(-)A17 Two Stage Charging
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Charging
Charging a system at commissioning or after a repair is one thing. Walking up to a unit and slapping gauges on is a different story. You must remember, above all, the charging chart is to be used for charging. You can't put your gauges on, and look at pressures when you don’t know what stage the compressor is running, what airflow is being delivered, or if the load is what it needs to be. Read on...
Are you looking for the charging chart?
Factory Charge Values
| Size | Weight in Oz. (G) |
| (-)A1724JA | 128 Oz. (3628 G) |
| (-)A1736JA | 151 Oz. (4280 G) |
| (-)A1748JA | 204 Oz. (5783 G) |
| (-)A1760JA | 223 Oz. (6321 G) |
Charging Process
Charging a two stage system is pretty straight forward. You surely noticed these are not 8 or 10 SEER units with pistons or capillary tube metering devices, so we can’t treat them as such. The details are crucial to achieve proper charge. First and foremost a technician needs to understand is that there must be a specific load on the coil in order for pressures to line up with the ones on the chart. We’ve all seen in the past, on a high load, the suction is higher than it is on a low load, right? In other words, an 80 degree home will have a higher suction pressure than it would if it were 70 in the house, no?
So how do we ensure we have a load similar to that used in the development of the charging charts? It is safe to say, we don’t have control over the outdoor temperature, so the charts are made to accommodate various outdoor temps. We do however have the ability to control the indoor temperatures. After all that’s what we do for a living.
Airflow, or the quantity of air that carries heat, is also crucial to achieve a proper charge. This may mean turning off any airflow adjustments and de-humidification to provide design airflow. You can turn all that back on after you have completed the charging task at hand. None of those were in play when the charts were developed for the sake of consistency.
Finally, since these are two stage units, it is important to have the compressor running at full capacity. For the (-)A17, this is basically a call for second stage.
The following criteria must be achieved in order for charging to work.
• For Charging in Cooling, the Outdoor Temperature must be between 55 and 99 degrees.
• Indoor temperature must be 80 degrees F +/- 2 degrees. Yes, this is important.
• For Charging in Heating, the Outdoor Temperature must be between 40 and 60 degrees F.
• Indoor temperature must be 70 degrees F +/-2 degrees
• The system must be matched
• The coils and filters must be clean
• Airflow Adjustments and De-humidification must be turned off.
• Compressor must be running in second stage
• Verify Airflow is where it needs to be for the equipment.
o 2 Ton – 800 CFM
o 3 ton – 1200 CFM
o 4 ton – 1500 CFM
o 5 ton – 1700 CFM
If the system has been wired legacy or an RH2T air handler is being used, you will need to verify airflow in a traditional manner. This does not lessen the importance even though it’s more work.
When applied with an EcoNet thermostat (601 series, rev 42 or later, or 700 series). You may go to the menu and find the service icon, and select OD checkout. You may still access the status screens. Once the compressor is running at full capacity, verify airflow.
Allow the system to operate on the factory charge for a while. I would love to say 15 minutes, and while that may be the case occasionally, it won’t hurt to go 30 or 40 minutes before attempting to adjust the charge.
Measure the outdoor temperature (the temperature of the air going into the outdoor coil) and plot this on the charging chart. It’s important to remember all the key bullets listed above. It needs to be 80 degrees, plus or minus 2 degrees entering the indoor coil. If the head pressure is lower than the chart add refrigerant. If it’s higher, remove some refrigerant. Wait for a period of 15 minutes before making additional adjustments. Once you have pressures similar to those on the chart, allow the system to stabilize and then you can look at the subcooling. Do NOT go straight to subcooling.
Subcooling is used to overcome a pressure drop in the liquid line. The smaller or longer the line, the more pressure drop the subcooling must overcome. If you have a short line set, chances are you can be a little short on subcooling and be just fine. Overcharges can cause failures related to high discharge pressures, flooded starts, and others. Do NOT overcharge the system. The old adage of “If a little is good, a lot is better” does not, and never has applied to refrigeration.
IF you are commissioning the system in temperatures below 55 degrees or above 99 degrees, you will most certainly have to get it close and plan a return trip to finalize the charge when conditions are suitable. There may be data in the charging chart for conditions outside of these ranges, but they are not going to be 100% correct
