With the introduction of Microchannel condenser coils, some service practices have changed from traditional copper tube and aluminum fin coils. It is important to understand that Microchannel coils hold a relatively small volume of refrigerant. First thing to remember is because of this, you cannot pump down Microchannel coils when you need to service other parts of the system like you have done in the past with tube and fin coils.
Secondly, gauge sets should be connected only when necessary for service. Again, since the systems only hold a small volume of refrigerant, every time you put your gauges on the system and fill your hoses with part of the system charge, you could be affecting the performance of the system. A slight (ounce to a few ounces) under-charge can impact unit performance. Use your gauges sparingly. When you do have to attach a gauge set, if you have bled off the “charge” in your hoses for transport, re-charge the hoses and purge them before attaching them to the unit and make sure your low loss fittings are working. By charging your hoses, you are keeping both non-condensables out of the system and assuring you are not affecting the unit charge. Keep in mind that the gauge set hoses should be considered when charging a unit. (1.5 ounces per 3 foot hose typical). Also, make sure you turn off or close the low loss fittings on your gauges before removing them from the system to avoid loss of refrigerant from the system.
(Source 1 part number for set of 36″ hoses with low loss fittings S1-CCLV-36)
If you find that you do have to adjust a unit charge, a complete recharge by weight to the nameplate lb./oz. is the best method for “charge adjustment”. A slight (ounce to a few ounces) over-charge, or any non-condensables can cause high pressure trips, especially at times of compressor starts in higher ambient.
WHY IS THE REFRIGERANT CHARGE ON “MICROCHANNEL” UNITS SO CRITICAL?
At the risk of being called redundant, I need to emphasize the point that because the condenser holding volume is so small, these units can be unforgiving when the refrigerant charge is not precise.
Let’s look at an example — a YCJD30 13 SEER unit’s charge. The unit holds only 3# 14 oz. of refrigerant for the unit, coil, and 15 feet of line set. If you add just an additional 4 oz of refrigerant (1/4 lb.), you have just over-charged the unit by 10 %. If your line set is longer than 15 feet, be sure to calculate the “charge adder”for the size tubing you are using for the additional line length and carefully weigh it in. All the charging information is readily available on our Tabular Data Sheets that come with each unit. Also remember, that you need to charge with liquid refrigerant with R-410A. You may want to invest in a “quick charge” adapter that turns the liquid into a saturated vapor in order to avoid “slugging” the compressor. They do work very well.
You must use a Digital Scale to weigh in, or out, the refrigerant when you are adjusting the charge. Add or remove only 2 oz. per adjustment and wait at least 15 minutes per adjustment. This allows the refrigerant in the system to circulate and equalize itself in the system.
Old practices of charging like, “BEER CAN COLD AND SWEATING” or “OUTDOOR AMBIENT +30″ need to be thrown away. The use of “fishing scales”, “bathroom scales”, or “guessing” for weighing in charge, need to be thrown away. As mentioned above, you need to invest in a good digital scale to work on Microchannel coil systems since the charge in these systems is critical as the example above shows. Of course, you do need to use the charging charts on the unit and set either the super-heat or sub-cooling to the proper point depending on the type of metering device in your system. You also need to make sure the indoor unit is set for the proper CFM (400 CFM / Ton). Remember, when you add or remove charge, keep it small (2 oz. at a time) and allow 15 to 20 minutes to expire before making any further adjustments. It is a “slow” process but it will prevent recalls for over charge pressure switches tripping, or under charged and a “not enough cooling” calls. It all goes back to do it right the first time.
Of course, good brazing practices with a nitrogen trace running through the system and evacuation to 500 microns with a micron gauge is also critical to a properly performing system. Since moisture is a non-condensable, and since the charge is critical, even a little bit of moisture left in a system can cause nuisance head pressure trips.
Some other points about Microchannel coils that you need to know:
- Even though Microchannel coils are all aluminum, they can be repaired with our repair kit and solder alloy.
- If a coil ever does need to be replaced, the coils come with copper stubs on them so you only have to make copper to copper brazed joints. (see attachment at the end of this posting)
As I have posted in a previous Blog (May 4th), cleaning of the coil is also different from conventional tube and fin coils. To re-cap, that posting:
- the use of chemical cleaners is not recommended.
- coils can be blown out from the inside out or vacuumed off.
- coils can be washed out with a hose from the inside out
- pressure should not exceed 100 psig,
If you follow these procedures and those listed in the manufacturer’s installation instructions, you will have a properly working system with few to no safety trips or problems. Microchannel coils do provide high-efficiency in a compact design but can be unforgiving if not charged and serviced properly. Be sure to read some of my other postings on how Micro-Channel coils work so you have a better understanding of these systems.
SPECIAL FOR YORK CENTRAL CUSTOMERS ONLY — IN ORDER TO HELP YOU HAVE THE RIGHT TOOLS FOR THE JOB, WE ARE OFFERING A 10 % DISCOUNT ON MICRON GAUGES AND DIGITAL SCALES ON FIELD PIECE TOOLS GOOD UNTIL THE END OF JUNE. DON’T MISS OUT ON THIS OFFER!