DaHaMac
10-23-2007, 04:45 PM
Hi guys, its been sometime since I've been able to lurk or post, such is the nature of being a maintenance man.
We've got an 18,000btu Friedrich VTAC that in the last couple of weeks has severely frozen the evaporator coils. At first I figured that this would be an easy problem to find, either it would be a low air flow problem or a leak in the refrigerant circuit. Especially since the unit has run without freezing for the past two years.
The fan motor is turning fine, one fan motor for condenser and evaporator (In fact this thing is a glorified window unit). The unit has been recovered, vacuumed (don't know exactly how deep, I still can't convice my supervisor that we need a micron guage), and the charge weighed in per factory specs. The unit ran for about five days with no problems and over the weekend it froze again.
The unit is running according to the guages a 40F evaporator and 135F condenser temperature. The discharge air is 55 with a return of ~75F, at its best. The space has been converted into a campus convenience store with accompanying coolers from an office area. I know that the unit is grossly undersized as it was not designed to handle the heat load from the coolers. This unit runs in cooling mode 24/7 and 365 days a year because of the heat load from the c-store. The only factor that appears to be similiar both times the unit has frozen is the fact that the OAT dipped into the low 50F's and possibly into the high 40F's with high humidity.
I feel sure that I have eliminated everything except for the Low Ambient control on the unit. The unit has a Low Temperature Thermostat attached to the lowest part of the evaporator. This thermostat according to the schematic is supposed to shut off the compressor when the evaporator coils become too cold. However, the last time the unit froze it iced up the evaporator coils and the filter.
We removed the low ambient thermostat and borrowed one from another unit, and I have tried to test this thermostat. I placed the thermostat that is suspected of being bad in the freezer and it opened like it should. However, I then placed the lead of my temperature probe on the thermostat and placed it back into the freezer until it opened again and this time it opened when it reached 21F. Now this sounds like this would be far too cold for the evaporator to reach and is the reason the evaporator is freezing, especially when the OAT gets low and the unit continues to run because the space is producing so much heat.
Does it sound like maybe I am on the right track with this Low Ambient Control, or have I overlooked something simple?
Thanks in advance,
Daniel
We've got an 18,000btu Friedrich VTAC that in the last couple of weeks has severely frozen the evaporator coils. At first I figured that this would be an easy problem to find, either it would be a low air flow problem or a leak in the refrigerant circuit. Especially since the unit has run without freezing for the past two years.
The fan motor is turning fine, one fan motor for condenser and evaporator (In fact this thing is a glorified window unit). The unit has been recovered, vacuumed (don't know exactly how deep, I still can't convice my supervisor that we need a micron guage), and the charge weighed in per factory specs. The unit ran for about five days with no problems and over the weekend it froze again.
The unit is running according to the guages a 40F evaporator and 135F condenser temperature. The discharge air is 55 with a return of ~75F, at its best. The space has been converted into a campus convenience store with accompanying coolers from an office area. I know that the unit is grossly undersized as it was not designed to handle the heat load from the coolers. This unit runs in cooling mode 24/7 and 365 days a year because of the heat load from the c-store. The only factor that appears to be similiar both times the unit has frozen is the fact that the OAT dipped into the low 50F's and possibly into the high 40F's with high humidity.
I feel sure that I have eliminated everything except for the Low Ambient control on the unit. The unit has a Low Temperature Thermostat attached to the lowest part of the evaporator. This thermostat according to the schematic is supposed to shut off the compressor when the evaporator coils become too cold. However, the last time the unit froze it iced up the evaporator coils and the filter.
We removed the low ambient thermostat and borrowed one from another unit, and I have tried to test this thermostat. I placed the thermostat that is suspected of being bad in the freezer and it opened like it should. However, I then placed the lead of my temperature probe on the thermostat and placed it back into the freezer until it opened again and this time it opened when it reached 21F. Now this sounds like this would be far too cold for the evaporator to reach and is the reason the evaporator is freezing, especially when the OAT gets low and the unit continues to run because the space is producing so much heat.
Does it sound like maybe I am on the right track with this Low Ambient Control, or have I overlooked something simple?
Thanks in advance,
Daniel