It’s that time of year when we get calls on a hot afternoon and hear our customers complain, “Our unit isn’t keeping up, and we need a service call.” Although there are many reasons for this type of call, and we’re glad our customers call, it’s a fact that cooling equipment capacity actually decreases on a hot day. Let’s take a look at how much it decreases depending on the condition it operates under.
12,000 BTU per Ton
First of all, no equipment delivers 12,000 BTU per ton on a hot day. 12,000 BTU per ton is called nominal BTU. Nominal means “kind of, but not really” in this case. Actually, under the AHRI design conditions of 95F outdoors and 67F wet bulb indoors, the typical system is only rated for around 11,200 BTU per ton.
In reality, when you do an ACCA Manual J on a building and identify the heat gain of a building, the results should be 36,000 BTU. That doesn’t mean a three ton unit will do the job.
I remember being new to the industry and when I learned this for the first time I had a long talk with my Dad. It was like learning about the birds and the bees all over again.
There’s another step required to size equipment described in ACCA manual S (S stands for equipment selection) where we adjust for the decrease in capacity that’ll result as a typical system operates in the field on a hot summer’s day. That’s really what this article is all about.
So at 11,200 BTU a three ton is now a 2.8 ton.
Over 95 Degrees
So, we’re down to 11,200 BTU per ton. When the outdoor temperature goes over 95F, the cooling capacity of the equipment continues to decrease. The rough number is about 1% per every two degrees warmer. It doesn’t sound like much, but when you start adding all these losses up, they’ll make a significant difference in the system capacity.
When you’re out servicing or selling on a hot day, and you’re not aware of this reduction in capacity, you may take some actions that aren’t good for you or your customer.
So on a really hot day, a three ton is now a 2.6 ton.
80 Degrees Indoor Temperature
67F wet bulb, is the indoor temperature that manufacturer’s rate equipment capacity under. In plain talk, it’s around 80F dry bulb at 50% relative humidity. Now as I get older, I don’t keep my office at 68F anymore. I’m starting to smell bacon cooking at 80F and I’m hot and sweaty. Unless you’re 90 years old and quite ill, you’re hot and sweaty at 80F too. So we’ll have to knock off more capacity if our customer wants to actually be comfortable indoors. Most of the country designs for 75F indoors. At 75F, we lose more capacity.
Typically, say goodbye to another 7% system cooling capacity to adjust the inside temperature down to something most people can live with. Many prefer 73F indoors, and that will knock off another 5% for a total of 12%.
In addition, latent BTU removal will take a huge dive also. But we’ll reserve that topic for another article.
So reducing the indoor temperature to where most customers are comfortable, a three ton system is now a 2.3 ton.
400 CFM per Ton
Sure, we design for 400 CFM per ton, but the average US system delivers below 300 CFM per ton and that knocks off another 5%. It’s difficult to challenge your delivered airflow individually, but unless you have measured your system and verified for yourself that you’re getting more than 300 CFM per ton, knock off another 500 BTU per ton.
If you are considering low airflow, you’re down to just above 2.1 tons of capacity out of your three ton system.
For the sake of discussion, we need another tenth of a ton loss in this three ton system to make the name of the article work. Can we assume the refrigerant charge isn’t perfect? Unless 400 CFM per ton was verified by legitimate airflow measurement when the refrigerant system was charged, we finally discovered why a three ton may be a two ton.
A Ton and a Half?
Ducts in unconditioned spaces you say? Oops. That’s another loss. Those that measure find duct losses on a hot day with ducts in the attic may easily take another half ton bite out of our suffering system. Simply measure the temperature leaving the supply registers and compare it to the temperature leaving the equipment. Do the same on the return side of the system.
If you have a 20 degree drop over the equipment and the supply and return duct system loses 5 degrees, the two tons we had left is now a one and a half ton.
The Right Sized Equipment
The good news is that some of these losses are system defects and we can discover and fix them. Many of us are well aware of legitimate losses in equipment capacity and fully compensate for each of the losses and deliver systems that work like we say they’ll work.
Hopefully what we’ve learned here can help us identify some of the obstacles we’ll face on the next hot day in the field when our customers are concerned with their comfort and we’re one ones charged with solving the problem.
Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company with technical and business level membership organizations. If you're an HVAC contractor or technician interested in a generic table indicating how equipment capacity varies under changing conditions, contact Doc at firstname.lastname@example.org or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.