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Air elimination

Knots Member Posts: 11
Working in 1948 era monoflow system with Crane cast iron baseboard. I have an L shaped section that is about 12x14 that won't heat on the return end. Naturally, it is the highest section most distant from the boiler, second floor above basement. It appears to have all the air out of it. Anyone have any ideas or hints? I am guessing it is a flow issue not air entrapment problem. I have always been able to get it going in the past but not without lots of fiddling and cussing.
It had to be drained because I just replaced all near boiler piping.
All other radiators and baseboard bled out just fine. BTW, should you bleed from the closest radiator to the farthermost or vise versa? Local "experts" do not agree. I always, in the past, bled the closest first.
Any suggestions would be greatly appreciated.


  • Paul48
    Paul48 Member Posts: 4,469
    If, in the past you got it going, and then had no problems....it is an air problem. Like all baseboard system, they can be difficult(near impossible) to bleed if not set up correctly. They need to be set up to create flow in one direction. Monoflow adds another wrinkle to this because of the main connection between the supply and return. Try raising the system pressure, and if the problem doesn't fix itself, add a full port ball valve between the supply and return for that baseboard.
    Paul S_3
  • Steamhead
    Steamhead Member Posts: 16,454
    Are you Pumping Away?

    Which air separator are you using?
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  • Firecontrol933
    Firecontrol933 Member Posts: 73
    Other than piping at the boiler what else was changed? Circulator? Direction of flow through the system? Size of near boiler piping?

    Monoflow systems require a certain amount of flow in the main to create the proper pressure difference between the inlet and outlet of each branch. If it always worked in the past I'm suspecting that something changed the flow characteristics in the main that is no longer developing enough differential across that one branch.
  • Knots
    Knots Member Posts: 11
    Near boiler piping stayed the same (1.5"). I am pumping away from "point of no pressure change", circulator ( Grundfus 3 speed set on medium), and the separator is a Spirovent. Pump is on the supply side. I had the same problem with tis system with the old boiler. It always seemed to finally start working but not without a fight. Just looking to speed up the battle.
    Thank you guys very much.
  • jumper
    jumper Member Posts: 2,139
    Evacuate air before filling. It actually saves time.
  • Knots
    Knots Member Posts: 11
    How would you remove the air from CI baseboard without filling it? I must be missing something.
    Thanks for your reply.
  • rick in Alaska
    rick in Alaska Member Posts: 1,441
    Also make sure the baseboard is higher on the end you bleed. If the house has settled, or whatever, the bleed end might be low and you are not getting all the air out. I would try raising the baseboard up on that end and then bleed it to see if you get any more air out.
  • Knots
    Knots Member Posts: 11
    Thanks Rick
  • jumper
    jumper Member Posts: 2,139
    Knots said:

    How would you remove the air from CI baseboard without filling it? I must be missing something.

    Thanks for your reply.

    Presumed you've drained whole system.Shutoff makeup. Attach a tee to drain.Attach shutoffs to both forks. Vacuum generator to one fork and water supply,preferably degassed or at least heated,to other. Get the idea?

    One way to minimize air in fill water involves running a hose from water heater (max temperature) to funnel above high point of heating system and from there back down to where you're filling hydronic system. It sounds complicated but it's less pia than bleeding repeatedly.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,644
    I have done the evacuation method with a vacuum pump and it has worked well. Also try the circ on speed 3 for some time with raised pressure (29+ psi). Are there valves on the branches?

    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • Paul48
    Paul48 Member Posts: 4,469
  • Paul48
    Paul48 Member Posts: 4,469
    Isolate the boiler from the system , with full port ball valves. On the system side of those ball valves, put 2 boiler drains. Put a full port ball valve between the supply and return of the problem circuit, and close the valve. Run a hose from a sillcock to the boiler drain on the supply side. Run another from the boiler drain on the return, out on the lawn. Purge at city pressure.
  • 4Johnpipe
    4Johnpipe Member Posts: 479
    edited May 2016
    What Rick said. I have seen baseboard as well as radiators settle. Also would not hurt to raise system pressure. Check the pitch on the baseboard first though.
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