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Thread: Heat pump pressures

  1. #1

    Default Heat pump pressures

    Can suction and discharge be checked on a heat pump in the heat mode? I was told that it couldn't and you had to place it in cooling to check it like a conventional a/c.

  2. #2

    Default

    Ummm... sometimes. Some heat pump manufacturers put three ports on their units. Two are to the lineset, and one is hooked directly to the suction line of the compressor. This is handy, but most manufacturers only have the two valves open to the lineset. In heating mode, both lines are high pressure. If you'd hook your conventional gauges on in heating mode, you'd immediately ruin the low side gauge. I have a "heat pump gauge set" that I cobbled together with two high side gauges. To get the readings you need and to charge the unit, you need to put it in a/c mode. Generally, you also have to block off most of the condenser coil with cardboard or something to simulate a 90 degree day to get decent readings and to charge the unit properly. Charging a heat pump in the winter is tricky, tricky.
    "Nothing is foolproof to a sufficiently talented fool"

  3. #3

    Default

    Ok, what if the unit has the third port coming right off the suction line at the compressor is it still under high pressure in the heat mode? Also, do you think the hot gas method of checking the charge is good enough? I mean like to also check it with a Gage set to compare the two.

    It seems to me that the gage's would be a lot more reliable.

  4. #4

    Default

    Quote Originally Posted by Mr Roboto
    Ok, what if the unit has the third port coming right off the suction line at the compressor is it still under high pressure in the heat mode?
    Nope.
    Quote Originally Posted by Mr Roboto
    Also, do you think the hot gas method of checking the charge is good enough? I mean like to also check it with a Gage set to compare the two.

    It seems to me that the gage's would be a lot more reliable.
    I don't know what the "hot gas method" is. Using a gauge set and some temperature measuring equipment is the only way to properly charge a system.
    "Nothing is foolproof to a sufficiently talented fool"

  5. #5

    Default

    This is out of the manual I have for it. I have checked it this way but I not sure how accurate it is.

    Hot Gas Method
    The following procedure can be employed as a method to check
    for system charge in the heating mode by measuring the hot
    discharge gas at the compressor.
    1. Allow the system to operate at least 20 minutes.
    2. Attach and insulate an electronic thermometer's probe
    to the vapor service valve (large line) at the base valve.
    NOTE - Make sure that the probe is well insulated from
    the outdoor air.
    3. Allow the system to operate at least 10 minutes.
    Afterwards, use an accurate electronic thermometer to
    measure the temperature of the discharge gas at the
    probe.
    4. Using the electronic thermostat, measure the outdoor
    ambient temperature.
    5. For check purposes the temperature measured on the
    hot gas line should be equal to the outdoor ambient
    temperature plus 110F (+ or-4F). e.g: Outdoor Ambient
    45F then the temperature measured by the
    thermometer's probe should be 155F for a system that
    is properly charged. If the temperature measured by the
    thermometer's probe is higher than the outdoor ambient
    plus 110F, the system charge should be adjusted by
    adding refrigerant to lower the temperature. If the
    temperature measured is lower than the outdoor ambient
    plus 110F, the system charge should be adjusted by
    recovering charge to raise the temperature
    NOTE: When adjusting the charge in this manner allow the
    system to operate for at least 10 minutes before taking the
    next temperature reading.

  6. #6

    Default

    That's funny, but it will get you in the ballpark. Using that method, you'll have a perfectly functional system, but the charge will never be right on. The type of electronic thermometer required to do this on a cold day is more expensive than a gauge set. This company probably recommends this method, since you must be certified to install a gauge set. An uncertified person can face a fine of $27,500 per offense just for screwing on a gauge set, since it is considered accessing the sealed system.
    "Nothing is foolproof to a sufficiently talented fool"

  7. #7

    Default

    Ok, what should the suction pressure be just for knowledge purpose?

  8. #8

    Default

    There is no "one pressure" that will answer your question, since pressures vary with temperature. In cooling mode, suction between 60 and 80, and in heating mode suction between 35 and 50 would be some ballparks. Depends in large part on the ambient temperatures, and the engineering features of the equipment. Heat pumps are quite regularly seriously overcharged in the winter, resulting in refrigerant needing to be recovered in the cooling season for proper operation. It's not uncommon for HVAC technicians to reschedule a "check up" to revisit systems in the spring that they've had to charge over winter, just to make sure that they've not screwed up the charge.
    "Nothing is foolproof to a sufficiently talented fool"

  9. #9

    Default

    Ok, thanks for your input.

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