Air Purge from Hydronic Heating System

[justkev]

I was starting to get less and less hot water over the past couple of weeks and I was able to narrow the problem down to the mixing valve. After I got the TMV adjustment knob unstuck, I opened it up fully and I began receiving hot water. Now I'd like to remove the TMV to repair or replace. If I do that, however, the pipes that connect to the hydronic air handler will drain and I'll need to purge the air from the system. This has me stumped. The annotated photo below shows the pipes, valves and drains; and connecting a hose to either drain below the tankless water heater and opening up the ball valve wouldn't accomplish this. It could easily be done if there was another ball valve and drain valve, where I've indicated in red. Is there an easy solution without having to install another ball valve and drain? Maybe the solution is in the last photo? Any help would be appreciated.



At the air handler, there are ball valves in the upstream and downstream pipes as well as a flow sensor and a Honeywell air eliminator. I don't see anything here that helps.



Inside the air handler, there is a valve at the bottom of the radiator (bottom right in the next photo). Could that be used to purge the air? If it is, I'm a little confused: there would be air in both the supply and return pipes so to purge all the air, a fair amount of water would probably need to discharge, and the size and placement of that valve seems a little inconvenient.

 

[breplum]

The air purge is supposed to be automatically via the air separator valve (middle photo)... Good practice is to have actual purge valves that short length of hose can be attached to to drain into bucket.
FYI, combi systems like this are supposed to be separated by a completely-isolated heat exchanger (typically a plate system).
During months when your heating system is off, you face water stagnation of the branch piping mixing with domestic water and can also be a source to harbor legionella.

 

[justkev]

The air purge is supposed to be automatically via the air separator valve (middle photo)... Good practice is to have actual purge valves that short length of hose can be attached to to drain into bucket.
FYI, combi systems like this are supposed to be separated by a completely-isolated heat exchanger (typically a plate system).
During months when your heating system is off, you face water stagnation of the branch piping mixing with domestic water and can also be a source to harbor legionella.

For some reason, I was under the impression that the air separator only handled so-called microbubbles. I gather then that, due to the tiny orifice in the valve head, purging air from a system that's largely drained might take a bit of time...hopefully not long enough to cause any damage.

The system I have is apparently up to code. I'm frequently away for a month or so at a time, during which time I shut off heat and water. Whenever I return, and knowing about Legionella, I do make a point of running hot water for a while (and keep the temperature at 140). So far so good.