How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
What size in tons is a Miller air conditioner or heat pump from the model number?
Sunday, April 26, 2020
You can determine the “nominal” size of your HVAC system by examining the model number on the data plate at the side of the condenser (outdoor unit). Just like the nominal size of lumber or an air conditioning filter, the exact cooling capacity may be a little more or less than the nominal rating, but it’s close.
For Miller, you should look near the end of the model number, and it will be a number divisible by 6 or 12, which represents the nominal BTU of the system in thousands. Here’s an example of a Miller model number: Q5RD-048K. A ton of air conditioning equals 12,000 BTU, and 48 divided by 12 equals 4, so the system is 4 tons.
Here’s a rundown of the range you will encounter: 18 = 1.5 tons, 24 = 2 tons, 30 - 2.5 tons, 36 = 3 tons, 42 = 3.5 tons, 48 = 4 tons, and 60 = 5 tons.
If you are unsure whether you have found the right two numbers, you can double-check it by looking for the “RLA” rating on the data plate. RLA is an acronym for Rated Load Amperage, and is what the maximum amperage should be when the condenser is up and running. If you divide the RLA by 6 for older units and 5 or 6 for newer units, you should get a number that approximates (not exactly) the tonnage of the system. Make sure you use RLA and not LRA, Locked Rotor Amperage, which is the surge of amps necessary to overcome inertia and start the system. It averages around five times the RLA.
To determine the age of a Miller system, go to our blog post How can I tell the age of a Miller air conditioner or furnace from the serial number? For the age of another brand or manufacturer, go to our blog post How do I determine the age of my air conditioner?
To learn more about heating and air conditioning systems, see these other blog posts:
of Blog Posts
Top 5 results given instantly.
Click on magnifying glass
for all search results.