled for awhile with hissing sounds and water gushing out.

The day after I re-opened back the bleeding valves on the radiators upstairs and nothing comes out, as if the boiler isn't pumping water to the radiators upstairs, so thats when I took a pause and decided to get help. Could there be frozen lines upstairs or anything else on the radiator side that's preventing the circulation?

Thanks in advance Guys.

PS. (Both Zones/Floors are connected to a T pipe on the boiler).

Updated with Zones and Circulation pump.
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Update 2: Picture of the backfill and automatic/manual feeder?
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UPDATE: Sorry for the late reply, It checked out to be frozen pipes. The weather went up to 60 degrees last week, I went to feel the upstairs zone's pipes and it was hotter than normal; I then went upstairs and I could feel the heat coming from the base board heaters as I was walking up. That made me very happy haha. Thank you guys for all the input.

Best Answer

You could have a frozen pipe (actually, it would take two in this case to completely isolate the upstairs bleeders.) You might not. Don't know the layout of your pipes...

Circulation pump does not need to be running for bleeding. System pressure alone should suffice for bleeding. "Hissing" when you bled before sounds like trapped air. When you start to get water mixed in, close the bleeder, wait a few minutes and re-open. You should eventually get just water, no hiss, no bubbles.

I'm going to suspect that you have low pressure in the system, so air is not being forced out the top bleeders by water pressure in the system. You should have a pressure gauge (often on the same face as a temperature gauge) on the boiler - what does that read? Normally it should be 20-30 PSI. You may need to add water.

If you can't sort it out yourself, call for professional help sooner rather than later, as letting the system freeze can result in considerable expense.

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