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  1. #1
    Join Date
    Jan 2020
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    Marv Air Heat pump issues Model# AVHA48HPO5ONBU-A5-200

    I work for a school district in southern Cali,
    We recently purchased 14 of the above unit’s mentioned in the Title and since going into heating.
    We are experiencing lockouts under 35 degrees, Unit locks out in these temps and goes to heat strip heating only throwing low pressure switch code until, unit is reset at breaker. I’ve done all the factory recommendations to no avail and continue to work with them. The regional rep let me know that there are 5 other in units here in California experienceing the same issue’s. If there is any help, info or data anyone can recommend I would truly appreciate it. Thank you guys in advance!

  2. #2
    Join Date
    Aug 2009
    Location
    Beatrice, NE
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    Looking at basics it has to be either air or refrigerant. An improper refrigerant charge can cause low pressure or low air flow through the coil can cause the pressure to be low even with the correct charge.

    Are these units installed in a way where outside air is recirculated back into the coil or from one unit into another?

  3. #3
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    Jan 2020
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    Thread Starter
    They are installed on the back sides of modular class rooms,
    I’ve pulled the charges and weighed them back in at factory specs,Air Balanced the rooms, Moved the defrost sensors per factory recommendation checked operating pressures when running and they still lock out at 35degree outside ambient temperature(usually overnight) considering we’re in Southern California.
    Marv air recommended putting in another heat strip and switch to lock out the compressor when temps get to 35 degrees but here in California it is illegal to use electric heat as a main source of heating and defeats the purpose of us buying 14 new units for energy efficiency. They sent me a new defrost sensor to ohm out along with the current one installed to see if that may be a issue. I’m currently waiting to here from the rep he has sent a tech to a college experiencing the same problem but they weren’t locked out this morning on there arrival so in the meantime there tech installed a 5lb low pressure switch to see if that may solve the issue.
    Thanks for your reply

  4. #4
    Join Date
    Aug 2009
    Location
    Beatrice, NE
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    Does this bring in fresh air into the room? If so does it enter before the air goes through the inside coil? What is the inside temp overnight?

    Here is the thought. Temp sets back at night, fresh, cold, outside air is introduced before the indoor coil lower incoming air to the point where head pressure can't be maintained high enough to keep suction pressure up.

  5. #5
    Join Date
    Sep 2007
    Location
    edmonds wa
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    Does it have factory installed economizers?
    UA Local 32 retired as of Jan 2020

  6. #6
    Join Date
    Nov 2011
    Location
    East Side
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    your model number is conflicting with some of the marvair nomenclature. I can't tell if you have the manual damper or, the motorized 2 position.....also, not sure if you have the low ambient kit.

    if you have the low ambient kit, the condenser fan cycle controls are in the circuit below 50*, and out of the circuit above 50*. that is controlled by the outdoor temperature sensor.

    if they are having the same problems at other sites, they have a manufacturer caused problem. since these are new units, you're stuck with the factory until they are fixed.


    a 5lb. low pressure switch? that seems like a bad idea. the only way you are going to find out what's happening is by watching your gauges while the unit is running below 35*. there could be multiple problems.

  7. #7
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    Jan 2020
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    Thread Starter
    Thank You for the reply’s guy’s, The units we have, have the motorized dampener and No low ambient kit. I’ve disconnected one dampener and closed it out to cut outside cold air intake, Which didn’t help.
    @71CHOPS your exactly right we’re stuck with the factory and I haven’t been impressed with them.

  8. #8
    Join Date
    Nov 2011
    Location
    East Side
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    If you don't have the low ambient kit installed, that is your problem.

    you will have to find out if the low ambient kit was specified in the bid, AND actually ordered. If it was, and it isn't installed, the factory should supply and install it under warranty.

    If it wasn't ordered, you will need to order and install it on all the units.

  9. #9
    Join Date
    Jan 2020
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    Thread Starter
    The low ambient kit would make sense to me if cooling was desired in a low ambient condition(like a data room cooling server’s year round). But the problem I’m experiencing is in heat mode cooling worked great. I also have the same unit that was installed 2018 with no Low ambient kit either and it heats and cools with no problem.
    If you could explain the reasoning behind the low ambient possibly solving the heating issue and either way I will run that bye the factory Monday and ask if they can send one out to see if that fix’s it.
    Thank you again for your time

  10. #10
    Join Date
    Jan 2020
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    Thread Starter
    Are temps here in Southern California are lows over night 28-35* and 50-60 during the day.

  11. #11
    Join Date
    Nov 2011
    Location
    East Side
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    oh, I guess I missed the heating mode only part! LOL!

    well, could be excessive CFM combined with outside air damper too far open. when you did the air balance, what was your CFM and how did it compare to factory. what is your temperature rise in heating? discharge air temp.? have you inspected all the duct work to be sure your flow hood readings are accurate?

    Don't forget, you are running in air conditioning mode......whenever the unit defrosts. check to see if the OD fan is somehow staying on, or something else odd is happening during defrost.


    you've checked your temps and pressures. What is your SH and SC in heating/cooling?


    I've looked at quit a few modular buildings, they are by far the worst designed and executed systems. I've seen supply and returns with registers installed backwards, "return openings" that were 3"x20" for 3 and 4 ton units, duct torn loose, broken, and just not even connected from the from the factory. wiring wrong from the factory, fan speeds wrong, the list goes on. My point is that if you're just having problems with these few units, you've gotta dig in to every single detail, if you expect to solve the problem. This could even come down to design and engineering......incorrect coil sizing, fan sizing, pipe connections, distributor sizing, etc.

    If there are others with the same problems, and the units are wired correctly and following the correct sequence of operation, I would be leaning toward a design issue.

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