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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?

pump and valve pump purge

Addendum: This is the bleeder that used to be in the second floor bathroom, which they never replaced after rerouting the baseboard pipe:

old bleeder

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    I'd continue bleeding air until there is no more to be had. It seems possible that there may be an obstruction of some sort due to the work but until you have it purged there is no way to tell. Those circulating pumps don't tolerate air well.
    – jwh20
    Nov 10 '21 at 14:11
  • How do I know if there is more air in the system? The plumber seemed to think that once it's flowing smoothly, all the air is purged, but that obviously wasn't true since I got more air out. When I did it the second time, I ran it until the smooth flow again. Should I keep running it or do I need to heat it up again before purging each time?
    – hepcat72
    Nov 10 '21 at 14:14
  • And should I open the purge valve all the way or run it slowly? Does it make a difference?
    – hepcat72
    Nov 10 '21 at 14:18
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    That's the wrong place for a purge valve. The air is going to collect at the topmost point of the system. There needs to be a valve there or you will never get all the air out.
    – jwh20
    Nov 10 '21 at 14:29
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I take it your "so-called-plumbers" did not install vents on the high points of the loop?

Seems like they might be unfamiliar with hydronic heat, rather than water supply plumbing.

Air dissolves more in cold than hot water, so heating the water brings air out of solution (no bubbles cold become bubbles when heated) and thus, any bleeding procedure that involves dumping a lot of new water in is doomed, which is why you need vents on the high points.

With cast iron pumps/boilers, getting to a point where you are not needing to add water all the time is essential, since the only way those work without becoming rusty messes is if all the oxygen in the dissolved air brought in with new water is used up, and not replaced with more to cause more rust.

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  • Yeah, this is definitely not the first issue I've had with them. The whole thing has been cause for a lot of sleepless nights. This is just the latest frustration.
    – hepcat72
    Nov 10 '21 at 14:24
  • "Air dissolves more in cold than hot water" which is why boiling water bubbles.
    – FreeMan
    Nov 10 '21 at 14:53
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    No, boiling water bubbles because water is becoming steam. Water that is not yet boiling bubbles because air is coming out of solution.
    – Ecnerwal
    Nov 10 '21 at 15:22
  • So I had another plumbing company come it to give me an estimate to take over the job. 1. They confirmed that there is still air in the system (he heard it when in the bathroom). 2. he noted that the water getting into my pump is not from condensate (as I'd naively thought), it is from a pressure relief valve - since the water flow is hampered, I'm losing a lot of water and it is replaced by more oxygenated city water. 3. This is causing rust, which you see at the beginning of each purge. 4. The bleeders near the boiler are corroded shut and not working. You can see it in the first picture.
    – hepcat72
    Nov 15 '21 at 20:36
  • He didn't say I needed a bleeder at the high point - but I didn't ask about that either.
    – hepcat72
    Nov 15 '21 at 20:37

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