April, 2017
Eric Shidell; HVAC Service Mentor
If you are reading this in most parts of North America, you may be facing a string of cool, mild weather. This is definitely not air conditioning weather. For air conditioning service techs who need to be out doing service calls and planned maintenance visits, that is not good news. When there is cold conditions outside, and little to no demand for cooling inside, air conditioning systems just don’t perform worth a darn. It is very difficult to determine whether or not a system is performing adequately or has the correct amount of refrigerant in it under these conditions.
This causes a lot of technicians to stay home, watch cable, and worry about how they are going to make ends meet when there is no work to do. They also know that when the weather breaks, there is going to be hell to pay because the scheduled calls that got cancelled due to weather still need to get done somehow.
If you want to get out of the house and get your PM calls done even in poor weather conditions, here are some hot tips to make your life easier.
First, you need to have a dry outdoor coil. Even in a light rain, there is a good chance your outdoor coil is still dry. How do you know? If your liquid line temperature is lower than the outdoor air temperature, your coil is wet. For this reason, you want to make sure that washing the outdoor coil is the last thing you do on your PM visit in cold weather.
Manufacturer’s charging charts and slide charts will be accurate and helpful down to about 55 degrees outdoor temp. Below that, you need a little help.
First, when you are about to begin travelling to the job location, call ahead and ask your customer to adjust the thermostat to 80 degrees in Heat mode in order to cause the furnace to run and heat. This will allow some btus to start to accumulate in the house and give the air conditioning system something to do when it turns on. Many homeowners may express concern about that, but if you explain that it is helpful to give the AC something to work on, and that that will help your visit to be more accurate and productive, most people will happily do as you ask.
Next, do what you can to keep the heat running while you are there. If you can, perform your electrical inspection of the outdoor unit while the heat is on. Replace or clean the filter while the heat is on. Once it becomes time to turn the AC on and do the running tests including check refrigerant charge, switch off the heat and switch on the AC. Apply your refrigerant gauges and liquid and suction line temperature probes.
Let the unit run normally for about five minutes and allow the heat from the furnace to dissipate.
The next step is to simulate warm outdoor conditions. Find something you can use to restrict air through the outdoor coil. You can use spare pieces of cardboard, empty trash bags, sheets of plastic, or your jacket and a few convenient rocks.
You want to partially block air through the condenser in order to drive head pressure to a more normal level. Ideally, you want to achieve a consistent liquid saturation temperature of 100 degrees. For R-22 this is a head pressure of 200 psig. For R410a this is a head pressure of 320 psig. You will need to adjust your condenser blockade so that the unit runs and maintains that head pressure.  Let the unit run for about 10 minutes so that it reaches a stable operating condition.
Now, you can run your superheat and subcool and indoor temp drop like you normally would. For fixed metering devices, use 80 degrees for your outdoor temperature value when determining required superheat.
If there is an undercharge or overcharge situation, you will find it. You can be reasonably certain that if the unit is working well now, it will continue to do so in the heat of the summer. If it is having problems now, it will have problems later, too.  You can add adjust the refrigerant charge, find airflow problems, metering device problems, compressor problems just as if it was a nice, warm day.  You can also give the unit a clean bill of health if that is the case too!
Once you have finished all your checks, now you can wash the outdoor coil at the end.
This method is not exactly perfect. You may find there about 5% of the units that you just can’t make a definite conclusion about how they will perform in warm weather. Those are the ones you want to schedule a return visit to double check when it’s warm.
That’s a whole lot better than rescheduling all of them!
-Eric Shidell

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