Hydronic heat system with 3 zones, water still gurgling after flushing out twice.

[woodbutcher]

greetings,
we purchased a 1960 construction colonial style home in Michigan, with hydronic radiant heat set up on three zones. Purchased and moved in during spring 2014, so did not need to run heat during renovating. First experience with hydronic radiant heat.
Upon running system when temps cooled(Michigan) noticed water gurgling and rushing sound in several area of the system. Researched the issue and learned how to flush the lines using the city water supply via the pressure regulator to the boiler. Flushed the lines with a hose from bypass faucet on return line from zones, into a bucket. Cycled through the three zones manually in turn, until all air bubbles ceased escaping into bucket of water. I kept a continuos flow of water going through system as i switched through full buckets and replaced with empty. This seemed to correct the issue for a short while, then the issue would/has resurfaced in the upper level.
I am curious if the issue is that all the air in the system is not being expelled when being flushed with the city water pressure, and I should continue to do it until remedied?
Or is it due to the oxygen in the city water being released during heating, and then eventually building up in areas of the runs?
There are no leaks in the system I am aware of, and the boiler and pump seem to be working as required.
One possible solution is to add some type of bleed valve on upper zone runs to release trapped air?
Hoping to avoid that type of project.
Thanks for any advice.

[akaothermick]

In the high point of the system, you can add a manual Key type vent or auto vent though normally under the baseboard cover there should be a key vent to bleed the air out, but sometimes these are omitted to save money, the air always goes to a high point so flushing from a low point like the boiler is not always effective in removing all the air. The system if its copper pipe will needed to be soldered or a compression type fitting added to accommodate a vent, soldering for a novice can present a problem so a experienced individual might help. The noise you hear will not hurt the system although an air lock could effectively stop a loop from working, but normaly wont

[woodbutcher]

Thanks aka,
yes the lines are copper and unfortunately none of the upstairs runs(4 bedrooms, 2 bath runs) have any type of bleed vent.
The first time i flushed the system, i used only the system water pressure without opening the pressure reducer. Bubbles came out of the runs but not too much.
The second time I flushed it, i did open the pressure valve to allow more pressure from the city line to hopefully push the bubbles through more effectively and i did see what i thought was a lot of small bubbles for a bit.
All the zones are giving off heat but upstairs ones are just a tad gurgly sounding when pushing through.

[akaothermick]

Thats where the air goes, the high point. you can just wait till warmer weather if the system is working fine, and add a couple of vent Tees and key vents. The gurgling will not be harmful, practice up your soldering skills on scrap copper pipe and use a silver bearing solder rather than 95/5 solder, more money but much easier to use. Opening the pressure reducer is the way to go in a flush, flush the boiler too, it gets a sediment accumulation that reduces its efficiency. Hopefully you opened all the zone valves before flushing.