carrier package unit

[anthonyac1]

i work on alot of carrier package units, thanks to walgreens. roughly 10 tons 2 stage cooling , 2 seperate circuits. anyways i seen there is a pressure switch on the discharge line and one on the liquid line! its NOT a heat pump, they are all like this so i assumed one on liq. line was a fan cycling switch until i said let me trace the wiring. pressure setting is faded off anyways they are in series together tied to the famous carrier lockout board. Why 2? i called tech support the guy told me the one on the discharge line is hi press. cutout. the one on the liquid line is a loss of charge (similiar to a low press switch). he said they do that on all the heat pumps so in the heat mode it will sense low pressure. ok cool but this is not a heat pump then he told me no even on the straight cool units they still put it there from factory so there is no confusing on the location. he said in this situation it serves no purpose and basically i have no low pressure protection. i think that is really stupid who do they have installing them??????????

[timebuilder]

i work on alot of carrier package units, thanks to walgreens. roughly 10 tons 2 stage cooling , 2 seperate circuits. anyways i seen there is a pressure switch on the discharge line and one on the liquid line! its NOT a heat pump, they are all like this so i assumed one on liq. line was a fan cycling switch until i said let me trace the wiring. pressure setting is faded off anyways they are in series together tied to the famous carrier lockout board. Why 2? i called tech support the guy told me the one on the discharge line is hi press. cutout. the one on the liquid line is a loss of charge (similiar to a low press switch). he said they do that on all the heat pumps so in the heat mode it will sense low pressure. ok cool but this is not a heat pump then he told me no even on the straight cool units they still put it there from factory so there is no confusing on the location. he said in this situation it serves no purpose and basically i have no low pressure protection. i think that is really stupid who do they have installing them??????????

It's still a loss of charge, from what I can tell.

When I work on one (a repair that will cover the cost) I will install a proper low pressure switch on the low side, and move on.

[anthonyac1]

if its almost completely out of gas then i can see it serving its purpose, and it will probably cut out. but i agree just install one.

[Dallas Duster]

It all work correctly as designed. In theory where are the possibilities of a freeze up, right past the drier . The drier protects the metering devices. The switch has a cutout of 7 psi. The mist safety in this sequence is the freeze stat on the evaporator coils. That is basically your main low pressure protection.

[timebuilder]

In theory....

When I see a 7 psi switch on a liquid line, I have to ask myself: how long will that compressor be able to run at 8 psi before the compressor takes its last breath? At 8 psi on the liquid line, what is the low side pressure? Oil return?

I'm going with a low pressure switch.

[anthonyac1]

It all work correctly as designed. In theory where are the possibilities of a freeze up, right past the drier . The drier protects the metering devices. The switch has a cutout of 7 psi. The mist safety in this sequence is the freeze stat on the evaporator coils. That is basically your main low pressure protection.

if your txv bulb gets stuck closed the evap wont freeze due to no refrigerant passing through, also the location on the freeze stat is about 3/4 up from the bottom of the evap coil. lack of ref. will not freeze all the way up, at times it might not trip. but there are many different scenarios we can go with, but my main point was that the guy from tech support said its just there so at the factory they dont get confused. and it is useless on this application.

[Dallas Duster]

if your txv bulb gets stuck closed the evap wont freeze due to no refrigerant passing through, also the location on the freeze stat is about 3/4 up from the bottom of the evap coil. lack of ref. will not freeze all the way up, at times it might not trip. but there are many different scenarios we can go with, but my main point was that the guy from tech support said its just there so at the factory they dont get confused. and it is useless on this application.

Well since there aren't any TXV's on the units in question I don't see that happening.

[Dallas Duster]

In theory....

When I see a 7 psi switch on a liquid line, I have to ask myself: how long will that compressor be able to run at 8 psi before the compressor takes its last breath? At 8 psi on the liquid line, what is the low side pressure? Oil return?

I'm going with a low pressure switch.

Hav you ever seen a liquid line run 7 psi?

[anthonyac1]

Well since there aren't any TXV's on the units in question I don't see that happening.

i know carrier has the liquid line going into the header. but ive also seen models with txv's. that i had to replace because the transmission tube on the txv rubbed and caused the powerhead to lose its charge causing the txv to close.

[Dallas Duster]

i know carrier has the liquid line going into the header. but ive also seen models with txv's. that i had to replace because the transmission tube on the txv rubbed and caused the powerhead to lose its charge causing the txv to close.

And those units with TXV's have a real low pressure switch that cuts out at 27 psi plus a coil freeze stat.