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whats best practice on bleeding unitheaters

jacobsond Member Posts: 89
3 story dormitory. Heat with hot water in winter in summer cool with cold water. Single coil in the unit heater. Each room has pneumatic thermostat. When we switched to cooling this year a lot of air got introduced in to the system. The usual procedure here was to just go to each unit and bleed the air out while the system is running. Well its not working very good. To much air. Ive been to each room (about 40) 4-5 times getting the air out only to come back and do it again. Ive re pressurized the system 4-5 times also. Finally fed up I shut the circulator down pressurized to about 40lbs static and with my 5 gal bucket I have been bleeding. I'm running a lot of glycol out on each and when the bucket is full going back down and pumping it back in. 2 times around with that. About 15 gal of glycol put back in. Still getting some air but better. When I left for lunch all units were 55-60 coils. Is the better procedure turning off the circulator. When I do that the other floors will stop cooling but not that long and building is not occupied anyway.
coming to you from warm and sunny ND


    EBEBRATT-Ed Member Posts: 13,437
    So I take it that this is a two pipe changeover system from heating to cooling. The problem is the air was introduced when the chiller was put on line. The problem is back at the source. Make sure the pumps are pumping away, proper expansion tank expansion tank pressure and system pressure and a main air elimination device.

    If the changeover is done right you shouldn't have air issues.

    You mentioned glycol, it the chiller drained during the winter??
  • jumper
    jumper Member Posts: 2,055
    Low pressure lets air out. One of the few advantages of rooftop boilers & chillers. A non-bladder expansion tank at high point saves many man-hours. I wonder if old gravity HHW with open expansion tanks even had bleeders on radiators?
  • hot_rod
    hot_rod Member Posts: 19,273
    What type of air purger is on the system. Once you have flow thru every emitter, the central air scrubber should finish the job.

    Properly placed and sized of course.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • jacobsond
    jacobsond Member Posts: 89
    Your right on when the changeover from heating to cooling went bad.The guy that had done it for the last 5yrs found a better job. We were left with by guess and by golly to make the changeover. Since we had to Isolate the heating side from the cooling side Some valves were left closed and some open. No real auto vents. There is a couple small ones in the mech room in the basement. The unit heaters have a valve and small hose on the top. Nothing automatic. I found I got more air out when I just pressurized to 30-40 lbs static and left the circulator off. Going to every room bleeding then on to the next. Then circulator back on for awhile then repeat. Last friday all were still cooling after the circulator was on for a few hours so I called it good for now. I left the system at 25lbs static. I know I will be back to bleed more.
    coming to you from warm and sunny ND
  • jumper
    jumper Member Posts: 2,055
    When cooling there's usually less air issues because fluid temperature fluctuates less. Just wait until heating time returns. Few owners bite the bullet to pay for piping modification. Instead they pay for man hours to bleed all over the place.