I hired some plumbers to reroute baseboard heating to hug rearranged walls on the second floor (as a part of a bathroom remodel). They drained the system, turned off the heat, and rerouted the pipe. It was disconnected for a few weeks while other work was being done. They hooked it back up a week ago but did not bleed the air out of the system and just turned it back on. Well, that didn't work. No heat. We have two zones: first floor and second floor. The first floor was working just fine but the pipes for the second floor just did not get hot. Whenever I heard the thermostat click to turn on, I would immediately hear a gurgle in the system, but heard no other ticking or water flow through the pipes as usual. Eventually, I stopped even hearing the initial gurgle.
I learned from googling that this can be caused by trapped air in the system after it has been drained, so I had them come back and try to purge the system to get rid of the air. They closed the valve leading into the pump from above and drained water from the valve above until it flowed smoothly. However, when the system kicked in again, same problem. No circulation. You could feel the vibration in the pipe and actually hear the pump whirring when you put your ear up to it, the pump itself got hot and there appeared to be some backflow of the hot water up above the pump a few feet up the return pipe (they told me the hot water should be flowing in the other direction). But further up, that pipe remained cold.
They told me that the pump must be bad and that I needed to replace it. However, after they left I tried purging air again, having watched and learned how to do it, and I did get more air out. When the system kicked on after that, there seems to be a small amount of flow, but very slow compared to prior to this work. The pipes on the second floor did eventually get hot after a half our or so, but it took a long time and did not get anywhere near as hot as they used to. The pump itself still gets very hot compared to the first floor pump, but after my second purge, the pipe above it did not immediately get hot like it did before the second purge. Though it did eventually get warm from some weak circulation.
I'm hoping that there's just still more air trapped in the system, but getting it out seems difficult. Is there something more I should be doing to get the remaining air out or is this simply an iterative process? I never had a problem with the pump before I had this work done since we bought the house in 2018, though I don't know how old it is. Should I be doing the purge while the water is hot or after it has cooled down? Or is it simply possible that the pump is failing and not operating at full capacity?
One last thing: When they rerouted the pipe in the bathroom, and replaced the length of pipe, they did not reinstall the bleeder valve that was in the corner. I had asked the inspector when he was here whether that should be replaced, and he said that those bleeders are no longer necessary to be to code. Is it this lack of a bleeder valve at the top of the circulating system that could be causing the problem?
Addendum: This is the bleeder that used to be in the second floor bathroom, which they never replaced after rerouting the baseboard pipe: