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Heat Pump Minimum Ambient To Protect Compressor

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Hello All!

My question concerns the minimum safe operating temperature for a heat pump outdoor unit. Our unit is 17 years old and has only been used for cooling. We heat hydronically. Ideally, I'd like heat pump mode available should the hydronic system be shut down for repair.

My refrigeration work life was limited to chiller units and refrigeration over 100 tons. Small units, like my house's, are not something with which I'm familiar. My R-22 experience is also limited to friends-and-family freebies and installing this unit.

I hope you don't mind lending guidance to amateurs like me.

The outdoor unit is Haier brand. it's 2-1/2 tons and uses R-22 with mineral oil. The compressor (Bristol H20J293ABCA) is reciprocating (piston) type and has an immersion crankcase heater (wattage is unknown). Expansion is by a TXV and there is a suction accumulator for cooling cycles. The 'evaporator-when-cooling' coil is in a Hi-Velocity brand air mini-duct handler. It's their RCM series thick style coil.

Haier no longer offers a Low Ambient Kit, which, if my memory is correct, was a headmaster valve.

My thought was to install a Snap-Disc type of control that would open at the appropriate temperature. I'd splice it into the compressor/outdoor fan combined contactor's coil.

I realize that in cooling season, I'd have to bypass it or swap it for an open at 65-F unit.
(Haier specifies 65-F in cooling mode and does not mention safe minimum temperature for heating mode, other than to say to use their low ambient kit).

What is a good temperature for the control to open? Also, rather than have the control inside the electrical compartment sensing air temp, would it be better on the suction-when-heating line?

Thank You for helping & sharing knowledge.
Paul

Comments

  • yellowdog
    yellowdog Member Posts: 157
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    Was the unit sized for heating? It may be a trial and error basis to find what outdoor temperature the system is still capable of covering the heat loss of the house.
    PGB1
  • PGB1
    PGB1 Member Posts: 77
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    yellowdog said:

    Was the unit sized for heating? It may be a trial and error basis to find what outdoor temperature the system is still capable of covering the heat loss of the house.

    Thank You for taking time to reply, yellow dog.
    No, it's not sized for heating. It's sized for cooling and is heat pump capable only by circumstance.

    I will Only use it in heating mode if the hydronic system is down for repair or maintenance.
    I think of it as an ~8,800 watt space heater. On a cold day, the building won't reach design temp., but it'll be ~30-k BTU closer.

    My ambient temperature worry is to prevent damaging the compressor, such as from backwards liquid slugging or oil migration. The easiest way would be a thermostatic device, such as a snap disc that opens at whatever temperature is safe.

    Paul
    PS:
    The reason that the outdoor unit is heat pump capable is because, when I was to purchase a condensing unit for cooling, the supply house asked me to try a Haier for evaluation purposes. They supplied the heat pump model. It came in about half the price of their lowest, builder's model (Goodman). The only extra I had to spend was the swap for a bi-directional filter drier.


  • Hot_water_fan
    Hot_water_fan Member Posts: 1,850
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    I'd imagine the 17 year old manual would have information? The new models have a temperature at which they won't restart, maybe -18F if I recall correctly?
    PGB1
  • PC7060
    PC7060 Member Posts: 1,160
    edited February 26
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    Pretty common to use a control circuit capable of controlling switchover from heat pump to boiler based on outside temperature (30-40f range)
    PGB1
  • pecmsg
    pecmsg Member Posts: 4,852
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    The compressor really doesn’t care what temperature it is. It does care that the oil is warm. Make sure the crankcase heater is operating. How much heat you’ll get below 30 depends on the equipment. 
    WMno57PGB1
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,529
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    You have an accumulator that should protect the compressor. I would not go below 20-25 degrees outdoor air temp. You will know when the unit spends more time in defrost than in heating
    PGB1
  • PGB1
    PGB1 Member Posts: 77
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    Thank You All for helping and for the advice. It is very much appreciated.

    I found it odd that the manual didn't specify a minimum outdoor temperature for heating mode, other than to sell the Low Ambient Kit (a Headmaster valve, if I remember correctly).

    PC7060 mentioned a control circuit to handle the switchover. Incorporating this would be a rather simple project if we decide to use the heat pump for primary heating. (We're spoiled and prefer the silence of the hydronic. And there's nothing like hugging a radiator after working outside all day.)

    Thank You Each Again!
    Paul

    PC7060