The Hotline: 07/30/2007

Low Ambient

From Dave Anderlik, Retired From Carrier

In past Hotlines, questions have been raised on fixed orifice low ambients. Blocking the condenser and charging to 105°F saturation temperature has been practiced and discussed for years. I would recommend that those involved in the servicing should go back under better outside/inside conditions and recover some of the refrigerant that was put in.

Knowing what the indoor conditions were, would also affect the heat and suction pressures.

One manufacturer has a superheat calculator that shows charging by superheat down to 55° condenser entering air DB and 50° entering air WB. If this was a residential application why would anyone be adjusting the charge below these conditions? If it was a spring maintenance call, for the benefit of the customer, it should not have been made.

Charging Old A/C Units

From Steve Sederstrom, York

I am responding to a Service Hotline article on charging an old a/c unit. It was stated that not all techs in the field had the equipment to do superheat and that they could also charge a unit with pressures and feel. This is wrong and should never be done under any conditions.

First of all, any tech that is out in the field had better have the equipment to charge with superheat and sub-cooling. If not, then they are robbing the customer of maximizing the performance of their equipment no matter how old it is. Any good tech knows that even if the temp is the same outside and you charge two houses on the same day, the pressures will not be the same.

You have to take into account the indoor temp, the rh inside. This will change from house to house and that changes the pressures. You must inform the techs out there, especially the young ones, that they must charge a system properly using superheat or sub-cooling or it may void any warranty left on the unit.

The difference between a professional and a fly-by-nighter is the tools. Every professional tech has the tools to do the job properly and thoroughly. The days of HVAC have changed. A tech cannot perform by the seat of his or her pants. We are professionals.

From Dan Kramer P.E.
Specialist Grade Member of RSES, Professional Engineer

Past discussions on refrigerant charging have not addressed the case of a system having a TXV fed by an in-line liquid receiver. I suggest that this type of system can be best charged by reference to a liquid sight glass positioned ahead of the TXV.

Yes, I am aware that there are cases such as liquid line bubbles caused by a high-rise from the receiver to the TXV; or by a restricted filter drier where an unknowing technician may overcharge the system. But in these cases, sole reliance on the sub-cooling method is also highly likely to lead to overcharge.

Regarding a statement that subcooling measurement needs to be employed to charge a TXV system, I wonder how many manufacturers routinely facilitate that measurement by providing a pressure tap in the liquid line upstream of the TXV for that purpose.

Lacking such a pressure port, I would continue to recommend that technicians learn how to correlate the feel of the liquid line temperature with the actual measured temperature as they are charging by scale according to the manufacturers’ recommendations. This will provide them with the experience to do it right, even lacking the manufacturers’ data.

As an aside, I am aware of one commercially available instrument that reads subcooling and superheat directly. It was made by Concept Technology and sold by JB Industries in Aurora, Ill. It has built-in and plug-in pressure temperature tables for various refrigerants. It makes the superheat and sub-cooling measurement job easier, however, you still need a pressure port where you want to measure each.

If you have a technical question, fax it to 815-654-7278 or submit it online by visiting The NEWS’ Extra Edition page and clicking on The Hotline link in the left-hand column.

Publication date: 07/30/2007

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