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  1. #1
    Join Date
    Nov 2003
    Location
    South Carolina
    Posts
    1,167
    I just installed a roof top full of carrier equipment. 3 condensers and (2) 7.5 ton gas packs. The 2 large gas packs had no existing return and draw in 100 percent outside air on the return. The previous units were ducted the same way and always worked well..it looked like on the previous units they had slowed the fans down but that was the only alterations made to the units. I started my 2 new units up without slowing the fans down to see how they operated and on both of them pressures ran at 40 on low side and around 190 on high side...that confused me because i figured i would be running high pressure instead of low pressure. Low side lines were freezing up and line temp was around 32 degrees. I opened the compressor compartment door to bypass some of the condesing airflow and temperatures went closer to normal at 60 on the low side. Ran the units all day and they are cooling the building great. I guess I will install 2 condenser cycling controls on both the units to conteract the low pressure. Can anyone else give me any ideas other than that and an explanation as to the pressures im getting? The way they are running I dont think I will slow the fans down if I dont have to. Outside ambient on the roof top at 90 degrees today and im in the southeast with high humidity. Supply temp running at 47 degrees.

  2. #2
    Join Date
    Nov 2000
    Location
    Upstate, SC
    Posts
    2,920
    Am I understanding correctly that you were running the units pulling in 90F outside air and got those temps and pressures? I don't think you should have had to bypass any condenser air on a 90F day. Do you have any ductwork at all on the return side of the unit? I have had to add some ductwork even on 100% outside air units to get the blowers to load up and move the proper amount of air. Whatever is causing it, I am thinking your problem has to do with the airflow across the evap.

    Bobby

  3. #3
    Join Date
    Nov 2003
    Location
    South Carolina
    Posts
    1,167
    Bobby thats exactly the same thing I thought about bypassing condenser airflow. Its really messing with me that im getting these pressures. Both units are brand new and I get same exact pressures with both units. They operate at a more normal range when I open the service panel allowing bypass air. No way its a problem with return air flow as its completely open to 100 percent outside air. I figured I would get higher than avg pressures but not low pressures..this is so weird. The only thing I can think of to do is to cycle those fans to keep the head up. It is not possible to add ducted return air to the building for these units.

  4. #4
    Join Date
    Nov 2003
    Location
    South Carolina
    Posts
    1,167
    By the way bobby Im right up the road from you in greenville.

  5. #5
    Join Date
    Jun 2004
    Location
    Richmond Virginia
    Posts
    1,078
    You are fixated on your suction pressure, it's the temperature and the pressure you should be concerned with. If that is the pressure and temps you have at 90 degrees ambient, what are you going to have at 75 degrees? Something else is wrong, you had mentioned slowing the fan down but it may need to be sped up instead or you could have a blockage in the supply duct.

  6. #6
    Join Date
    Nov 2000
    Location
    Upstate, SC
    Posts
    2,920
    Is there any duct at all on the return side? I still think we have an airflow problem with both units doing the same thing. Could be supply duct related, but it would have to be effecting both in the same way. Have you checked superheat and subcooling? That might give us some idea what the freon is doing. Something ain't right though, I think we all agree on that.

    Greenville, huh? I make it over there fairly often. I think I am going to be in Laurens and Clinton tomorrow.

    Bobby

  7. #7
    Join Date
    Nov 2003
    Location
    South Carolina
    Posts
    1,167
    Im not fixated on the suction pressure but by getting that reading on initial start up it let me know ive got some issues here. The units I replaced were working units...they were just old and heat exchangers were rusted out due to the years run with 100 percent outside air. The supply goes straight down the entire 4 story building with one supply vent in each of the 4 hallways...each 7.5 ton unit cools one hallway on each floor. The air flow is great coming out of the supplys on each of these floors and the previous 7.5 ton units cooled the halls fine without having the pressure problems im having with these new units. However the old units fans appeared to have been altered by slowing the fans down somewhat and thats what i had intended to do with the new units. So it took me by surprise when I started getting these readings. Even with panels off my head was only 215 with an outside ambient of 80 at 6 pm today. With the head raised the units have stopped freezing up and are cooling the hallways fine so thats why Ive considered going with fan cycling switches to keep head up. You have to remember...the duct situation in this hotel is non reversible without tearing the hell out of the hotel and thats just not gonna happen so I have to work with what I have here so I was posting to see if anyone else can think of other alternatives to the problem at hand. Ive had situations in other building where fire dampers had closed down causing lower pressures so I know what your saying about the supply but in my experience when supplys got restricted the pressure were very unstable...the pressures im getting with these units is very stable.

  8. #8
    Join Date
    Nov 2003
    Location
    South Carolina
    Posts
    1,167
    Bobby I agree since both units are doing this then only reason states that the supply duct has to be the problem but that cant be changed so its not an option. No there is no ductwork for the return at all...just basicly a rain shield for the return but strangely enough it cools fine and doesnt pressurise the building because there are other avenues of escape for the air in the building...also maintains a good humidity level inside the building. Its strange but thats how the engineers designed this thing...its in a hampton inn hotel. So since I cannot change this duct situation Im thinking the only option i have here is to cycle the condensing fans unless any of you can think of anything else i might try.

  9. #9
    Join Date
    Aug 2003
    Posts
    49
    Low highside presure is due to very low evaporator temperature. I Agree that there is an air flow problem. Check the blower fan rotation. I had a not very similar case which was fixed by switching L1 and L3. Leave L2 as your wild leg.
    I'm learning

  10. #10
    Join Date
    Aug 2003
    Posts
    242
    Originally posted by alex1960
    Low highside presure is due to very low evaporator temperature. I Agree that there is an air flow problem. Check the blower fan rotation. I had a not very similar case which was fixed by switching L1 and L3. Leave L2 as your wild leg.
    I also agree, check the blower rotation. You definitely have a problem with airflow across the evap. Did a piece of insulation get caught up in a set of turning vanes? Is a fire damper closed? Some of those Carrier units are both side discharge and bottom discharge. Is the blank off plate installed on the bottom return connection so you dont recirculate air? What is the static pressure on the discharge side of the blower at the unit? What is the static pressure in the ductwork inside the building?

    You dont want to retrofit the duct work because it is code that you have 100% outside air in the hallways of a hotel. Thats why that system is designed that way.

  11. #11
    Check and see the static pressure is high and for proper roatation. I bet a fire damper has shut.Recently had this happen to me .A new unit was installed smoke detector wiring that powwered fire dampers below were removed from old unit. New unit had smoke detector wired from factory so installler never pursued. powered up dampers bing air flow.Just a thought.

  12. #12
    Join Date
    Nov 2003
    Location
    South Carolina
    Posts
    1,167
    Thanx guys...silly me during start up did not check to see if blower rotation was right. I went back up there just to check because i thought it was right and low and behold that boy was spinning backwards. I havent changed the leads yet but will tomorrow and do a new start up from scratch. Thanx again.

  13. #13
    Join Date
    Nov 2000
    Location
    Upstate, SC
    Posts
    2,920
    Okay, well, guess that should take care of it. I started a couple fan coils up in Haywood Mall a few weeks ago and left them running fine. Three days later they call and say one has stopped cooling the area. As soon as I walked in I noticed that I couldn't hear the thing running like I could when I left it. Nobody would admit to changing the phase, but it didn't to it by itself.

    Bobby

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