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What’s So Wrong With “Beer Can Cold”?
First off, if you've never heard the term “beer can cold,” you are either not in the trade, or you have been living a pretty sheltered existence. I started as a tech apprentice when I was 17 years old, and on my first day in the truck, my trainer grabbed the suction line of a running split system and said, “She's running good! Beer can cold.” Now, before you freak out, my trainers were primarily a couple of guys named Jimmy Wells and Dave Barefoot, and these old school techs would JOKE about beer can cold, and then they would proceed to connect their gauges and properly check superheat and subcooling.
There are two things to know about old sayings like “beer can cold,” listening to your vacuum pump, or feeling the air velocity out of a register.
#1 – They Can Be Useful Tools of An Experienced Tech
When I was in trade school, my instructor taught me to “feel my way” through the refrigerant circuit to identify the liquid line, suction line, and discharge line by touch. This resulted in some minor burns and a perspective on the “qualitative” or intuitive understanding of the refrigerant circuit.
Using your senses to hear, feel, and smell the system are really important tools an efficient and effective tech builds over time to alert themselves of slipping belts, a vacuum pump that isn't operating properly, a burned board or transformer, a bad bearing, or even an underfeeding or overfeeding evaporator. This is where “beer can cold” (grabbing the suction line to get an approximate temperature) isn't always a bad thing—but only when used as an initial qualitative test.
#2 – Senses Should Lead to Measurement
A good diagnostic technician finds THE problem first; whatever is primarily causing the problem is the first order of business. Once that primary problem is identified, THEN a good tech moves on to inspecting the entire system and making more measurements as possible to identify additional issues. Once the initial set of know issues have been rectified, then a good technician will always verify proper system performance using real measurements that PROVE that the system is operating properly.
So, let's be 100% clear.
You cannot charge a system by “Beer Can Cold.” It is nothing more than a long-running inside joke that refers to grabbing the suction line and it feeling cold like a beer on a functioning A/C system.
But….. (Warning, I'm about to take this WAY TOO FAR)
Depending on the type of beer and the preference of the drinker, beer can be anywhere from 36°F (2.22°C) for a good old can of American Lager all the way to about 55°F (12.77°C) for a British stout kept at cellar temperature. Craft beer enthusiasts will tell you that about 45°F (7.22°C) is a good compromise between flavor and temperature.
On average, your evaporator temperature will have a 35°F (19.25K) DTD (design temperature difference), which means the coil temperature will be about 35°F (19.25K) colder than the return air DB temperature. This means if it's 75°F (23.88°C) in the return, the evaporator will be at about 40°F (4.44°C). We then need to add in superheat, which will vary quite a bit on a fixed orifice system. On a TXV or EEV system, it will be between 5°F and 15°F (2.75°K-8.25°K) on a properly functioning system. This means that the suction line indoors could range from 45°F to 55°F (24.75K-30.25K) by the time you account for the TXV superheat range, the uncertainty of the temperature measurement, and the variability in DTD. If you are grabbing the suction line outside, you will also need to account for anywhere from a 1°F to an 8°F (0.55K- 4.4K) rise in temperature on the suction line by the time it gets from the coil to the outdoor unit where “beer can cold” is taken. Now, the range is all the way from an acceptable beer temp of 46°F (7.77°C) all the way up to a putrid 63°F (17.22°C) that even the British would find unacceptably warm.
All of this, just at a 75°F (23.88°C) return temperature WITH a TXV
I don't know about you, but my hand is only calibrated to within +/- 4°F (2.2K), so when you add that to the mix, I find that using my hand to feel the suction line gives me only the roughest estimation of what is going on and if 50°F (10°C) is the average. That is too warm for my taste in beer anyway.
Beer can cold, like most “rules of thumb,” is far too inaccurate to be useful (at the risk of overstating the obvious).
What I do recommend is becoming fully familiar with:
- The design CFM of the system and the sensible/latent requirements of your area
- The efficiency of the equipment you are working on (to help anticipate condensing temperature)
- Type of metering device (to understand target superheat)
- Evaporator coil design temperature difference (DTD)
- Condensing temperature over ambient (CTOA)
- Delta T
- Static pressure
Add a good understanding of all of these readings, and when and where to take them in the mix, and THEN and ONLY THEN have you earned the right to make jokes about beer can cold. If you have not yet understood the concepts, I would advise starting by reading THIS and then listening to THIS.
P.S. – Just for fun, we created some “beer can cold” and other inside joke gear that you can find at HVACSchoolshop.com