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  1. #1
    Join Date
    Jan 2005
    Went to a system today,low side pressure was 90 psi. Warm air coming out of registers. Newly installed system,don't know why HO didn't call the company that installed the unit. Lowered suction pressure to 70 psi.,should have a 40 degree coil correct? Suction line at compressor was sweating a little,temp. was around 55. Probably a little less didn't get that accurate of a temp. Went in house and temp. out of registers was 60 degrees. 84 air temp. in house. HO called back and said not cooling enough. Unit is cooling second floor and 2 attic rooms,large open stairway to downstairs that is not air conditioned. I could feel cool air coming downstairs as I walked up them. If you have to add R22 to system because of a long run,this is not a long run,the pressure should still be around the 68-70 suction line shouldn't it?

  2. #2
    Join Date
    Jan 2004
    All depends on the metering device, and the indoor wetbulb temp, and outdoor drybulb.

    Always explain to the customer that it may take hours for the temp to come back down.
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    How many times must one fix something before it is fixed?

  3. #3
    Join Date
    Mar 2004
    Grottoes VA
    I am thinking this is a piston evap coil. If that info is correct you need to charge by superheat with a SH calculator, or by the charging chart for the unit. With a 24 td across coil airflow maybe a little low.

    It will take a long time to bring the temp. of the house down to an acceptable level. My house will only cool down 1 an hour at 90 oa, and my unit is 1/4 ton over sized.
    Karst means cave. So, I search for caves.

  4. #4
    Join Date
    Aug 2003
    Fort Worth, TX
    If kartshuntr's hunch is right and this is a piston fed evap, you gotta nail that charge to get this puppy to perform like the customer is looking for. How's the airflow over the coils? Both coils clean? Blower wheel clean? Filter fresh and clean? Your 24 degree TD would indicate you may be not moving enough air.

    Only way you'll really know once all the above ? check out is to measure the suction line superheat and, if the OEM's charging specs are available, see if you can approach that with your given conditions. It takes very little to either leave a piston system undercharged, thereby guaranteeing an evap that will always run a little to a lot starved, to overcharging it, where it will always run either almost right when heat loads are average or run flooded when outdoor ambients are higher. On high heat days it won't run worth a flip. The mass flow rate due to higher head pressure will kill coil performance and leave the homeowner wondering why he even bothers to run the thing at all.

    Oh yeah...pistons suck!

    [Edited by shophound on 06-12-2005 at 10:56 PM]
    Building Physics Rule #1: Hot flows to cold.

    Building Physics Rule #2:
    Higher air pressure moves toward lower air pressure

    Building Physics Rule #3:
    Higher moisture concentration moves toward lower moisture concentration.

  5. #5
    Join Date
    Jan 2005
    Sysytem is brand new. Everything is clean. Calling HO today to set up another visit.

  6. #6

    high temps cause higher low side preasure

    If you have to add R22 to system because of a long run,this is not a long run,the pressure should still be around the 68-70 suction line shouldn't it?

    If you are getting 16 to 20 difference comming out you are close very close..

    You can not just set low side preasure to 68-70. That is a mistate. The preasure could be from 68-83 roughly. Depends on temp across the coil. high temps cause higher low side preasure. If you go to this house for example the air comming into the furnace is 80 degree and your low side preasure is 68 degrees you are low on preasure you need to add. if you have 80 degrees comming across the coil I would say your low side should read somewhare around 75 and 80.

    Another way of checking to see if you are low on freon is to goto the furnace and listen to liquid line. If it sounds like its not enough freon comming through then its not.
    Pratice listening to the liquid line.


    [Edited by klyons20 on 06-13-2005 at 08:06 AM]

  7. #7
    Join Date
    Nov 2003
    With it being 84* inside,the original 90 lbs. might have been closer.

    No way to tell without a superheat reading.

    Is the blower on high speed?
    Is it an electric furnace?
    Heat strips stuck on?

    Do you have an air balance hood or anyway to check the actual airflow?

  8. #8
    Join Date
    Jan 2005
    It's an air handler in attic. I'm going back Saturday to check the piston. I should have checked that first. That has happened to me before,wrong piston size. Sometimes the coil is 1/2 ton bigger and installer will forget to change piston. When the pressure was 90 the air was warm coming out of the registers. Pressure at 70 air temp. was 60.

  9. #9
    Join Date
    Nov 2003
    "Pressure at 70 air temp. was 60."

    Like it was starting to freeze up from a low charge?

    You need to get away from that 70PSIG thingy you're doing.

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