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technical reason why ac condenser/compressor can't run in cold weather?

have a ductless system where the unit mounts on wall in the room and the 2 refrigerant lines run from it to the outside unit. On a particular system it's operating environment says outside temperature must be greater than 55°F for cooling. Can someone explain the reason why?
I don't mean specifically for this make/model, but what technically would cause an air conditioning system to have an operating condition where outside temperature needs to be greater than 55°F for it to work?

Assuming there is a temperature sensor on the outdoor condenser unit preventing it from operating below 55°F,
what would happen if that was disconnected or removed and the unit operated in colder conditions, down to 32° and down to 0°F ? Why would the unit not work or fail?

And what parts/features make it possible for an outside condenser unit to operate in cold weather?
Please don't just say "cold weather kit". looking for a technical reason.


  • Jamie Hall
    Jamie Hall Member Posts: 21,093
    The technical reason is simple enough -- when an air conditioner (or heat pump) is operating, the cold side (evaporator) drops below ambient -- sometimes well below. If it drops below the dew point (which it usually will) you will get condensation. If it then drops below freezing -- which it surely will if the ambient is less than freezing, and very likely will even with much higher ambients, such as 55 -- that condensation will freeze and block air flow in the evaporator.

    Which can produce anything from simply not working, to failing the unit completely.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,225
    What Jamie said, the evaporator will freeze first due to low system pressures. The next thing that happens, as it gets even colder, the system will pyramid due to the condenser being stacked full of liquid refrigerant. At this point the evaporator temperatures will rise again and your superheat will increase exponentially due to the decrease in mass flow rate. It won't provide sufficient cooling. If the compressor is outside, it may run for a while if it gets enough ambient cooling and doesn't loose all it's oil right away. If the compressor is inside it will fail rather quickly due to lack of cooling.
  • ron
    ron Member Posts: 402
    I'm a little confused and disagree with the indoor evaporator freezing.

    the situation is a computer room and there is a massive heat load, so there is no chance of air dropping below dew point in that room with the evaporator.
    i can basically guarantee air in the room is always > 80°F .
    That said, what then would be the reason why the service manual says the system can function only when outside temperature is greater than 55°F ?

    forgot to mention, it is an R-410a system.
    i did some googling, and the 2 most likely reasons are (a) the lubricating oil is spec'd around warm weather operating and there's a chance for lubrication failure, and the more likely reason of

    (b) is the pressure/temp of r410-a and high side pressure not being higher than low side pressure at metering device? Anyone know if this is true?

  • bob_46
    bob_46 Member Posts: 813
    That is true. Low ambient kits raise head pressure in one way or another. Some reduce condenser fan speed , some turn cond. fan off, some flood cond and dump hot gas into receiver. If your trying to cool a computer room get the good stuff google Liebert.
  • njtommy
    njtommy Member Posts: 1,105
    edited February 2016
    There is no other manufacturer that comes to mind or worth the money when it comes to server rooms. Liebert is the way to go. They are capable of dehumidifying, humidifying, and cooling the space down no matter what the out side temp is.
  • Matt_67
    Matt_67 Member Posts: 266
    there must be refrigerant flow for air conditioning to take place. Without low ambient controls the discharge pressure falls to the point where there is inadequate volume of refrigerant to accomplish necessary cooling. In comfort cooling systems with fixed orifices it usually not as much of a problem because the cooling load typically drops with the outside temp so reduced capacity is not a problem. In refrigeration systems, server rooms and industrial motor control centers or electrical substations the load is constant regardless of outside temp so there needs to be a means to ensure adequate refrigerant flow under low ambient conditions. The current best practice that Liebert uses is a special valve that backs refrigerant into the condenser to effectively decrease the amount of condenser available. another valve then bypasses hot gas into the receiver to keep the discharge pressure where it needs to be. Most of these type of systems use 407c refrigerant.
    JUGHNE Member Posts: 10,562
    I have modified a Marvair unit for a phone server building.
    Added a crankcase heater and fan cycling switch. The outside coil is protected from direct wind by its location on the east side of the building.
    This unit will use outside air for building cooling if conditions are right. But the compressor can operate if needed.
  • SWEI
    SWEI Member Posts: 7,356
    Lots of options, especially for free cooling. The optimal design depends on the climate. Pay attention to where Google, FaceBook, and Apple are building.
  • Techman
    Techman Member Posts: 2,144
    That kinda depends on the brand of ductless you are talking about. What do you have?
    80* in a server room is a little on the high side, why so high?
  • aircooled81
    aircooled81 Member Posts: 205
    78 is the new norm for idf and server rooms out here in the bay area.
    Back to original post, the lower the head pressure, the more efficient the unit works. Unfortunately, when the head pressure tanks, it becomes sooo efficient, it lowers the pressure in the condenser toooo much. This can cause poor metering device reaction, and you loose your capability of cooling at your indoor coil.
    its not like you have a fixed orifice or cap tube with your system, but you can try a few varietys of head pressure control otherwise.
    Either get a switch to cut the condenser fan out, a controller to slow it down, or a damper to block air off to the coil while its cold.
    Ive seen guys do a winter swap on the condneser fan blade, laughable i know, but sometimes you gotta do what you gotta do. His idea was, turn the blade around, so its not moving the same air around it used to, basically upside down. It probably moved far less air, and ran into trouble on warm days!
  • Techman
    Techman Member Posts: 2,144
    edited March 2016
    Or, lets talk oil burner nozzles for a sec or two. The "stamped rating" of the oil nozzle is at 100psig,so, a 1.00 80* B ,nozzle delivers 1 gal of oil an hour. Now increase the oil press up to 125psi and now the oil nozzle is delivering 1.18 gph. Thats 18%. Then drop the oil press down to 75psi and now, what is the gph delivered?

    OK, back to AC and its "nozzle" , or, it's "metering device". The lower the ambient temp the lower the press!Same thing! KindaSorta, depending on the type of "metering device", depending on the "type" of Ductless Split equipment. So, depending, the 80* room, w/ the "lower than allowed amount of Freon" being fed into the evap coil causes a "lower than allowed " suction press. The "new" suction press/temp is = 25*F. KindaSorta.

    And the lower press makes for LARGER atomized liquid droplets leaving the metering device which adds to a "lower mass" of the suction gas which leads to "leaving the comp oil behind" causing the comp to run low/out of oil and WHACK! Another innocent comp DIES!!!!!

    Just a Q here on the server room temp. Is this "new" proper temp of 78* or so allowed by the Computer Mfrgs? I went to a 3day Liebert Class back in 77' in Columbus, Ohio ,so I'm still using that "old school" info.
  • njtommy
    njtommy Member Posts: 1,105
    On regular splits that run in cold weather I use an ICM325HN fan speed control. Along with an icm175 bypass timer for the low pressure safety. Also crank case heater for for the compressor.
  • SWEI
    SWEI Member Posts: 7,356
    The big guys are running them hotter than that.
  • Techman
    Techman Member Posts: 2,144
    Hi SWEI, back in the day there was "gold scavenging" and "silver migration" to worry about thru proper indoor conditions. I was told ,today, that the "newer" computers don't have those two problems, anymore. How high have you seen? How high of a temp causes damage to the server/computer? Tks!
  • SWEI
    SWEI Member Posts: 7,356
    My knowledge is about a decade old at this point, but I know Apple, FB and Google are all doing a lot of work in the area, and that both motherboard and chip makers are working to keep up. Check out the DC locations -- they're picking mild climates with nearby water sources for 2-stage (and even single stage) evap coolers.
  • aircooled81
    aircooled81 Member Posts: 205
    Server rooms, dont go below 50*F or above 82*F. Here in california, energy conservation and title 24 run all. The data you get from the big boys is likely coming from a server room around 78*F to 80*F. The small rooms like idf and a couple racks might like 68*F to 71*F for optimum performance, or the guy who works in there just likes it cold and says its gotta be that low.
    When they are building a room to hold servers, are they insulating the ceiling tiles and the walls, not likely. So no use driving the temp lower than the outside rooms and ceiling, or your wasting energy.
    More common now are economizers, which was worry back then because of humidity, but in this area 40% rh is fairly typical. Most of our economizers are dry bulb, not enthalpy controlled.
  • robsimpson
    robsimpson Member Posts: 3
    edited March 2016
    Ductless mini splits are not designed for server rooms. First is they don't cool well at low ambient temps (clearly addressed above) without a low ambient cooling kit added. Second is that they are almost all designed with a 70/30 sensible heat ratio. As stated by njtommy above: use a Liebert.